Evolution of walking leaves

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Diversity of female walking leaves within and between species. The two individua
Diversity of female walking leaves within and between species. The two individuals on the left belong to Phyllium elegans from Papua New Guinea, followed on the right by Phyllium letiranti from Peleng Island in Indonesia and Phyllium hausleithneri from Malaysia. Photo: Bruno Kneubühler
Göttingen research team creates phylogenetic tree of leaf insects An international research team led by the University of Göttingen has studied the evolution of the walking leaves. Walking leaves belong to the stick insects and ghost insects that, unlike their approximately 3,000 branch-like relatives, do not imitate twigs. Instead, they wear large-scale extensions on their bodies and legs to mimic deciduous leaves in shape and colour. They have perfected this camouflage, which serves as protection from predators such as birds and mammals, by imitating feeding spots and withering leaf elements. In addition, the animals imitate leaf veining by special veining of the large forewings. In the genetic family tree the researchers now created, the relationships of almost 100 species are determined. That is about two-thirds of all known species. The results were published in.

Until now, the majority of the walking leaves were grouped under the name Phyllium. However, numerous species were united here that are not at all closely related to each other. The scientists now introduced some new genera in order to be able to describe the natural relationship. "Many of the species studied have not even been described scientifically," explains Sarah Bank, first author of the study and a doctoral candidate in the Biodiversity and Ecology program at the University of Göttingen. "We also identified many cryptic species - those that are not outwardly distinct but genetically very different."

Göttingen evolutionary biologist Dr Sven Bradler adds, "A few years ago, just 50 species had been described. Through our studies and especially through the collaboration with the New York expert on walking leaves, Royce Cumming, this number has now doubled. This shows that our knowledge of biodiversity is often heavily dependent on the expertise of individuals. For many groups of organisms, such knowledge is simply lacking."

The team also reconstructed the phylogenetic distribution of the insects. According to the study, the origin of the walking leaves is in the Southern Pacific, from where they spread throughout tropical Asia over the past 50 million years - as far east as Fiji and as far west as the Seychelles. "The age of the walking leaves is a scientifically controversial topic," Bank says. "Other studies had placed their temporal origin in the Cretaceous or even Jurassic, 100 to 150 million years ago. At that time, however, the angiospermous flowering plants whose leaves so perfectly mimic insects were not even common. Consequently, the results of our work seem more plausible to us, favoring a more recent origin."

Original publication: Sarah Bank et al. A tree of leaves: Phylogeny and historical biogeography of the leaf insects (Phasmatodea:Phylliidae). Communications Biology (2021). https://doi.org/10.1038/s42003-0­21-02436-z