The pitcher trap is a striking example of convergent evolution: unrelated lineages of pitcher plants have independently evolved remarkably similar traps as adaptations to growing in nutrient-poor environments. In fact, traps derived from leaves that attract, trap and retain prey, have evolved independently at least six times in the Plant Kingdom. The best-known examples are Nepenthes, Cephalotus and the Sarraceniaceae which grow in the Old World Tropics, Australasia and the Americas, respectively. Attractive nectar, slippery surfaces, and glands which secrete digestive fluids to absorb nutrients, all feature prominently across these three pitcher plant groups.
Convergent evolution in pitcher plants is a relatively well-established phenomenon. But scientists are now discovering that within groups of related pitcher plants, species also show intriguing patterns of divergent evolution. Divergence in Nepenthes is linked to novel strategies for obtaining nutrients from specific sources – insects for example. Nepenthes gracilis which grows in Borneo, exploits the impact of rain drops for capturing insect prey. The pitcher lid acts as a rain-driven torsion spring, flicking insects into the pitcher during heavy rain. Related N. albomarginata produces a white ring of tissue on its pitchers to mimic lichen which is attractive to termites – a valuable source of nitrogen for the plant.
Meanwhile, other species have diverged from a staple diet of insects almost completely. Nepenthes ampullaria grows in dense forests, and forms dense carpets of ‘composting bins’ – pitchers ideally positioned to capture leaf litter fall. Most peculiar of all, are a handful of Nepenthes species which grow where insects are scarce. These plants produce pitchers which trap animal faeces – a rich source of nutrients. Scientists have demonstrated that the size and geometry in the pitcher mouths of N. lowii, N. rajah and N. macrophylla, all closely match the body size of tree shrews, which feed on the pitchers’ nectar which is secreted on the under-surface of the lids. These ‘tree shrew toilets’ have concave, reflexed lids oriented to position the animal for optimal faeces capture. Tree shrew toilets may have evolved from pitchers with broad pitcher mouths adapted for enhanced water or leaf litter capture, like the pitchers of Nepenthes ampullaria, described above. Finally, Nepenthes hemsleyana produces long, slender pitchers which capture few insects, but provide a day-time roosting site for bats. The pale-coloured tubular pitchers resemble the flowers of bat-pollinated plants and are easily located by the bats in dense, tropical vegetation.
The bizarre assortment of shapes and sizes of pitcher trap in Nepenthes adapted to specific nutrient sources, appear to be analogous to the well-known examples of adaptive radiation seen in animals; the beak shapes of Darwin’s finches and adaptations of cichlid fish in the African Great Lakes, for example. Faeces capture in carnivorous plants was only discovered relatively recently, and in species already known to science. By contrast, new Nepenthes species are described every year and little or nothing is known about trapping strategies of these plants. This opens up an exciting new area of research into adaptive radiation in pitcher plants, and raises questions such as: which genes are under selection? What is the role of hybridisation in forming new species? To what extend has adaptive radiation shaped the diversity of pitcher plants we see today? In our review, we suggest that pitcher plants, and Nepenthes in particular, are an ideal group with which to explore such questions.
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Read the Tansley insight: Thorogood, C. J., Bauer, U. and Hiscock, S. J. (2017) Convergent and divergent evolution in carnivorous pitcher plant traps. New Phytologist. doi: 10.1111/nph.14879
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Chris Thorogood is Head of Science and Public Engagement for the University of Oxford Botanic Garden and Arboretum. Chris’s research interests centre on evolutionary genetics, plant taxonomy and biodiversity hotspots. Specifically he is interested in speciation and adaptive radiations in cryptic parasitic and carnivorous plant groups, as well as taxonomic diversity in biodiversity hotspots including the Mediterranean Basin region and Japan. Chris won a scholarship in 2005 to carry out his PhD research on speciation in parasitic plants at the University of Bristol for which he won the Irene Manton Prize for botany in 2009; Chris is a Fellow of the Linnean Society of London.