The phasmatodean tree of life: resolving evolutionary patterns of diversity and disparity in a mesodiverse insect lineage

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Stick and leaf insects, or Phasmatodea, are a mesodiverse lineage of large terrestrial herbivorous arthropods with predominantly tropical and subtropical distribution. They are usually nocturnal and exhibit remarkable forms of masquerade crypsis, imitating various parts of plants, such as twigs, bark and leaves to deceive visually hunting predators. Phasmatodea comprises ~3100 extant described species and serves as a model system to address important questions in evolutionary biology, such as speciation, evolution of asexuality, loss and regain of morphological traits, and clade diversification or, more precisely, repeated adaptive radiations in geographic isolation. Until recently, stick and leaf insects stood out as one of the last remaining insect orders without a robust higher-level phylogenetic hypothesis. Our latest molecular studies based on extensive transcriptomic and Sanger sequence data have now resolved the early evolution of these insects and corroborate the finding that the extant diversity is the result of a surprisingly recent rapid radiation. Our comprehensive temporally calibrated phylogenetic analysis is based on data selected for maximum phylogenetic coverage from over 1200 taxa – more than one third of the known species diversity. The tree provides the evolutionary framework for (i) tracing the global historical biogeography of phasmatodeans, (ii) assessing rates of speciation and extinction, and (iii) reconstructing ancestral character states and transformations of crucial traits that are involved in adaptive radiations and have an effect on diversification. These traits comprise flight capability and the presence of associated structures such as wings and ocelli, body size and form, tarsal attachment structures and the capacity for asexual reproduction. Yet, for some crucial evolutionary lineages of which no transcriptomic data is available our analyses still produce conflicting phylogenetic hypotheses. However, robust evolutionary scenarios for the outlined research questions can only be inferred from a reliable phylogeny. To reach this goal, we will expand our studies and apply a further phylogenomic approach, which is based on target enrichment of selected exons for 200 carefully chosen informative species of stick and leaf insects. The project is designed to significantly advance our understanding of adaptive clade diversification across space and time in a previously neglected group of insects.