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St Francis Xavier University, Antigonish, Nova Scotia, Canada   |
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Speakers Historical biogeography of circumarctic plants The present-day arctic flora comprises approximately 1500 species and is of relatively recent origin. Throughout most of the Tertiary (65-2 Ma), forests grew at high latitudes in the Arctic and tundra did not appear until the late Pliocene. Initially tundra was distributed discontinuously, but a circumarctic belt was present by 3 Ma. Little is known about the origins of arctic plants or the migration routes they followed during the initial colonization of the Arctic. In addition, uncertainty remains over where circumarctic plants survived Pleistocene glaciations. A phylogenetic analysis of chloroplast DNA variation in the Purple saxifrage (Saxiftaga oppositifolia) indicates that this plant first occurred in the Arctic in western Beringia before it migrated in east and west directions to obtain a circumpolar distribution. Moreover, the geographical distribution of cpDNA variation in the species supports the hypothesis that refugia were located in Beringia during Pleistocene glaciations as well as in other periglacial areas outside the main ice sheets. The results of recent phylogeographic studies on two other circumarctic species (Saxiftaga cernua and Vaccinium uliginosum) have extended further our understanding of the historical biogeography of circumarctic plants. Hybridization, polyploidy and speciation in Spartina (Poaceae) Hybridization and polyploidy are well illustrated in genus Spartina where at least three cases of recent interspecific hybridization have been well documented, with well-known evolutionary and ecological consequences. Spartina species are perennial tetraploid, hexaploid or dodecaploid plants colonizing salt marshes. Most Spartinas are native to the New World and only four species originate from Western Europe. Three of them are of hybrid origin, being the result of the introduction of the Eastern American Spartina alterniflora (2n = 62) into Western Europe at the end of the 19th century, and its subsequent hybridization with the indigeneous Spartina maritima (2n = 60). In Southern England, hybridization resulted in a sterile perennial hybrid, S. x townsendii that gave rise to a vigorous fertile allopolyploid Spartina anglica (2n = 120, 122, 124). This young and successful species has rapidly colonized British salt marshes and is now introduced on several continents. In southwest France, hybridization between S. alterniflora and S. maritima led to the formation of another hybrid species, S. x neyrautii. The mode of formation of these taxa, as well as the genomic divergence of the parental species and their phylogenetic relationships will be discussed in the light of various lines of molecular evidence. Verne Grant and Louisiana Irises:
is there anything new under the sun?
The impact of Verne Grant’s 1981 edition of Plant Speciation reverberates as strongly today as it did 22 years ago. His insights into the field of plant evolutionary biology continue to inform current and future research programs. For example, Grant’s work pointed the way for those who have argued that the consequences of natural hybridization are of primary importance in the evolution of many plant and animal species complexes. It is thus now widely accepted that numerous, important outcomes are afforded by crosses between divergent lineages. In the present paper we highlight some of the concepts that appear in Grant’s seminal work. We address these in the context of their application by Grant to a plant species complex that we have studied for 15 years, the Louisiana Irises. We consider Grant’s inferences in light of new data and our own conclusions. In the main, we find ourselves supporting Grant’s (1981) conceptual framework. Selection in an Ipomopsis hybrid zone: implications for
ecological speciation The fitness of hybrids relative to parental species plays an important role in models of speciation. In flowering plants, ecological speciation may be driven by divergent selection of floral traits by pollinators, at the expense of intermediate phenotypes. An alternative model involves physiological adaptation of plants to distinct environmental conditions, which may be accompanied by reproductive isolation due to environment-dependent selection against hybrids. We are examining the lifetime fitness of F1 and second generation hybrids, using Grant’s classic example of Ipomopsis aggregata and I. tenuituba as a model system. Pollination studies reveal some disruptive selection on a floral trait (corolla width), but there is considerable geographical variation in the strength of this pollinator preference and hybrids do not generally suffer lower pollination. Reciprocal transplants show that survival of hybrids depends on both genotype and environment. Combining survival and reproduction, certain types of hybrids are more fit than one or both parents in parts of the hybrid zone. Ecophysiological studies show that hybrids can be more water use efficient than either parent, perhaps contributing to success in dry hybrid habitats. The dynamics of this hybrid zone involve a mixture of selection mediated by pollinators and other sources. Evolution in spatially structured populations Recent models of speciation have incorporated population structure and migration into the class Dobzjansky-Muller model of speciation. The results have shown that speciation may occur despite incomplete isolation among demes. It has also been shown that, in some cases, low levels of migration may actually enhance the rate of speciation among demes. In this paper, we expand upon these models to incorporate increasingly realistic types of population structure and gene flow into these models of speciation.
Reticulate evolution in
diploid and polyploid perennial soybeans (Glycine subg. Glycine)
Glycine subgenus Glycine is a group of about 20 perennial species, mostly native to Australia, that is the sister group to the subgenus that includes the annual cultivated soybean (G. max). Phylogenetic incongruence between chloroplast DNA (cpDNA) and nuclear genes suggests hybridization and introgression both between distantly related diploid species and between diploid taxa that have diverged much more recently. Reticulation also has led to allopolyploidy in the subgenus, involving at least eight different genome combinations. Allopolyploid taxa appear to be very recent, perhaps having evolved since the arrival of humans in Australia around 40,000 years ago. Some genome combinations have arisen at least six different times, and in some but not all cases origins have been bidirectional, involving different maternal progenitors. Reticulate evolution at the polyploid level can have a profound impact on the evolution of gene families, as it has in the case of copy number and expression in the 18S-26S nuclear ribosomal gene family of subg. Glycine. Geographic cohesion, parallel adaptive radiations, and
consequent floral evolution in Calochortus (Calochortaceae) Calochortus has undergone extensive diversification in floral morphology, habitat, serpentine tolerance, and chromosome number, and many species are restricted to small geographic areas. A cpDNA phylogeny identifies seven major, geographically cohesive clades, centered in the Bay Area, Pacific Northwest, Coast Ranges-Sierra Nevada, San Diego area, Southwest California, Great Basin, and Central Mexico. Geographic cohe-sion also operates at lower taxonomic levels, with sister species often being peripatric. The genus arose in California, in the Coast Ranges or Sierra Nevada, which were them-selves uplifted during the last 3-5 million years. Three of the four major floral syndromes evolved at least twice, in association with particular kinds of habitats. Serpentine tolerance evolved at least seven times. A base chromosome number of x = 9 is ancestral; divergence among clades in chromosome number helps create reproductive isolation in most cases where different clades overlap. We argue that limited dispersal ability led to narrow endemism of individual taxa, the geographic cohesion of clades, and parallel adaptive radiations in different areas. We propose that floral syndromes have under-gone consequent radiation, reflecting adaptation to local pollinators and physical conditions as species invaded different habitats, rather than divergent selection to partition pollinators or attain reproductive isolation. Speciation in plants: how much has been learned since 1950? My talk will review how speciation in plants has been studied since 1950, when Ledyard Stebbins published his important book, “Variation and Evolution in Plants.” Between 1950 and 1980, the leading American students of plant species were Verne Grant, Jens Clausen and Harlan Lewis. They emphasized different aspects of species biology and, not surprisingly, came to different, though overlapping, conclusions about how speciation occurs in plants. I will discuss their important results and show how their ideas have stimulated recent research, particularly that making use of the attitudes and techniques of molecular genetics and molecular systematics. In addition, I will suggest several topics having to do with species divergence that now seem appropriate for focused analysis. Fern speciation: different mechanisms meet different
challenges and opportunities Long considered to be poster children for allopolyploid (“secondary” sensu Grant) speciation, recent studies have demonstrated that homosporous fern lineages have also experienced a wide range of mechanisms leading to “primary” species. Outcrossing mechanisms and simple breeding systems of ferns predisposes them to form hybrids, thereby launching the cascade of events resulting in allopolyploid species. Because such species are hosts to genomic diversity, they are primed for diversification via reciprocal gene silencing. At the diploid level, mechanisms promoting fern diversification have been described but are largely untested. By combining DNA sequencing and isozyme electrophoretic techniques, an improved perspective on primary speciation in ferns has been obtained. Comparing speciation pathways in a temperate triad of sister species to those in a tropical quartet indicated that classic allopatric models followed by reinforcement are likely among the temperate species whereas diversification by adaptation to ecologically distinct habitats prevails among the tropical ones. Temperate sister species tend to be molecularly quite distinct from each other but remain morphologically very similar. The tropical sister species, on the other hand, are distinct morphologically but nearly identical molecularly. Such contrasts indicate that ferns, as other plants, follow a variety of pathways when initiating new evolutionary lineages. Genetics of reproductive isolation between Aquilegia
formosa and A. pubescens Verne Grant used Aquilegia formosa and A. pubescens as his first specific example of how species may be reproductively isolated due to floral differences. In order to understand the extremely rapid radiation of species in this genus, we have been investigating the genetic basis of shifts in floral morphology and their consequences to reproductive isolation. Floral differences between these two species have a strong impact on which animals visit their flowers and the efficiency of pollen transport. In order to estimate the genetic architecture of these floral differences we constructed a genetic linkage map using 330 F2 individuals and both microsatellite and AFLP markers. Quantitative trait locus (QTL) mapping has revealed that some floral characters (e.g., floral spur color and flower orientation) are controlled by few QTL of large effect while others (e.g., nectar spur length and petal blade length) are controlled by many QTL each of smaller effect. In addition, our analysis has revealed that many segments of the genome had distorted segregation ratios of which nearly all favored alleles derived from A. formosa. Furthermore, some segments were distorted only through the male parent and others only through the female parent. This detailed genetic analysis has therefore revealed new characters that are likely important in causing reproductive isolation between these species and provides the basis for future investigations into their genetic basis. Evolutionary dynamics of diploid-polyploid contact zone: insights into polyploid
speciation Polyploidy is viewed as an important and rapid mechanism of speciation in plants, yet our understanding of the process by which polyploids arise and establish is poor. In theory, new polyploids will experience a strong frequency-dependent mating disadvantage, raising the question of what ecological and genetic factors are responsible for their establishment. Here I explore these questions using research from the diploid-tetraploid contact zone in Chamerion angustifolium (Onagraceae). We find that indeed tetraploids experience a frequency-dependent mating disadvantage, which is reinforced, rather than counterbalanced by the moderate viability of tetraploids. Two factors that may account for the success of tetraploids are explored: assortative mating and polyploid formation. We find that polyploids are produced at a surprisingly high rate, but this alone cannot compensate for the minority cytotype disadvantage. Assortative mating between ploidies is very strong and includes several prezygotic reproductive barriers. Furthermore, reproductive isolation is assymetrical, favouring the establishment of tetraploids within mixed populations. These results suggest that population processes leading to polyploid establishment are often restrictive and, depending on rates of assortative mating and polyploid formation, polyploid speciation may be a slower and more complex process than once thought. Historical inferences from the self-incompatibility locus Ancestral polymorphism preserved
at the self-incompatibility locus provides information over a much longer time
scale than neutral polymorphism. This enables us to draw inferences about
historical occurrences far older than extant species. In this paper we outline
the ways in which studies of the S-locus can provide useful insights into
patterns of speciation. First, we review the evidence concerning the prevalence
of founder events in speciation. A dramatic population size reduction is
expected to leave a pattern of reduced sequence diversity at the S-locus that
is diagnosable for tens of millions of years following the relaxation of the
bottleneck. Generally, the lack of evidence for such bottlenecks preserved at
the S-locus indicates that this mode of speciation is rare. Second,
uninterrupted polymorphism at the S-locus provides a novel opportunity to
reconstruct with certainty ancestral states of an important mating system
character, the presence or absence of incompatibility. We demonstrate this
approach using a phylogenetic analysis to find that transitions from
self-incompatibility to self-compatibility are common and irreversible in the
Solanaceae. Currently, self-compatible taxa outnumber self-incompatible taxa.
Therefore,
self-incompatibility is either slated for extinction or the presence of
incompatibility increases the net diversification rate, maintaining a mixture
of self-incompatible and self-compatible species in equilibrium. We briefly
outline how phylogenetic approaches can be used to determine the effect of
incompatibility on diversification. Self-incompatibility, present in many plant
lineages, provides a unique opportunity to test the effects of a mating system
character on diversification rate. Contrasts between quantitative traits and molecular markers suggest that quantitative traits typically diverge in response to local selection pressures more than do individual genes. However, it is debated to what extent migration affects adaptive divergence. I will review theory showing that under diversifying selection on quantitative traits, covariances develop among allele frequencies at additive loci underlying quantitative traits. These covariances contribute a substantial fraction of the among population trait variance permitting considerable trait divergence with limited divergence of allele frequencies at the underlying loci. Thus adaptive trait divergence can be accomplished in the face of substantial gene flow. Simulations suggest that gene flow influences within population variation to a greater extent than it affects divergence. Thus in some instances, gene flow may enhance, rather than constrain, the ability to adapt to local or novel environments. However, the contribution of covariances among loci to the divergence of traits depends upon there being multiple loci underlying quantitative trait variation. Empirical work in our lab is directed to mapping genes underlying ecotypic differences in Avena barbata in California and towards studying the fitness consequences of hybridization between the ecotypes. The cytoplasmic factor in
plant speciation The role of nucleocytoplasmic interactions in the genesis of post-zygotic isolation has been given little attention by plant evolutionists. I present evidence from reciprocal crosses, cytoplasmic substitution lines, and cell fusion lines that hybrid weakness and sterility often arise from interactions between the nuclear genome and the chloroplast and mitochondrial genomes. These interactions are much more important in the origin and isolation of species than we appreciate. The strength of the post-zygotic barriers tends to be a function of cytoplasmic divergence. I also review evidence indicating that the properties and evolutionary potential of allopolyploids and diploid hybrid derivatives may be influenced by cytoplasmic factors. Hybridization and ecological divergence in sunflowers Entry into new and discrete ecological niches is theoretically difficult because it often requires simultaneous changes at multiple traits. One possible mechanism by which this difficulty might be overcome is transgressive segregation, which refers to the generation of extreme traits in segregating hybrid populations. Transgressive segregation is observed for many traits in artificial hybrids, and some authors have suggested that it might contribute to adaptation in nature. In annual sunflowers, the three wild species found in the most extreme habitats (sand dunes, desert floor, and salt marshes) all happen to be of hybrid origin. Possibly, transgressive segregation contributed to niche colonization. This hypothesis was tested by assessing whether (1) synthetic hybrids exhibited the transgressive phenotypes thought to be necessary for colonization of novel and extreme habitats; (2) transgressive phenotypes were favored by selection in nature; and (3) QTL combinations contributing to transgressive phenotypes in synthetic hybrids also occurred in the natural hybrid species. Results indicate that extreme traits found in natural hybrid species can be accounted for by transgressive segregation in synthetic hybrids. As expected, transgressive segregation in wild species is largely due to complementary gene action and the transgressive phenotypes and individual QTL are under strong directional selection in hybrid habitats. Finally, transgressive QTL combinations in synthetic hybrids are found in the natural hybrid species. Thus, transgressive segregation likely facilitated major ecological transitions in annual sunflowers. Processes and patterns of intrinsic
reproductive barriers in the Solanaceae Intrinsic, postzygotic reproductive isolating barriers have long been considered the gold standard for defining species because they demonstrate a closed gene pool, allowing independent evolutionary tracks. These barriers can arise from differences in individual loci that cause incompatibilities (genic barriers) or from karyotypic rearrangements (chromosomal barriers). Despite the importance and seeming inevitability of these barriers, little is known about what kinds of genes are involved in incompatibilities and how and when they arise in the course of speciation; nor do we know how chromosomal rearrangements that cause impairment of fertility can become fixed differences between species. My research focuses on both of these questions using different genera and species within the Solanaceae as model systems. In my talk I will present data addressing the different manifestations and molecular basis of genic incompatibilities between the cultivated tomato, Lycopersicon esculentum, and a wild relative, Lycopersicon pennellii. In addition, patterns of chromosomal rearrangements within the Solanaceae will be used to address the role these rearrangements have played in adaptive divergence between species in the genus Solanum. Hybridization as a mechanism of dispersal in oaks Orbitally-induced climate changes have forced species to shift their ranges. This has had long-lasting genetic consequences. In European oaks, the footprints of postglacial colonization can still be detected, in the form of patches several tens of kilometers in diameter where virtually all trees descend from the same maternal founder individual. Remarkably, this local spatial structure is shared across oak species. These patches have been interpreted as the consequence of founder events due to long-distance seed dispersal during postglacial expansion. Such a genetic structure, once established, is remarkably resilient, due to the high population sizes of mature oak forests and to the prevalence of pollen movements. On the other hand, adaptive traits related to species status may be disseminated through pollen flow. The resurrection of relatively pure oak species as a consequence of intense disruptive selection eventually results in ‘nuclear capture’ of incoming oak nuclear genomes by resident cytoplasms. Hybridization and introgression may therefore be viewed as mechanisms of dispersal. In fact, the succession of oak species (with the more pioneer species being replaced by late-successional ones) can have both a demographic and a genetic component, since hybridization is often asymmetrical. Other plant species may take advantage of related species to extend their ranges. Integration of
populations and differentiation of species The study of speciation is largely founded on two conceptual advances. The first was Darwin’s proposal that species differences are caused by natural selection. The second was Ernst Mayr’s formulation of the biological species concept, which emphasized the importance of gene flow for holding species together and reproductive barriers for keeping them apart. However, the primacy of selection has been challenged by speciation models emphasizing population bottlenecks, and the importance of gene flow has been disputed because migration seems too limited to keep conspecific populations from diverging. Here, I review evidence from recent genetic studies to evaluate these claims. With respect to selection, the majority of loci affecting species differences have effects in the same direction. This pattern is most consistent with divergence through directional natural selection; neutral divergence would result in a high proportion of loci with antagonistic effects. Likewise, there is sufficient gene flow in essentially all species to enable the spread of strongly favorable alleles, the most likely agents of collective evolution. Thus, gene flow probably does hold species together, but its traditional role as a force that constrains differentiation has been over-emphasized relative to its creative role as a mechanism for the spread of advantageous mutations. Adaptive radiation and regulatory gene evolution in the Hawaiian silversword
alliance (Asteraceae) The Hawaiian silversword alliance is a premier example of adaptive adiation,
with the species exhibiting extensive and rapid diversification in reproductive
and vegetative form. Molecular evolutionary analyses of the floral regulatory
genes ASAP1 and ASAP3/TM6 indicate a significant acceleration in nonsynonymous
relative to synonymous substitution rates in the rapidly evolving Hawaiian
lineage. Analyses further indicate that the Hawaiian species are allotetraploids,
deriving from an ancient interspecific hybridization event involving species in
two lineages of North American tarweeds. Molecular population genetic analyses
of the duplicated ASAP1 and ASAP3/TM6 genes indicate that two homoeologs
(ASAP1-A and ASAP1-B) appear to be evolving in a similar fashion, whereas two
other homoeologs (ASAP3/TM6-A and ASAP3/TM6-B) have patterns of nucleotide
diversity consistent with divergent evolutionary trajectories. This divergence
suggests that selection may be partitioning the functional trajectories of these
two regulatory gene copies. A multilocus study of six genes (ASAP1-A, ASAP1-B,
ASAP3/TM6-A, ASAP3/TM6-B, ASCAB9 and ASNAD1) among several Hawaiian silversword
alliance species further indicates that recently-derived sibling species that
exhibit significant morphological divergence show incomplete haplotypic
divergence and genetic distances similar to levels expected between intraspecific
populations. The extent of genetic divergence and historical demographics
influencing diversification are also examined. Pedunculate oak (Q. robur L.) and Sessile oak (Q. petraea (Matt.) Liebl. are closely related species with a widely sympatric distribution in Europe. When the two species coexist in the same stands, they show clear ecological preferences and morphological differences. A whole body of literature data shows that the phenotypic differentiation is associated to an extremely low genetic differentiation. One hypothesis for the discrepancy was related to the distribution of differentiation within the genome. When numerous markers were tested, it became apparent that a rare proportion exhibited larger allele frequency variation between the two species. These results clearly suggested that genomic regions involved in species differentiation are rare and explain why earlier molecular investigations using a low number of loci proved to be inefficient. We revisited previous genetic surveys conducted in natural mixed populations with different markers systems and computed interspecific Gst (Nei’s genetic differentiation) values for all markers. Their distribution follows an L shaped curve. In a second step we detected QTLs for leaf morphological traits exhibiting interspecific phenotypic differences. The comparison of the distribution of interspecific Gst values and QTLs revealed some interesting colocalisation. These two independent approaches confirmed the existence of hots spots of species differentiation in the oak genome. Phylogenetic estimation of speciation and extinction rates In the last 10 years quantitative methods for estimating speciation rates in clades have improved significantly. Better estimates for patterns of rate variation across plant groups have been obtained by combining these methods with new information about the divergence times of species, based on analysis of sequence data and the fossil record. In this talk I review statistical approaches to estimating diversification rates and evaluate their dependence on the accuracy of divergence time estimates. I also discuss two critical complicating factors: the role of extinction in causing biased inferences, and the difficulty of modeling the diversification process in a realistic fashion. Finally, I discuss examples of estimating speciation and extinction rates at two very different phylogenetic scales: among major clades of angiosperms, and between selected recent radiations for which good geological time calibrations are available. Adaptive divergence and speciation
in wild rice (Oryza) Adaptive divergence plays an essential role in phenotypic evolution and speciation. The populational genetic model of adaptation predicts that the phenotypic sizes of favorable mutations fixed during an adaptive walk decrease exponentially as the population moves toward the fitness optimum. The wild progenitors of the cultivated rice (Oryza sativa), Oryza nivara and Oryza rufipogon offer an ideal experimental system to test this hypothesis. Phylogenetic analyses based on multiple gene sequences suggested that O. nivara was derived from the O. rufipogon-like ancestor as a result of habitat shift from deep-water swamps to seasonally dry habitats. Despite the recency of divergence (less than 0.3 million years ago), the two species have shown remarkable differentiation in life history, reproductive allocation, breeding system, seed dispersal, and photoperiodism. The genetic basis of the adaptive differentiation is being studied through QTL mapping.The loss of photoperiod sensitivity must have been among the earliest mutations fixed during the adaptive walk leading to the origin of O. nivara. Being a short-day plant, Oryza rufipogon does not flower early enough to produce seeds before the dry season and would have zero fitness in the seasonally dry habitats. In rice cultivars, the loss-of-function mutation of the Hd1 gene explains a large portion of the phenotypic variation of photoperiod sensitivity. Our greenhouse experiments and DNA sequencing analyses indicated that the loss-of-function mutation of the Hd1 gene is correlated with the photoperiod insensitivity in O. nivara accessions. A transgenic experiment has been conducted to determine whether the single-gene mutation accounts for the loss of photoperiod sensitivity of O. nivara, and consequently to test whether favorable mutations fixed initially in adaptation have major phenotypic effects. The dynamics of polyploid establishment in flowering plants Nearly half of all flowering plants species are the result of whole-genome doubling, i.e. polyploidy. The mechanisms by which a neopolyploid is able to invade the population of its progenitor differ from those involved in the fixation of a new adaptive mutation in a homoploid population. Unlike the selective advance of a new adaptive mutation, a neopolyploid is nearly or entirely reproductively isolated from its progenitors. While the more abundant progenitor will mate principally within a cytotype, inter-cytotype matings will be the rule for the neopolyploid, and these typically produce far fewer progeny which suffer from low viability and fertility. Although a new cytotype may be ecologically differentiated from its progenitor and may achieve superior viability or fertility, frequency-dependent mating success results in the exclusion of the minority cytotype. The dynamics of polyploid establishment has been investigated in a number of models using a variety of approaches. All models examine the dynamics of a single population, and all identify the conditions for coexistence of a polyploid with its progenitor, as well as those leading to the replacement of one cytotype by another. The models generally assume annual populations and deterministic dynamics and it is assumed that intercytoype matings fail, e.g. triploids are not produced in a mixed population of diploids and tetraploids. The results from these theoretical efforts demonstrate that by virtue of their low initial frequency, and the low success of intercytotype matings, neopolyploids possess an inherent disadvantage that can be overcome by a variety of mechanisms, including: 1) fitness differences, 2) ecological differentiation, 3) selfing or apomixis, and 4) a high frequency of unreduced gamete formation by the progenitor cytotype. Nevertheless, most authors conclude that the conditions under which polyploids can successfully coexist with their progenitor cytotype are quite restrictive. Coexistence often requires parameter values that are very different from those observed in natural populations. A fundamental assumption shared by all published models is that the invasion and spread of polyploids occurs within a single population. Here we provide the results of computer simulations that investigate the dynamics of polyploid establishment. We propose that the distinguishing feature of polyploid establishment not included in previous models is the potential for neopolyploids to possess such profound ecological differences as to allow them to occupy habitats unsuitable for their progenitor. Such ecological differentiation could provide substantial reproductive isolation, and thus overcome the frequency-dependent minority disadvantage. Polyploidy and
speciation Grant dedicated four chapters of Plant Speciation to the topic of polyploidy, covering range and frequency of polyploidy, types of polyploids, factors promoting polyploidy, and the polyploid complex. Much of Grant’s text remains relevant today, but some areas have seen dramatic changes in the past 20 years. Genetic and genomic investigations have provided unparalleled insights into the genetic architecture of polyploids. It now appears that all angiosperms are polyploid to some degree. Some polyploid genomes are highly dynamic and have experienced considerable rearrangement, while others have been relatively stable. Genomic downsizing has occurred following polyploidization in some cases, but not others. Genetic tools have clarified duplicate gene evolution in polyploids, as well as the extent and mechanisms of gene silencing in polyploid species. Epigenetic phenomena may play a major role in gene silencing, and the potential role of subfunctionalization of duplicate genes is becoming apparent. The prevalence and genetic consequences of autopolyploidization were unappreciated 20 years ago. Similarly, multiple origins of polyploids, largely uninvestigated prior to the 1980s, are now known to be the rule, with important genetic consequences for natural populations. Despite new understanding of the genetic and genomic attributes of polyploids, progress in other areas has been minimal. For example, little is known about the ecological and physiological attributes of polyploids relative to their diploid progenitors. Hybridization and speciation in Gossypium The cotton genus includes approximately 50 species distributed in arid to semi-arid regions of the tropics and subtropics. Gossypium species exhibit extraordinary morphological diversity, ranging from herbaceous perennials to small trees and having a great variety of floral and vegetative features. A parallel level of cytogenetic and genomic diversity has evolved during the global radiation of the genus, with the eight diploid (all n = 13) genome groups varying 3.5-fold in genome size. Phylogenetic analysis and molecular clock calculations suggest that the genus originated 5 - 10 million years ago, and that the major genome groups arose in rapid succession following formation of the genus. This evolutionary history has included multiple episodes of trans-oceanic dispersal, invasion of new ecological niches, acquisition of specialized reproductive syndromes, and a surprisingly high frequency of interspecific hybridization and hybrid speciation among lineages that presently are inter-sterile. One modern diploid appears to have a complex history of cryptic trysts with lineages from two hemispheres. A chance biological reunion among diploid lineages isolated in different hemispheres for millions of years led to the monophyletic origin of American polyploid cottons, which subsequently radiated into lineages represented by five extant species. The ecology of plant speciation The role of ecology in plant speciation, whether acting directly to bring about reproductive isolation through ecological speciation or indirectly by allowing ecological persistence, is generally poorly characterized. Minimally, niche shifts can facilitate the establishment of reproductive isolation. In addition, under ecological speciation, divergent natural selection acting on ecological tolerances can drive the evolution of reproductive isolation. Verne Grant (among others) noted strong divergent selection is easily imposed by the edaphic environment. Indeed, edaphic specialists such as occur on serpentine or highly saline soils provide the best evidence for ecological speciation in plants. Although there are relatively few well-studied systems, these suggest two trends. First, reproductive isolation may be a common byproduct of ecological divergence. For example in Lasthenia californica, levels of reproductive isolation increase along with ecological divergence between populations. Second, parallel evolution of reproductive isolation is also a likely outcome of ecological selection. These trends suggest that the contribution of divergent natural selection to the evolution of reproductive isolation in plants merits considerable attention as it is likely substantially under-appreciated. Furthermore, I suggest that the study of edaphic adaptation may be among the most fruitful avenues for future research to clarify the extent of ecological speciation in plants. Mating system evolution and speciation in monkey flowers The genetic basis of species differences provides insight into past evolutionary change and has long been a subject of contention among evolutionary biologists. In this study, we investigate the genetic architecture of phenotypic differences and reproductive isolation between two flowering plant species with highly divergent mating systems: Mimulus guttatus (outcrossing) and M. nasutus (selfing). Using a large number of molecular markers, we identified a total of 24 quantitative trait loci (QTLs) underlying seven floral traits associated with the divergent mating systems. Variation in each floral trait was caused by at least 11 QTLs, and almost all QTLs affected more than one floral trait. Nearly all of the QTLs had very small effects. We also used this map to investigate the genetic basis of partial postzygotic reproductive isolating barriers. We found evidence of Dobzhansky-Muller incompatibilities and nucleo-cytoplasmic interactions responsible for partial male and female hybrid sterility, and have begun to characterize the underlying loci. These investigations provide an unusually detailed view of the complex genetic changes that occurred during the evolution of self-fertilization and reproductive isolation, and set the stage for future investigations into the molecular genetic basis of these traits. |
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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 |
