Towards an evolutionary history of European-Alpine Trechus ground beetles: Species groups and wing reduction

The evolution of flight triggered the rise of pterygote insects, but secondary flightlessness has evolved numerous times and is often associated with reduced gene flow among populations and patterns of diversification. With 85 species most of which are wing reduced, the ground beetle genus Trechus in the European Alps may be one such example. Here, we reconstructed a molecular phylogeny using 72 of these species based on mitochondrial and nuclear DNA sequences as a basis for reconstructing their evolutionary history. We rearranged the species into 20 monophyletic species groups, of which five are novel and 15 were already established but with different species allocated. Wing measurements revealed a strong tendency for wing reduction but also variation within and among species, with the few fully-winged species distributed across multiple species groups containing also wing-reduced species. Using character mapping and phylogenetic independent contrasts, we found that neither distribution area, body size, pigmentation, elevational zone, nor hygrophily explained wing status in our sample. Assessing five completely sampled clades, we inferred that each of their ancestors had most likely already been wing reduced. We discuss putative scenarios explaining this pattern and the presence of wing polymorphism across the phylogeny. One plausible scenario would be an already wing-reduced last common ancestor of all Trechus species and multiple regains of full wing length via back mutation and/or hybridisation. Alternatively and possibly more likely, the ancestors were either fully winged, with subsequent rapid and repeated wing reduction explaining the current wing-status pattern, or polymorphic, with long-term polymorphism or reselection acting on standing genetic variation explaining the recent fully-winged species. Either way, Trechus ground beetles are a promising, taxonomically and ecologically diverse system for studying the evolution of flightlessness. Areas for future research include morphological assessment of flight muscles, functional analysis of flight capability, and exploration of the mechanistic and genetic bases of wing and flight evolution.