Biodiversity and evolution of Malagasy stick insects: Ancient lineages or recent adaptive radiation?
Final Report Abstract
Madagascar is one of our planet’s biodiversity hotspots, exhibiting an extraordinarily distinct and diverse, but also severely endangered biota. Many elements of its fauna are still poorly documented and remain largely unexplored. For example, the evolutionary history of the Malagasy stick insects, large terrestrial herbivores of the insect order Phasmatodea, has been entirely obscure for a long time. Traditionally, these insects were considered to belong to four distinct lineages of different categorical ranks ranging from family to tribe (+ one taxon incertae sedis). These groups have been considered to be unrelated to each other and were assumed of being relic lineages, which survived on this long-term isolated landmass. However, based on a combination of nuclear and mitochondrial sequence data we demonstrate that these morphologically and ecologically diverse stick insects are in fact closely related and most probably represent a single adaptive radiation. We found high support for a sister-group relationship between the giant-sized winged Achriopterini and the flightless and ecologically diverse Anisacanthidae, both previously placed in the two different suborders Areolatae and Anareolatae. The remaining Malagasy lineages are also phylogenetically grouped with these taxa. It was demonstrated before that geographical distribution rather than traditional taxonomic grouping and morphological similarity appears to reflect the evolutionary relationships among stick and leaf insects – and the Malagasy stick insects are no exception. This is impressively demonstrated in a comprehensive morphospace comparison of the Malagasy stick insects with Australasian taxa, revealing a high degree of convergent evolution. Our analyses also dispute the assumption of Madagascar harbouring relic phasmatodean lineages. In fact, Madagascar was colonised from Africa via transoceanic dispersal by the ancestral stick insect approximately 42 million years ago, long after this fragmentary island became geographically isolated from any other landmass around 90 million years ago. Most surprisingly, the Malagasy stick insects do not show any affinities to phasmatodeans of adjacent islands such as the Mascarene archipelago or the Seychelles. Our comprehensive analysis of members from these landmasses revealed another fully unexpected radiation of stick insects on Mauritius and Réunion and supports an Australasian origin of Mascarene stick insects, meaning their ancestors voyaged over 5000 km to reach these islands off the African coast. They also arrived before the current islands even existed thus providing evidence for the long-term existence of former islands in that region, the product of volcanic activity during the Eocene and Oligocene. Further surprising results associated with this project comprise the report of the oldest fossil stick insect from the Mesozoic of China, which is pivotal for dating our new evolutionary trees and providing details on the paleobiology of these insects in preangiosperm times, and the first report of a phasmatodean ootheca reported from the Korinninae, an enigmatic taxon among stick insects that we included as an outgroup in our analyses for the first time. These three latter studies were extensively covered by local newspapers (Göttinger Tageblatt, Northeimers Neueste Nachrichten, Hessische Niedersächsische Allgemeine), by the national and international press (Rheinpfalz am Sonntag, LABOonline, Spiegel online, National Geographic, Washington Post) and were also highlighted in numerous science blogs.
Publications
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2011. Stick insect on unsafe ground: does a fossil from the early Eocene of France really link Mesozoic taxa with the extant crown group of Phasmatodea? Systematic Entomology 36: 218–222
Bradler S, Buckley TR
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2014. A molecular phylogeny of Phasmatodea with emphasis on Necrosciinae, the most species-rich subfamily of stick insects. Systematic Entomology 39: 205–222
Bradler S, Robertson JA, Whiting MF
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2014. Under cover at pre-angiosperm times: a cloaked phasmatodean insect from the Early Cretaceous Jehol biota. PLOS ONE 9: e91290
Wang M, Béthoux O, Bradler S, Jacques FMB, Cui Y, Ren D
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2015. Extreme convergence in egg-laying strategy across insect orders. Scientific Reports 5: 7825
Goldberg J, Bresseel J, Constant J, Kneubühler B, Leubner F, Michalik P, Bradler S
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2015. Single origin of Mascarene stick insects: ancient radiation on sunken islands? BMC Evolutionary Biology 15: 196
Bradler S, Cliquennois N, Buckley TR
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2015. The Phasmatodea Tree of Life: Surprising facts and open questions in the evolution of stick and leaf insects. Entomologie heute 27: 1–23
Bradler S
