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St Francis Xavier University, Antigonish, Nova Scotia, Canada   |
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Mechanisms of ecogeographic isolation between Mimulus cardinalis and
M. lewisii Reproductive isolation is critical for the origin and maintenance of species differences. Because isolating barriers act sequentially throughout the life history of an organism, late-acting barriers to mating prevent only that fraction of gene flow that is not prevented by earlier acting barriers. For this reason, ecogeographic isolation is an important though frequently overlooked reproductive barrier. Here we examine the mechanisms contributing to ecogeographic isolation between the interfertile sister species Mimulus cardinalis and M. lewisii. In the central Sierra Nevada Mountains of California, M. cardinalis occurs from 0 – 1800 m whereas M. lewisii occurs from 1200 – 3000 m. Reciprocal transplant gardens show that each species achieves highest fitness within its elevation range. M. lewisii experiences 100% mortality at low elevation. Although M. cardinalis survival is high at all sites, its reproductive success drops to 0 at high elevation. Growth chambers simulating low and high elevation temperature regimes replicate the fitness differences observed in reciprocal transplants. Taken together, ecogeographic isolating mechanisms related to temperature tolerance reduce the probability of co-occurrence and the opportunity for hybrid formation between M. cardinalis and M. lewisii throughout their ranges. The Phlomis X composita hybrid group viewed
under the recombinational speciation evolutionary scenario Phlomis is a Mediterranean and Irano-Turanian genus in the Lamiaceae where interespecific hybridization seems to be common. Polyploid and allopoplyploid species are rather frequent in the plant kingdom; contrarily, diploid hybrid species are scarce, and in fact, only a few cases have been rigorously documented in nature. We are now examining population genetics’ theoretical predictions for the likelihood of homoploid hybrid speciation in the P. X composita group (P. lychnitis X P. crinita) in Spain (Iberian Peninsula). Morphometric analysis, ecological data and allozyme variation supported an ecogeographical pattern of distribution and their frequencies of hybrid forms. Moreover, reproductive biology experiments revealed that hybrid forms are relatively fertile with very weak or even null reproductive isolation barriers to parental plants, and chromosome structure differences were also minimal. Thus, backcrossing, introgression and gene flow are, at present, the rules in this case study. Given that homoploid hybrid speciation is a punctuated process, all the interpretations in this hybrid group should be focused within this evolutionary scenario where rapid chromosomal reassortment could raise effective isolation reproductive barriers. Study of chloroplast and mitochondrial DNA diversity in the
natural populations of Gaultheria fragrantissima in the two hotspots in
India Gaultheria fragrantissima occurs only in the two hotspots in India, Western Ghats (WG) and Northeastern Himalayas (NE). It is a medicinally important plant where the oil from its leaves known as ‘Oil of Indian Wintergreen’ is used in most of the pain balms. PCR- RFLP technique was used to study chloroplast and mitochondrial DNA diversity in its natural populations. Eight cpDNA and seven mtDNA haplotypes were detected from eight populations. None of the cpDNA haplotypes is shared by all the populations. Four cpDNA haplotypes are unique to WG and four to NE region. One mtDNA haplotype is shared by all the populations and is the most common haplotype. Three mtDNA haplotypes are common to both the hotspots, three are unique to WG and one to NE region. CpDNA diversity (hT= 0.639) is higher as compared to mtDNA diversity (hT= 0.582). However within population diversity is higher in mtDNA (hS= 0.467) than cpDNA (hS= 0.224). The level of population subdivision for unordered and ordered alleles is much higher in cpDNA (GST= 0.650; NST= 0.841) as compared to mtDNA (GST= 0.197; NST= 0.252). Higher value of NST than GST in both cp and mtDNA reveal high differentiation among population. Use of cpDNA sequences for studying the native and introduced taxa
of the Poa pratensis complex (Kentucky bluegrass) The Poa pratensis complex (Kentucky bluegrass) is taxonomically difficult and characterized by high polyploidy, aneuploidy, hybridization, and few useful morphological characters. Previous restriction site data (an important indel and site variation) from the chloroplast DNA region previously known as rbcl-ORF106 separated North American taxa into two groups, an indigenous arctic/alpine complex comprising P. pratensis subspp. alpigena and colpodea, and a widespread non-arctic low elevation complex comprising taxa of European origin, such as subspecies pratensis, and cultivars. These markers show promise for recognizing native P. pratensis in North America; for example, many floras do not distinguish native alpine from introduced taxa. Sequence data show that a deletion of approximately 180bp in the cpDNA rbcL-psa1 region (which overlaps the rbcL-ORF106 region) characterizes the arctic/alpine P. pratensis subsp. alpigena. In addition, the rbcL-psa1 region possesses a small region of high variability, which will be useful in studies of taxonomically difficult complexes in Poa. The aims of the present study are to determine by sequencing the cpDNA rbcL-psa1 region the molecular basis of the restriction site characters, to confirm the relationships between the native and introduced North American taxa, and to provide diagnostic tools to differentiate them. The relationship between European and North American lineages of the P. pratensis complex is also explored. Haplotype diversity patterns in autogamous Packera pauciflora Packera pauciflora, is one of two wide ranging autogamous species in a genus where most species are both allogamous and prone to hybridization. Its distribution is centred in the northern Rocky mountains but it also occurs at high elevations in California and in the Gulf of St. Lawrence region of eastern Canada. Chloroplast DNA haplotype variation was examined in populations from northwestern North America and California. Haplotype diversity values within populations were generally low, especially compared to other Packera species. Most populations were monomorphic and most, but not all, populations contained haplotypes that are similar to each other and to other coastal and northern Packera populations from different species. California populations contain haplotypes not found in the northern populations. The lack of hybridization resulting from the autogamous breeding system is thought to have contributed to the reduced levels of haplotype diversity present in this species when compared with other widespread Packera species like P. cana. The high level of interpopulational variation is thought to be the result of a combination of drift and possibly some limited ancient hybridization. Biochemical adaptation to soil moisture in a Rocky Mountain
wildflower Functional studies in evolutionary genetics are required to distinguish between competing hypotheses regarding which evolutionary processes generate patterns of genetic variation. Such investigations foster mechanistic insight into the consequences of allelic variation for organismal fitness in the field. Here, we present evidence that natural selection by water stress maintains an allozyme polymorphism in 6-phosphogluconate dehydrogenase (6PGD2) alleles in Ipomopsis aggregata. Observations that favor an adaptive explanation include: 1) reproducibility of a correlation between the 3 allele (6PGD2-3) and soil moisture in two years, 2) presence of the correlation in a set of geographically isolated populations and in local subpopulations subject to heavy gene flow, and 3) discovery of two isolated high-frequency sites. We also demonstrate in situ an allele-specific effect in the response of plant performance to different soil moistures. For only those genotypes carrying 6PGD-3, plant performance - as measured by plant size - significantly correlates with soil moisture, with carriers performing better in wet sites, and worse in dry sites, relative to non-carriers. The strong selection observed, as indicated by population divergence despite gene flow and measurability of allelic impacts on performance, demonstrates that this is a promising system for the study genotype-phenotype-environment interactions and their role in evolutionary process. Maximum-likelihood
estimates of hybrid index based on molecular markers Hybrid zones are natural settings for the study of barriers between species. Much of the research in hybrid zones benefits from estimates of the relative contribution of parental genomes to the individuals encountered. Various methods exist for estimating the hybridity of individuals based on genetic and morphological characters. This presentation develops a maximum-likelihood estimate of a hybrid index based on dominant and codominant genetic markers. Dominant markers have an inherent ambiguity because homozygous present and heterozygous genotypes can not be distinguished, and this lack of resolution is incorporated in the calculation of the hybrid index. In contrast, genotypes of codominant markers can be fully resolved and typically involve a larger number of alleles per locus. The potential information content of each class of marker is described, as is the effect of the number of loci on confidence intervals for the hybrid index. The calculation of the hybrid index is implemented in stand-alone software and is available from the author. Catching the rays: detecting hybridisation and introgression in the
genus Senecio (Asteraceae) The introgressive or hybrid origin of a number of plant species has been proved using molecular markers. Such markers are generally thought to be neutral and therefore their role in the new species is unknown. Here we use cycloidea genes as a marker for introgression in the genus Senecio. Two members of the cycloidea gene family are very tightly linked to the gene controlling presence/ absence of ray florets in Senecio. We have used these genes to examine the origin of two disjunct subspecies of the tetraploid Senecio mohavensis. Secondly, we have used cycloidea genes to determine the parentage of the hexaploid S. teneriffae, a Canary Island endemic, closely related to the British neohexaploid S. cambrensis. Phylogenetic reconstruction of the genus Fragaria using intron
sequence from a protein encoding nuclear gene The history of the genus Fragaria involves two important evolutionary phenomena, hybridization and chromosome doubling (polyploidy). The octoploid, cultivated strawberry, Fragaria x ananassa, is a hybrid between two wild octoploid species, F. chiloensis and F. virginiana; however, its ancestry has not been traced to the diploid level. We performed parsimony and Bayesian inference phylogenetic analyses on intron sequence data from the alcohol dehydrogenase gene to reconstruct the phylogeny of the genus and identify the diploid progenitors of the cultivated species. Our analyses included data from thirteen diploid and five polyploid Fragaria species, and one outgroup species, Potentilla fruticosa. Multiple alleles for each allopolyploid species were detected and all unique sequences were included. Our data contain several phylogenetically informative gaps, and we have explored methods to incorporate them into the analyses. The resulting trees form two clades, clade A and clade B, with limited resolution at the terminal branches. Each wild octoploid has alleles in both clade A and clade B, suggesting at least one diploid contributor from each clade. Although not thoroughly resolved, these trees implicate diploids F. vesca from clade A and F. iinumae from clade B as likely progenitors of the octoploid species. Speciation in facultatively apomictic Hieracium subgen. Pilosella
taxa: a bottom-up approach Facultatively apomictic taxa present special difficulties for the elucidation of speciation processes. A vast number of (micro)species and a virtual continuity of forms make their identification and treatment a challenging task. Therefore, we started at the clonal level by multilocus fingerprinting in order to assist with their proper determination and taxonomic delimitation as a foundation for further studies and to determine the number, origin and geographic distribution of clones present in different apomictic taxa. A mixture of basic species, old fixed intermediate forms and recent hybrids was found. Next, we studied hybridization and introgression phenomena of the confirmed taxa at population and species level using sequences and structural properties of the trnT-trnL intergenic spacer. Despite multiple origins of clones, most of the presumed hybrids obtained their chloroplast DNA from the same parent and often, the facultative apomict even proved to be the maternal parent rather than the sexual species. Finally, a molecular phylogeny of all Pilosella basic species was established based on chloroplast as well as nuclear (ITS) sequences. Discrepancies between them reflected either morphological/cytological or distributional/ecological features. The molecular approaches were accompanied by morphological and cytological analyses as well as by studies of reproduction systems and experimental hybridizations. Characterizing the genetic basis of quantitative trait differences
between ecotypes of Avena barbata using recombinant inbred lines To investigate the genetic basis of differences between naturally occurring ecotypes we have constructed a population of 188 recombinant inbred lines (RIL) from two genotypes of Avena barbata that show strong association with moist and dry habitats. An initial survey of all 188 lines was conducted in the greenhouse, alongside their parental ecotypes, for a suite of early and later growth traits such as germination time, tiller number, flowering time vegetative vs. reproductive biomass and spikelet number. Broad sense heritability estimates for all traits were high (>.50) among the RILs and significant genetic correlations among traits were detected. The RILs were subsequently genotyped with 150 AFLP markers found to be polymorphic between the parental ecotypes. Linkage analysis reveals the markers group into several small linkage blocks that span 360 centimorgans of the A. barbata genome. Single marker quantitative trait locus (QTL) mapping analysis has identified marker loci that show strong association with at least one trait and several markers showing association with multiple traits. Preliminary interval mapping has revealed one position where QTL for flowering, spikelet number and vegetative biomass co-localize and another where QTL for spikelet and tiller number co-localize. Evidence for an adaptive
radiation in Agave species In this study we aim to understand the phylogenetic limits and time of origin of the family Agavaceae and to explore the possibility that there has been an adaptive radiation in its most species rich genus, Agave sensu lato. To this end, we present an original analysis of the phylogenetic relationship of the monocots using 334 sequences of rbcL obtained from genbank, 24 dicot and 1 gymnosperm sequence to root the tree. We generate the topology of the tree using maximum parsimony and estimate branch lengths using maximum likelihood. This information is used to estimate the age of all internal nodes using penalized maximum likelihood and the age of the gymnosperm/ angiosperm split (132 MYA) to calibrate rates. Next we combine some original and genebank sequences for both nuclear (ITS intergenic region) and chloroplast (two intergenic spacers) genes to examine patterns and rates of speciation in the highly diverse group, Agave sensu lato, that includes more than 200 species. These analyses of molecular data are combined with information on the life-history characteristics of Agave species, in particular the existence of monocarpy, giant rosettes and pollinator syndromes. We find that the genus Agave has a relatively recent origin (~ 8MYA ± 2 MYA) in the Americas and that it appears to have undergone an adaptive radiation early in its evolutionary history. The origin of Helianthus deserticola: survival and selection in a
desert habitat The diploid hybrid species Helianthus deserticola inhabits an extreme environment relative to its parental species H. annuus and H. petiolaris. Adaptation to the arid desert floor may have occurred via the acquisition of novel phenotypes resulting from transgressive segregation in early hybrids. We have explored this possibility through a field experiment designed to test the direction and intensity of phenotypic selection, using crosses between the parental species as proxies for the ancestral genotype of the ancient hybrid species. Helianthus deserticola, H. annuus, H. petiolaris, and early-generation hybrids between H. annuus and H. petiolaris were all grown in native H. deserticola habitat, and a selection analysis revealed that several traits were subject to strong selective pressures. Several of the traits under selection were also extreme or transgressive in H. deserticola, and the range of variation present in BC2 hybrids suggests that many aspects of the H. deserticola phenotype are easily recreated. Thus, transgressive segregation may have contributed to the adaptation of H. deserticola to the desert habitat. Annual fluctuation of
pollinator-mediated selection on floral traits in a natural hybrid population
of daylily (Hemerocallis fulva) and nightlily (H. thunbergii) Pollinator-mediated selection is probably a major force that drove speciation through diversification of floral shapes. To test this idea, we studied a natural hybrid population between daylily (H. fulva) and nightlily (H. thunbergii) that show extraordinarily high level of variation in floral traits. H. thunbergii is pollinated by hawkmoths at night while H. fulva is pollinated by butterflies and bees. In 2001 when a hawkmoth, Theretra japonica was dominant, fruit set was higher in more H. thunbergii-like plants having longer flower tube and shorter pistil. On the other hand, in 2002 when diurnal bees were abundant, fruit set was also higher in H. fulva-like plants having wider petals and longer stamens. This finding suggests that the high level of variation in floral traits in the hybrid population is maintained under annual fluctuation of pollinator composition and resulted fluctuating selection on floral traits. In the habitat where either hawkmoths or diurnal pollinators are dominant, a set of phenotypes that characterize parental species will be favored. A hypothesis on the origin of Agave tequilana Weber var. azul, the
raw plant material for the tequila production Hardly found in wild form and widely cultivated in Jalisco, Mexico, Agave tequilana Weber var. azul (Agavaceae) is the only species of the genus allowed for the production of the mexican alcoholic beverage known as tequila. This plant shares many morphological characteristics with A. angustifolia Haw., making difficult in many cases its identification. The previous Gentry’s suggestion (1982) about the possible origin of A. tequilana Weber var. azul as a selected variety from A. angustifolia, seems to be tenable. Thus, based on a review of the extant literature and on field observations, we propose a hypothesis for the origin of A. tequilana Weber var. azul. The history and consequences of self-incompatibility: Solanaceae Gametophytic self-incompatibility (SI) in Solanaceae is one of the best-understood systems of self-recognition and rejection. Our knowledge of its molecular basis and history of allelic lineages provide unequivocal evidence that distantly related Solanaceae genera share SI ancestry. A phylogenetic analysis combined with molecular genetic data from extant taxa allows accurate ancestral state reconstructions. Transitions from SI to self-compatibility (SC) are unidirectional, and SC taxa currently outnumber SI taxa. Two hypotheses are proposed to explain this finding. SI is either slated for extinction or, more likely, is associated with a greater net diversification rate relative to SC. This higher diversification rate must be greater than the transition rate from SI to SC for the two states to coexist in equilibrium. SI systems present a unique opportunity to test the effect of a widespread mating system character on diversification rate. Fitness consequences of hybridization in Avena barbata: Dominance,
epistasis and GXE interactions The fitness consequences of hybridization will be influenced by dominance, epistasis and GXE interactions. These are expected to differ between early and late generation hybrids, and among segregating genotypes. We examined the long term and short term consequences of hybridization between ecotypes of Avena barbata using 6 F2 families and 25 F6 families. We also examined genotype X environment interactions in a subset of 12 of the F6 families. Fitness (spikelet number), flowering time and biomass were measured in both experiments. No evidence for hybrid vigour or hybrid breakdown was found in the F2 generation. Hybrid breakdown for all three traits was found in the F6 generation, with only 2 families outperforming the parents in spikelet number and flowering time. Estimates of dominance, epistasis and additivity indicate that dominance may be counteracted by the break-up of co-adapted gene complexes in early generation hybrids. Preliminary analysis of 12 F6 families in 8 different environments shows genotype X environment interactions do occur. Although two families do exhibit high fitness in all environments the rank order of fitness of genotypes does change across environments. Unusual patterns of nucleolar dominance in natural and artificial
allopolyploids of the Glycine tomentella (Leguminosae) complex Nucleolar dominance (ND) is an epigenetic phenomenon where the ribosomal DNA (rDNA) of only one parent is transcribed in a hybrid. Although ND has been well studied in some model species, still very few plants have been seriously investigated on a molecular basis and there is an overall lack of studies of ND in natural populations. Results on ND in the allopolyploid Glycine tomentella complex (2n=4x=80) are reported here using a semi-quantitative RT-PCR approach. Individuals from genetically distinct natural allopolyploid races and from artificial crosses have been investigated for ND. In one allopolyploid race (T5), all individuals show ND towards the same parental rDNA, but individuals of another race (T4) show ND toward either parental rDNA and some even lack ND. Variation between plant tissues also was observed, ND sometimes being less in earlier developmental stages such as cotyledons and hypocotyls. In artificial crosses, all resynthetised polyploids (F2) investigated lack ND, while about only half of F2 homoploid hybrid individuals (diploid or tetraploid) show ND. Some of these results are unprecedented and may help to understand the mechanisms underlying the establishment and maintenance of ND as well as its evolutionary significance in natural populations of Glycine tomentella. Spatial modeling of plant's genes dispersal and western Alps postglacial
recolonisation: a new methodology for the case of Biscutella laevigata (Brassicaceae) Spatiality influences genetic diversity. Population genetics could gain accuracy by coupling them together with GIS potential habitat and dispersion modeling. Identification of anisotrope gene flux in the landscape seems of primary importance to better explain the genetic diversity in natural populations. Biscutella laevigata L. (Brassicaceae) is a Mediterranean perennial herbaceous species growing in the Alps. It is supposed to have colonized the Alps before the last glacial maximum with 2n=18 chromosomes and to have recolonized this area after the Würm in a doubled chromosomes form (2n=36, autotertaploid). The recolonisation would have started from ice free parts where the original population survived. The identification of migration pathways used by this species could help in explaining the mutation and in understanding the genetic implications of polyploidy in relationship with migration processes. A new GIS-based approach provides predictive habitat's maps as likelihood spaces. Transportation vectors maps (wind, gravity, hydrography) are then overlayed with these areas to make genes migration pathways appear. This approach should help population geneticists in discriminating between processes shaping genetic diversity. Habitat modeling will generates insights about the environmental factors playing a selection role by comparing them with the optimal range of the whole variables acting on species survival. Identifying migration pathways of genes will be of great interest for biodiversity management and conservation purposes, as well as for topics related to plant distribution under changing climate. Evidence for reinforcement of speciation in pollinator-sharing Neotropical
Costus (Costaceae) I investigate reproductive isolating factors in the understory herbaceous genus Costus (Costaceae) for a closely related sympatric pair of species and in a comparative study of mechanisms across the genus. Costus pulverulentus and C. scaber are studied at two sympatric sites, Barro Colorado Island in Panama and La Selva Biological Station in Costa Rica, where they overlap extensively in spatial distribution, phenology, and the identity of their hummingbird pollinators. Experimental arrays at La Selva show that interspecific pollen flow may be substantial, yet few hybrids are produced in hand pollinations due to strong interspecific pollen-pistil incompatibility that inhibits pollen germination. This incompatibility mechanism acts locally between sympatric populations, does not function in interspecific crosses between Barro Colorado Island and La Selva populations, and therefore is consistent with sympatric reinforcement of prezygotic isolation. A comparative study of crossing relationships throughout the Neotropical subgenus Costus shows that regardless of genetic distance, hybrids are easily produced except for sympatric species pairs sharing the same pollination syndrome. I suggest that for these pairs, which have the greatest possibility of interspecific pollen flow, direct natural selection has led to pollen-pistil incompatibility to prevent hybridization. This study provides the first strong evidence of reinforcement in plants. Hybridization and variation in Johnsongrass and related
weeds The evolutionary relationships among species of Sorghum sect. Sorghum (Poaceae) are not well understood. Sorghum sect. Sorghum includes cultivated sorghum [Sorghum bicolor (L.) Moensch] and johnsongrass [Sorghum halepense (L.) Pers], one of the world's most noxious weeds. Sorghum halepense is a polyploid (2n = 4x = 40), thought to be derived from S. bicolor and S. propinquum (Kunth) Hitch. Sequence data from the waxy locus (granule-bound starch synthase) are being used to construct an allele phylogeny to determine the genome origins of S. halepense. In addition, the allele phylogeny will allow a determination of the minimum number of polyploid events that likely occurred in the evolution of this species. Other single copy nuclear loci are being used to investigate introgression from cultivated sorghum and weedy diploid relatives of cultivated sorghum (including S. bicolor ssp. arundinaceum and S. bicolor ssp. drummundii) into S. halepense. Preliminary analyses suggest that S. bicolor and S. propinquum are the diploid progenitors of S. halepense. However, in some accessions, the genome of S. halepense may be more similar to that of S. bicolor, as a result of introgression from S. bicolor. Hybridization, polyploidy, and rarity: a molecular systematic study of
speciation in Delphinium Delphinium (larkspur, Ranunculaceae) is a large (~400 species) genus with centers of diversity in Asia, Mediterranean Europe, and western North America. Hybridization is frequently observed and several species may be of hybrid origin. Many species are polyploid, and others are of conservation concern. Sequences of the ITS and trnL-F regions were used to reconstruct the phylogeny of Delphinium section Diedropetala to test the hypothesized hybrid origins of Delphinium gypsophilum and the endangered D. luteum. Population data were also collected using allozyme and either AFLP or RAPD markers. Little phylogenetic signal was obtained from the trnL-F sequences and analyses of the ITS data recovered several small clades, with the majority of relationships unresolved. Delphinium luteum appears in a clade with both of its putative parents, and D. gypsophilum appears in a clade with one of its parents. In both cases, the sequence data are consistent with hybrid origin, but are also consistent with the putative hybrids being related to one or both of their proposed progenitors. Population-level data refute the hypothesis of hybrid origin of D. luteum, and are equivocal concerning the hybrid origin of D. gypsophilum. These data may indicate that Delphinium radiated recently in North America. Demographic and genetic analysis of the rare Oconee Bell (Shortia galacifolia)
throughout its known range Shortia galacifolia T. & G. (Diapensiaceae; Oconee Bell) is endemic to a narrow mountainous region of the Carolinas (five counties total) in the eastern United States. The other species of Shortia are Asian. First collected in 1787, Asa Gray spent almost 40 years trying to find the type locality and additional populations with no success. Historically, the largest concentration of S. galacifolia and its possible center of distribution were considered to be the confluence of the Horsepasture and Toxaway Rivers, with the greatest abundance found in the Jocassee Valley of South Carolina. Morphological variation between populations within the two drainage systems has lead some taxonomists to split the species into two subspecies. The creation of the Keowee and Jocassee reservoirs in the mid-1970s eliminated an estimated 60% of S. galacifolia populations. Today, the areas around these new lakes have become even more developed, further threatening the remaining populations. Before genetic work can be started, updating the distribution of extant populations must be completed. Genetic diversity of this species will be measured using RAPDs, and these data will be used to develop strategies for preserving this unique plant. Genetics of postzygotic isolation in Mimulus guttatus and Mimulus
nasutus Understanding the genetic basis of reproductive isolation is necessary for insight into the process of speciation. Genic incompatibilities (Dobzhansky-Muller (D-M) interactions) have been shown to be the major cause of postzygotic isolation between many plant and animal species, and studies suggest that these incompatibilities are predominately recessive. Here, we investigate a form of hybrid inviability between geographically isolated populations of Mimulus guttatus and between populations of M. guttatus and M. nasutus. We performed multiple crosses between and within populations of M. nasutus and M. guttatus to produce F1 and outcrossed F2 reciprocal hybrids (both interspecific and intraspecific). We found inviable, chlorophyll-deficient seedlings in several F2 hybrids between populations of M. guttatus, and in several M. guttatus X M. nasutus F2 hybrids. No F1 hybrids or outcrossed parental plants were chlorophyll-deficient. These white seedlings, when observed, were in proportions similar to 1/16, suggesting that this form of hybrid breakdown results from two-locus, recessive X recessive D-M incompatibilities. Additional studies suggest that different genic incompatibilities result in similar chlorophyll-deficient phenotypes. Furthermore, the alleles responsible for producing these incompatibilities are not fixed within each population, suggesting that this form of hybrid inviability still appears to be in the early stages of evolving. Sub-specific diversification and incipient speciation among sympatric
populations of an endemic member of the Hawaiian Silversword Alliance, Dubautia
laxa (Asteraceae) Dubautia laxa Hook. & Arn., a member of the extremely diverse Hawaiian Silversword Alliance, shows a wide geographic and ecological distribution occurring on five of the six major Hawaiian islands . Unlike many of the species within the Hawaiian Silversword Alliance, D. laxa has maintained a high level of morphological cohesiveness as it has colonized younger islands. However, there has been some morphological and habitat specialization that has lead researchers to recognize four sub-species of D. laxa. Two of the sub-species of D. laxa are endemic to the island of Oahu while the other two are found on multiple islands. Under the current classification, all four sub-species of D. laxa are sympatric on the island of Oahu often with close physical proximity between populations. Distinct, easily recognizable populations of different sub-species of D. laxa have been found within 1km of each other with no obvious signs of hybridization as judged by unique morphological characters. This study investigates the levels of genetic differentiation between populations of D. laxa on the island of Oahu using 8 microsatellite loci. By examining populations of each of the sub-species that are isolated and populations that are found close to one another, we hope to determine if the sub-species of D. laxa represent unique evolutionary lineages possibly deserving recognition as individual species. Species cohesion: implications of
gene flow and selection for the spread of favorable alleles The traditional view that species are held together through gene flow has been challenged by observations that migration is too restricted among populations of many species to prevent local divergence. However, only very low levels of gene flow are necessary to permit the spread of highly advantageous alleles, providing an alternative means by which low-migration species might be held together. Our literature review of gene flow, the strength of selection on phenotypic traits, and effect sizes of QTL supports a model of species cohesion through the spread of favorable alleles. Specifically, we show that although low levels of gene flow are present among many taxonomic groups, selection coefficients for leading QTL underlying phenotypic traits may be high enough to permit their rapid spread across populations. Preliminary simulation models also suggest the potential for quite rapid spread of alleles of moderately large effect size. Thus, species may evolve collectively at major loci through the spread of favorable alleles, while simultaneously differentiating at other loci due to drift and local selection. Plant-specific patterns of interspecific reproductive isolation:
predictions under the Dobzhansky-Muller model The inviability and sterility of species hybrids can be explained by between-locus “Dobzhansky-Muller” incompatibilities inappropriate interactions between newly derived alleles that have accumulated in independently evolving lineages. The theoretical predictions of the Dobzhansky-Muller model have been well-explored for dioecious animal systems, however little work has been directed towards non-animal systems, despite the fact that angiosperms exhibit biological features that might substantially influence patterns of reproductive isolation between species. Three such features are gametophytic gene expression, formation of triploid endosperm following double fertilization, and nuclear-cytoplasmic interactions. Each involves characteristic interactions between haploid and diploid genomes from parental individuals. We present a model of two-locus Dobzhanksy-Muller incompatibilities that distinguishes between different classes of hybrid haploid-diploid interactions. We predict the relative fitnesses of hybrid genotypes by calculating the expected number of each type of incompatibility. Of particular interest is the pattern of asymmetric reproductive isolation – where species are substantially less fit (more reproductively isolated) in one direction of a cross compared to the reciprocal direction. Isolation asymmetry appears to be common and taxonomically widespread in angiosperms. We show that asymmetric reproductive isolation can result both from stochastic processes, and from deterministic interactions, depending on the dominance of alleles affecting hybrid fitness. Molecular analysis of South Indian endemic tetraploid wild
rice, Oryza malampuzhaensis: focus on its phylogenetic relationships and
putative progenitors Oryza malampuzhaensis Krish.et Chand (2n = 4x = 48; Poaceae, Oryza) is endemic to Western ghats, South India. Taxonomy and phylogeny of this taxon is poorly understood. It is currently treated as a tetraploid race of diploid O. officinalis. Three distinct accessions of O. malampuzhaensis were analysed together with 21 accessions of eight related Oryza species using RAPD markers and genomic DNA hybridization technique. Cluster analysis based on RAPD data separated O. malampuzhaensis into a distinct group. The genetic distance between O. malampuzhaensis and other species were comparable with the distance between the species examined. Bgl II or Hae III restricted genomic DNAs of the accessions examined were hybridized individually with the labeled genomic DNA of O. malampuzhaensis, and diploid BB and CC genome species. The size and signal intensity of the bands generated in O. malampuzhaensis were mostly identical to that in O. punctata (BB), whereas its profile did not show any specific affinity with any of the CC genome species. The results demonstrate that i) O. malampuzhaensis should be considered an individual species ii) its ‘B’ genome progenitor is O. punctata iii) its ‘C’ genome progenitor can be any of the diploid CC genome species. A maximum likelihood method for testing whether intermediate outcrossing
is unstable It has been theoretically suggested that obligate outcrossing or obligate selfing is ESS and intermediate outcrossing rates represent transit stages. To test this hypothesis using phylogenetic data, we develop a maximum likelihood method based. Outcrossing rates are classified into 3 categories: High, moderate, and low (H, M, L). We test the adequacy of the following two models by a likelihood ratio: two-parameter model in which transition probability between H and M is defined as α and that between M and L is defined as β and 4-parameter model in which transition probability from H to M, from M to H, from M to L, from L to M are defined as α ,α'f, β and β'f, respectively. If intermediate outcrossing rates were unstable, is expected that four-parameter model is fitter and transition probabilities from L and H to M is expected to be larger than reversal ones. We applied this method to our data set of Mutisieae (Asteraceae). The likelihood ratio test did not reject two-parameter model significantly. However in four-parameter model, the estimates of transition probabilities between M and L showed a consistent trend with the hypothesis. South
African Cape Erica spp.: fire, rain and diversity The genus Erica, with over 600 species and a high level of narrow endemism, represents theparadigm of plant bio diversity in the mediterranean Cape region of South Africa. Both floristic diversity and endemism in this genus are markedly associated to the seeder post-fire response, and are concentrated in the southwestern Cape. In Erica species having both seeder and resprouter forms, resprouters consistently have more starch in root xylem and a higher proportion of specialized storage tissue than seeders. These differences might have a genetic basis and are detected from earliest seedling stages. Thus, seedlings of resprouter lineages will have a comparatively slower growth rate than those from seeder, and will consequently be more sensitive to a mild seasonal water stress. We have developed a computer simulation model to assess the effects of summer drought severity and winter rainfall reliability in the recruitment success and long-term persistence of seeder and resprouter lineages, considering 15-year fire intervals and assuming fire-induced germination in both seeders and resprouters. Our model illustrates the selection and thriving of Erica seeder lineages, more prone to speciation, under high and reliable winter rainfall and mild summer drought, conditions found in areas within the southwestern Cape region. Morphological evidence for diploid hybrid origin of Arctostaphylos gabilanensis Arctostaphylos gabilanensis is a newly discovered auriculate-leaved species of this large genus. The species is found in two separate populations in central to northern parts of the Gabilan Mountains, San Benito County, in central California. The species appears to represent a potentially common mode of speciation in this genus, hybrid origin. A. gabilanensis combines characteristics of the auriculate-leaved lineage of Arctostaphylos, a group of species found in summer fog areas from the Channel Islands to the San Francisco Bay Area, and characteristics of Arctostaphylos glauca, a common species of the more xeric areas of the inner coast ranges in central and southern California. Four auriculate-leaved species are found in geographic proximity as well as A. glauca. Here we test the hypothesis of a diploid origin using a multivariate approach to morphological characters. We used 50 characteristics of leaves, stems, inflorescences and fruit of A. gabilanensis, A. auriculata, A. pallida, A. andersonii, A. pajaroensis, and A. glauca. Molecular sequence cladistic analysis is also in progress. Insect
pollination, flower morphology, heterostyly and speciation in Narcissus
(Amaryllidaceae) Heterostyly is a genetically based floral polymorphism which has appeared repeatedly in angiosperms. It represents an ideal model system for studies on the relative advantages of inbreeding and outbreeding via assortative vs. disassortative mating. The genus Narcissus has been critical to test competing hypotheses on the evolution of heterostyly. Current findings and available evidences so far fit Lloyd & Webb’s model, which stresses the importance of pollination biology in driving evolutionary steps from style monomorphism to style dimorphism and, ultimately to heterostyly. For unknown reasons, style dimorphism, which is rare in taxonomic groups with heterostyly, is widespread in many species within the genus Narcissus, whereas only two species present true heterostyly: N. albimarginatus (distyly) and N. triandrus (tristyly). The section Apodanthi, comprises seven species which show different forms of style polymorphism, from monomorphic to polymorphic species, including the distylous N. albimarginatus. We have selected this section Apodanthi as a study system to assess the role of pollinators in driving style polymorphism. Specifically, we have taken detailed flower measurements in some of these species (in addition to the tristylous N. triandrus, within the section Ganymedes), including style length variation, stamen whorls location, and corolla tube length and width, in order to find correlates between these flower features and insect pollination. Genetic differenation in Anthericum liliago populations Genetic differentiation in plant populations is determined by an interplay of gene flow (pollen and seed dispersal) and selection. In central Germany, Anthericum liliago L. (Anthericaceae) is a hernicryptophyte found on dry grassland, dry shrub or it is saxicolous. Development, forestation, modernization and intensification of agriculture have reduced the distribution of A. lifiago and increased the distances between the remaining populations. We investigated all 127 A. liliago ramets found in a small and isolated natural population (Halle, Saxony-Anhalt). Analysis of 54 RAPD markers revealed a relatively low level of genetic polymorphism (34.6%) and show a siginificant (p<0.001, r-0.423) correlation between genetic and spatial distances within this population which may be a result of restricted pollen and seed dispersal and of a high selfing rate. The occurrence of an unusual pollinator species, the ant Myrmica sabuleti, may have caused a shift in the reporducion system. An investigated large Anthericum population (Rothenburg, Saxony-Anhalt) with some thousand individuals and an unusual pollinator assembly (syrphid flies, bees) shows a significant higher genetic variability. At this time we are investigating the genetic structure of some more Anthericum populations considering pollinator assembly and population sizes. Inheritance and diversity of
chloroplast DNA (cpDNA) in Fragaria Chloroplast DNA (cpDNA) sequence analysis is a useful tool to study phylogenetic relationships. Unlike nuclear DNA, cpDNA is not affected by differing ploidy levels, making cpDNA a useful tool for tracing the ancestry of the octoploid, cultivated strawberry, Fragaria x ananassa. Although cpDNA is generally found to be maternally inherited in angiosperms, exceptions to the maternal inheritance are known, as in alfalfa (biparental inheritance) and kiwifruit (paternal inheritance). The lack of published empirical data supporting the assumption of maternal cpDNA inheritance in strawberry prompted my study. A polymerase chain reaction (PCR) based approach was used, in some cases employing a technique called mixed-template heteroduplex analysis, to compare cpDNA sequences in parents and progeny of several intra- and inter-specific crosses. All results supported the hypothesis of maternal cpDNA inheritance. A new indel (insertion/deletion) polymorphism discovered in the trnL intron of the cpDNA will be a useful tool in future studies of geographic variation in Fragaria vesca, a wild diploid relative of the cultivated strawberry. In the course of speciation: ecogeographical divergence of Oncocyclus
Irises Natural selection may lead toward one of two directions: radiation and divergence into many entities, or stabilizing the present situation without producing clear separation. Recent studies in the Oncocyclus irises raised the question of the evolutionary trend in this group. High levels of genetic and morphological variation exist within populations, without clear separation between taxa. Morphological characters are correlated with latitude, performing a substantial phenotypic plasticity. Biologically, all the species can interbreed without post-zygotic isolation, and produce vital progenies. No species-specific exists in Oncocyclus irises and their Eucerinii bee pollinators. However, in cases of sympatric distribution no hybrids were found. On the one hand, the continuous nature of the morphological/genetic traits may indicate lack of differentiation. Moreover, gene flow between races by the common pollinators may prevent future isolation or divergence. On the other hand, each local race might be in its adaptive peak, where selection favors a specific morphology. The patchy distribution of the populations may create isolation de-facto between populations, and as a consequence, ecogeographical divergence is feasible. The whole picture that arises is of a group in the course of speciation, but without clear evidence in which direction: diversification or homogenization. This makes the group a perfect model system for evolutionary research. Does floral symmetry affect speciation rates in angiosperms? V. Grant’s postulate that pollination systems are an important mode of isolation in angiosperms has generated several important hypotheses about plant speciation. Studies of floral morphology suggest that increased floral complexity serves the dual function of limiting pollinators and preventing pollination by non-specific pollinators. Some types of increased floral complexity result in changes to corolla symmetry. The most common forms of floral symmetry are actinomorphy (radial symmetry) and zygomorphy (bilateral symmetry). Zygomorphy is thought to be the derived condition, and it has often been noted that angiosperm lineages that have attained zygomorphy tend to be more specious than their actinomorphic sister lineages. This study examines the hypothesis that increased mechanical and ethological isolation through changes in floral symmetry play an important role in the speciation process, and that lineages that undergo a shift towards greater complexity in symmetry should therefore be more specious than their radially symmetric sister lineages. Here we show that changes in symmetry are correlated with increased species diversity in angiosperm lineages. Progenitor-derivative speciation and hybridization in the serpentine
endemic Synthyris cordata (Scrophulariaceae) Morphological and molecular analyses were conducted with Synthyris cordata and S. reniformis to investigate the relationship of the two species. The presence of unicellular hairs in the corolla tube, and lower leaf length/width ratios consistently distinguish S. cordata from S. reniformis. Synthyris reniformis is a common widespread species from southern Washington to northern California. Synthyris cordata has a smaller range, restricted to serpentine soil in southern Oregon and northern California. We examined chloroplast haplotypes (trnT-trnL spacer, trnL-trnF spacer, trnK intron) and nrDNA internal transcribed spacer (ITS) sequence variation in seven populations of S. cordata and seventeen populations of S. reniformis. The two species have divergent ITS sequences and considerable cpDNA variation. A parsimony tree and a haplotype tree were created based on 15 cpDNA restriction sites. The parsimony results and cpDNA haplotype tree are consistent with S. reniformis as the progenitor of S. cordata. A single northern California population is intermediate between these two species. This population has S. cordata morphology, the ITS of S. reniformis, and does not occur on serpentine. This population may represent a direct ancestor of S. cordata. Another population in northern California has ITS sequences of both species, apparently a result of secondary contact and hybridization. The role of host-specificity in the evolution of the
terrestrial holoparasite Orobanche (Orobanchaceae) Orobanche (Orobanchaceae) is known to be notoriously difficult to determine. Therefore, the host is often used as an additional character for species delimitation. This is, however, problematic for several reasons: (1) the physiological reasons for host-specificity are hardly known, and therefore it is not sure, if host-specificity is a heritable trait that thus could be used as a character; (2) the influence of the host on the parasite is not known, hence it is possible that morphological variation seen between different parasite-races could be the result of growing on different hosts; (3) if host-specificity is a priori considered as a character, the detection of host-specific races can become a self-fulfilling prophecy. Such problems can be addressed by using a phylogenetic approach, in which host-taxa are pasted on a parasite-phylogeny. This approach is applied to Orobanche and related genera. The results should be dealt with caution for two reasons: (1) the existing phylogeny of Orobanche does not yet include all taxa, and accordingly key-taxa might be missing; (2) the data on host plants have to be treated carefully, since the determination of the actual host plants is in case of terrestrial parasites a non-trivial task, which would require either digging out the host-parasite-connection, which is hampered in certain soil types, or cultivation experiments, for which only a few parasite-taxa can be used. However, the first results indicate that (1) speciation events of the parasite are often accompanied by host-switches, but no conclusions about causality can be made; (2) a change from perennial to annual hosts is often accompanied by a widening of the host-range; (3) host-specificity is not correlated with phylogenetic age of the parasite lineages; (4) host-specific taxa occur over the whole phylogeny and are not restricted to certain clades, accordingly no predictions can be made in which groups of parasites host-specificity can be used as character. The
birds, the bees and plant sex: examining the associations between hummingbird
pollination, rainfall and the evolution of dioecy in Echinocereus coccineus
(Cactaceae) using GIS (Geographic Information Systems) and path analysis The role of pollinator behavior in plant evolution has been a topic of intense interest since Darwin’s first explorations of this subject. Echinocereus coccineus populations show intriguing patterns that indicate differences in pollinator behavior may be driving the divergence of the mating system. E. coccineus populations along the Rocky Mountains tend to be hermaphroditic, while those to the east and west tend to be dioecious. The migration patterns of hummingbirds also follow the Rocky Mountains, leaving populations to the east and west pollinated predominately by Halictid bees. I hypothesized that the patterns in the mating system were directly related to the behavior of the pollinators, with hummingbirds maintaining hermaphroditsm through high rates of outcrossing, and bees selecting for dioecy through high rates of self-pollination. An alternative hypothesis is that dioecy is selected in drier environments to improve resource allocation. To better quantify the associations between pollinator behavior, rainfall and mating system evolution in E. coccineus a GIS map was created with sex of flowers, annual rainfall, and hummingbird abundance. The data was then analyzed using multiple regression and path analysis to assess the relative direct and indirect influence of all factors on the presence of dioecy. Hybridisation between wild
sunflowers (Helianthus argophyillus and H. debilis) in Southeastern
Africa – morphological and molecular characterisation Populations of H. argophyllus and H. debilis plus their putative hybrids spontaneously growing in Mozambique and South Africa were collected and characterised by means of molecular markers and morphological traits. Putative hybrids show intermediate values for leaf morphology, despite AFLP markers show that putative hybrids are closer to H. argophyllus than to H. debilis. This suggests that hybridisation alters significantly development even when the genome is not fully intermediate between species. However, the two parent species show close relatedness and share chloroplast and mitochondrial haplotypes, in spite of morphological (and nuclear) differentiation. Seven chloroplast SSR markers out of nine show complete identity of the two species, while one is polymorphic H. argophyllus. Breeding lines of H. annus show the same haplotypes as H.debilis and none of the H. argophyllus variants, suggesting that the latter is somehow separated from the former two. Due to uniformity of haplotypes, maternally-inherited markers did not help to definitely establish whether the hybrid plants were the product of unidirectional pollination and/or introgression, as suggested by AFLP data. Since multiple fragments are produced by PCR in H. argophyllus samples, the data show hints of leakage between chloroplast and nuclear genome, a mean of genomic evolution. Chloroplast DNA variation in New Zealand vegetable sheep (genus Raoulia,
Asteraceae: Gnaphalieae) is consistent with widespread introgression A number of angiosperm groups appear to have undergone rapid endemic radiations in New Zealand after dispersal from other landmasses. Of these, the Gnaphalieae (Asteraceae) present perhaps the most bewildering array of forms. In order to develop molecular markers for phylogenetic and population genetic studies in this group we have examined variation in several chloroplast DNA regions. The psbAtrnH intergene is hypervariable, with informative substitutions distinguishing 18 haplotypes distributed among species of Raoulia and related genera in a preliminary sampling. Sequences of each haplotype were combined with sequences of the more slowly evolving trnL intron and trnLtrnF spacer region and subjected to median network analysis. Although a number of chloroplast lineages were resolved, these do not correspond with taxonomic or morphological units or with groups indicated by nuclear rDNA ITS sequences. One species, the famous “vegetable sheep” Raoulia eximia was found to possess at least four distinct cpDNA haplotypes, all but one of which was shared with other diverse species. These data, considered in the context of the highly complex pattern of morphological character state distribution in the group suggest a complex set of inter-relationships among extant species, perhaps resulting from successive cycles of differentiation and hybridisation. Pollinator-mediated
diversification of Andean Solanaceae Ecological studies have long recognized the close relationship between environmental variables (e.g., climate and elevation) and pollinator composition and abundance. Thus, when plant species disperse to new environments, they may encounter new pollinator fauna and undergo changes in floral morphology and pollination systems. This research will examines the hypothesis that transitions to high-elevation environments are correlated with shifts to hummingbird pollination and the evolution of colorful, tubular flowers using the Andean subtribe Iochrominae (Solanaceae), a clade of 28 species sister to Physalinae in the large subfamily Solanoideae. Iochrominae occur at wide range of elevations, and they display a great diversity of floral morphologies and pollination systems, making them an ideal group in which to investigate this hypothesis. The study is divided into three parts: a phylogenetic analysis of Iochrominae, field studies of pollination, and biochemical analysis of floral pigmentation. The preliminary phylogeny constructed using waxy and ITS reveals that tubular, hummingbird-pollinated flowers have arisen in high-elevation habitats at least twice in the subtribe, once in the northern Andean clade and again in the southern Andean clade. Furthermore, analysis of the floral pigments in Iochroma suggests that the evolution of red flowers is due to genetic change in the anthocyanin pathway as opposed to a change in vacuolar pH as in red-flowered Petunia. Reproductive isolation by breakdown of self-incompatibility
in Witheringia solanacea The transition from obligatory outcrossing, as enforced by biochemical self-incompatibility systems, to a predominantly selfing mode of reproduction, has occured repeatedly throughout the flowering plants. Selection on mutations causing loss of self-incompatibility must be very strong, as populations with polymorphism for such mutations are rare. We have identified two separate such mutations occuring in natural populations of Witheringia solanacea in Costa Rica. In both cases, the mutation causing lack of S-RNase activity has given rise to selfing lineages which have no detectable inbreeding depression, in contrast to self-incompatible individuals growing in close proximity, in which inbreeding depression is severe. Patterns of nucleotide diversity are affected by mating system and
asymmetric introgression in two species of Mimulus The evolutionary transition from outcrossing to self-fertilization has far-reaching implications for patterns of intra-specific genetic diversity and the potential for speciation. The divergence of two closely related species of Mimulus with contrasting mating systems was examined using the pattern of DNA sequence variation at two nuclear loci. To investigate the effects of mating system and introgressive hybridization on the outcrossing M. guttatus and the selfing M. nasutus, we inspected nucleotide diversity within and between natural populations spanning the species' geographic ranges. High sequence similarity among populations of M. nasutus points to a single evolutionary origin for the species. Consistent with their distinct mating systems, all genetic variation in M. nasutus is distributed among populations, whereas M. guttatus exhibits appreciable levels of nucleotide diversity within populations. Silent genetic diversity is extensive in M. guttatus (mean θsil/site = 0.077) and greatly exceeds the predicted two-fold elevation in neutral variation for outcrossers relative to selfers. The finding of several contiguous M. guttatus haplotypes that share complete identity with sequences from M. nasutus is suggestive of recent asymmetric introgression. Exceptionally high nucleotide diversity in M. guttatus is consistent with a long-term history of directional introgression from M. nasutus to M. guttatus throughout species divergence. Pollinator effectiveness on sympatric milkweeds (Asclepias): do
floral morphology and insect behavior influence species boundaries? Pollinators are a potentially powerful force in plant speciation. In theory, selection favors floral characters and pollinator behaviors that increase fitness by enhancing pollen donation or receipt, or reducing interspecific pollination. Milkweeds (Asclepias) provide a model for evaluating the role of floral differentiation and “generalist” pollinators in maintaining species boundaries. Asclepias bear pollinia that remain attached to nectar-foraging visitors, allowing estimates of pollinator effectiveness. We asked: 1) Do differing morphologies of co-occurring asclepiads alter the behavior or effectiveness of pollinators? 2) Can such differences act as pre-mating barriers? We analyzed pollinium loads, flight patterns, and videoclips of insects foraging on sympatric Asclepias. Although larger proportions of A. syriaca pollinia were potentially inserted in flowers, overall pollinator effectiveness was greatest for A. incarnata and A. verticillata. Significantly more pollinia occurred on tarsal hairs than on distal appendages except in large-flowered A. syriaca. The common visitor Bombus griseocollis differed in both approach and primary contact on various asclepiads. Differences in insect behavior and floral morphology suggest wide variability in pollination effectiveness and some differential adaptation that parallels phylogenetic relationships in Asclepias. Broad overlap among floral visitors and frequent interspecific flights in sympatric populations reveal little evidence of pre-mating isolation in Asclepias. Triploidy, apomixis and latitudinal genetic variation in the Slim-Leaf
Onion (Allium amplectens) in western North America Allium amplectens occurs in dry open habitats from southern B.C. to southern California. Diploids, triploids and tetraploids (2n = 14, 21 and 28 respectively) are known. Although the chromosomal races are similar in floral and vegetative traits, triploid plants produce asynaptic pollen dyads, in contrast to diploids and tetraploids which produce a tetrad of normal pollen grains. Northern (B.C.) populations of this species are exclusively triploid and appear to reproduce mainly by vegetative offsets, with few individuals recruiting from seed. In contrast, more southerly populations are commonly diploid or tetraploid and can reproduce abundantly by seed. Northern populations have relatively low levels of genetic variation (based on isozyme and preliminary AFLP evidence) and appear to possess a subset of the alleles found in more southern populations. Preliminary pollination experiments on plants from B.C. populations suggest that the triploids may be pseudogamous, producing embryos by apomixis but requiring fertilization to produce endosperm. Current investigations include (i) embryo sac development in plants of the three different ploidy levels and (ii) the geographic distribution of chromosome numbers and AFLP patterns. Rapid radiation in the Columbines: evidence from multiple nuclear loci Understanding the evolutionary forces responsible for adaptive radiations will provide insights into the process of speciation. Verne Grant suggested that adaptive radiation in Aquilegia is "essentially the evolution of different pollination systems" (Grant 1952). I have taken a phylogenetic approach to testing two components of this hypothesis. First, I tested whether speciation in Aquilegia was rapid and recent by comparing sequence variation among Aquilegia and among species in the closely related genus, Isopyrum. Based on 4 kb of sequence data, including three nuclear introns, the average genetic distance within Aquilegia is 4-7 times lower than between geographically comparable Isopyrum spp. suggesting an extremely rapid radiation. Second, I am currently testing whether shifts in floral morphology have driven speciation using a detailed phylogeny of North American Aquilegia spp. Specifically, I plan to make comparative tests of the frequency and distribution of shifts in pollination syndromes and habitat associations throughout the phylogeny. Although the depauperate levels of molecular variation among Aquilegia spp. makes phylogenetic reconstruction especially challenging, preliminary data from AFLPs and additional nuclear loci (including 3' UTRs) are promising. AFLP data indicate that species are monophyletic and the variation discovered at additional nuclear loci is likely to resolve the phylogeny. Examining population differentiation in the circumarctic fern species Athyrium
distentifolium using markers derived from gene transcripts The pteridophyte Athyrium distentifolium, the Alpine Lady-fern is widely distributed in montane areas throughout the northern hemisphere, growing in an arctic-alpine environment. In the UK however, this fern is classified as a ‘Scarce Plant’ (Stewart et al. 1994) where it is only found in Scotland. Here the fragmented populations grow at altitudes between 600-1000m, usually on open, acidic screes and gullies in the Central and Northern Highlands. In a few areas of the Central Highlands there is also an endemic variety: A. distentifolium var. flexile which is particularly well adapted to grow in more extreme environments with low nutrient status. As part of a wider programme examining the use of microsatellites to study several circumarctic / montane plant species of conservation importance in Scotland, we have developed SSRs derived from transcribed regions of the genome. Here we present data for A. distentifolium populations from Scotland, Iceland and the USA, assessed using these functional markers. We demonstrate the effectiveness of these markers for studying population differentiation in a species across a range of geographical scales and the potential transferability of these markers to other related species. Symmetry of hybridization in a natural Ipomopsis (Polemoniaceae)
contact zone Spatial variation in natural selection may play an important role in determining the genetic structure and evolutionary outcomes of natural hybridization. Previous studies have found that experimental F1 hybrids between naturally hybridizing Ipomopsis aggregata and I. tenuituba in central Colorado differ in fitness depending on genotype and environment: hybrids had higher survival if I. aggregata was the maternal parent, except in the center of the hybrid zone where both types of hybrids had high survival. This study used genetic markers to determine the cytoplasmic structure of the natural hybrid population, and tested the prediction that the majority of natural hybrids have I. aggregata cytoplasm, except in areas near the center of the hybrid zone. To identify the cytotype of natural hybrids, we developed three species-diagnostic cpDNA RFLP markers and analyzed 342 individuals from across the hybrid zone. Cytoplasmic gene flow is bidirectional, but contrary to expectation, most plants in the hybrid zone have I. tenuituba cytoplasm. This cytotype distribution is consistent with a hybrid zone in transition, with I. aggregata nuclear genes advancing into the contact zone. A more complete evaluation of the factors influencing the cytoplasmic asymmetry depends upon understanding the nuclear genetic composition of these hybrid individuals. Molecular phylogenetic studies of sexual and agamospermous plants of Stevia
in Mexico that are differentiated into annuals, perennials and shrubs Mexican Stevia (Eupatorieae; Asteraceae) consists of ca. 100 spp. that are mostly endemic and differentiated into annuals, perennials and shrubs. gamospermous polyploids are found in 45 spp. Our studies of Mexican Stevia focus on the following three questions in plant speciation. (1) Is there any difference in speciation pattern among annuals, perennials and shrubs? (2) How many times did agamospermous polyploids evolve and how long have they persisted? (3) Why are some widespread species particularly polytypic? To answer these questions, we constructed a phylogenetic tree using ITS sequences of 70 species, and carried out further detailed analyses on a polytypic species, S. origanoides, using ITS sequences and AFLP. Shrub and perennial-annual clades diverged at the base of a phylogenetic tree. Annuals evolved at least four times independently and showed markedly increased rates of molecular evolution. Speciaton in shrubs occurs more locally. Agamospermy evolved independently at least 15 times in perennial species examined, but not at all in annuals and shrubs. Stevia origanoides includes several sympatric cryptic species in three areas and disruptive selection probably accelerates the sympatirc or parapatric speciation between diploid sexual populations. Assessing patterns of reproductive isolation in an Aquilegia
hybrid zone Hybrid zones can provide an ideal setting to study processes of speciation, particularly the mode of reproductive isolation. In angiosperms, the two most common mechanisms of reproductive isolation are pollinator shifts and/or habitat differentiation. I examine two closely related species, Aquilegia formosa and A. pubescens that differ in their floral and habitat characteristics that has resulted in a controversy about their primary mode of reproductive isolation. I am studying a small hybrid zone (50 m x 40 m) between these two species in the Eastern Sierra Nevada, California. Within this small zone, approximately 550 individuals are found exhibiting an array of intermediate floral phenotypes and parental types. A detailed map of plant distributions reveals that parental and hybrid types group in distinct clusters. Because gene flow is likely to be greater than the physical scale of these clusters, it is likely that there is strong selection via habitat preference structuring this hybrid zone. Accordingly, I find a strong correlation of soil types with flower morphologies. I am also examining if pollinator preferences contribute to reproductive isolation. I am testing this hypothesis directly using a paternity analysis to determine if assortative mating occurs due to pollinator preferences for specific morphologies. Breakdown of prezygotic reproductive barrier under a secondary contact
of dayfily (Hemerocallis fulva) and nightlily (H. thunbergii) If young allopatric species come into contact and hybridize, it is expected that initial reproductive barrier will be broken down or reinforced. However, neither process has been fully documented In plants. Hermerocallis thunbergii and H. fulva provide an excellent opportunity for examining the outcomes of secondary contact. H. fulva flowers in daytime while H. thunbergii flowers at night. Those differ in many floral traits consisting of pollination syndromes. Populations of two species are largely allopatric, but we found a hybridizing population in Hirado Island where ranges of two species overlap. Using two species, we examined: (1) How strong is prezygotic barrier between two species? (2) Is the prezygotic barrier broken down or reinforced in Hirado Isalnd? Flowers of two species from Hirardo were hand-pollinated, using conspecific pollen of the same population, allopatric heterospecific pollen or sympatric heterospecific pollen. In both species, (1) seed set from conspecific pollination (23%, 24%) was higher than that from allopatric heterospecific pollination (17%, 5%), and (2) that the latter was lower than that from sympatric heterospecific pollination (23%, 19%). Consequently, initial prezygotic barrier is found between H. thunbergii and H. fulva and this barrier is probably broken down under secondary contact. Floral spur variation in Impatiens capensis: Effects
on pollinator visitation, pollen removal and pollen deposition Flowers of Impatiens capensis have a spur in which the nectar is produced and stored. There is considerable population variation in the length and shape of this spur, and it appears to be heritable (repeatability estimates from within and among plant measurements). I am seeking to determine if selection is acting on spur angle, via its affects on pollinator behavior. Flowers are visited legitimately by several species of Bombus, by Apis mellifera, and by hummingbirds. I examined the effect of spur shape on several pollinator visitation parameters (visit length, quantity of nectar removed, quantity of nectar remaining after a visit) and plant reproductive parameters (pollen deposition on the stigma and pollen removal from the anthers). Pollinator visit duration was significantly positively correlated with spur angle and longer visits resulted in the removal more nectar from the spur. However, spur angle was not correlated with pollen deposition or removal. A single pollinator visit, regardless how short, usually deposits sufficient pollen on the stigma for full seed-set. I found that bees prefer to visit flowers that have more nectar, suggesting that visitation will be more likely to flowers with curlier spurs. |
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Illustrations: Plant Speciation logo by Sam
Day. The logo depicts scarlet Gilia, of the Polemoniaceae - the systematics
of the Polemoniaceae was one of Verne Grant's specialist fields. Alpine flowers,
courtesy of T J Tschaplinski. Helianthus anomalus, courtesy of L Rieseberg.
Beech forest, courtesy of R J Norby. Last updated: January 17, 2007 |
