Macroevolutionary consequences of habitat transitions in aquatic Coleoptera

Abstract

Understanding the origin and evolution of novel traits is one of the unknowns on which evolutionary biologist work. These traits may influence the origin and diversification of new lineages, and some of them may also facilitate the transition between different habitat types. Focusing on the aquatic environment, some of the most important transitions occurred between lotic and lentic environments, and between ecosystems with different salinities. With reference to the first transition, it has been established that species living in standing water (less persistent than running waters at geological time scales) need greater dispersion capability than running water species. On the second transition, some authors have shown that salinity tolerance may appear in independent lineages during arid periods, likely as a byproduct of increased resistance to desiccation. We use different groups of aquatic Coleoptera as evolutionary models: the genus Ochthebius (Hydraenidae), and tribes Hydroporini (Dytiscidae) and Hygrotini sensu lato (Dytiscidae). It is expected that transitions between habitat types had some effect on the diversification pattern on the lineages. We have tested these predictions with Hygrotini sensu stricto, including 129 specimens from 53 species with a Holartic and Afrotropical distribution. Five gene fragments were used in the phylogenetic analysis (COI, 16S, 18S, H3 and 28S). We used molecular phylogenies, ecological data and physiologic experiments to reconstruct macroevolutionary patterns of the traits using bayesian methods. Our results show a clear preference for lentic environments in Hygrotini, which are ancestral to the lineage, with repeated independent emergence of isolated lotic species. Similarly, high salinity tolerance has emerged independently in several lineages from an ancestor with slightly saline tolerance. Preliminary analyses suggest that the transitions to higher saline tolerance occurred predominantly in geological periods with increased aridity, supporting the hypothesis that salinity tolerance in water beetles could be an exaptation of tolerance to desiccation.