Convergent evolution of aquatic life by sexual and parthenogenetic oribatid mites.

Convergent evolution is one of the main drivers of traits and phenotypes in animals and plants. Here, we investigated the minimum number of independent colonisations of marine and freshwater habitats in derived oribatid mites (Brachypylina), a mainly terrestrial taxon. Furthermore, we investigated whether the reproductive mode (sexual vs. thelytokous) is associated with the habitat type (marine, freshwater) where the animals live. We hypothesized that continuous resource availability in freshwater systems fosters asexual reproduction. We used 18S rDNA sequences to construct a molecular phylogeny of oribatid mites from terrestrial, marine and freshwater habitats. The results indicate that aquatic life in oribatid mites evolved at least 3×: once in Limnozetoidea (including only freshwater taxa) and at least twice in Ameronothroidea. In Ameronothroidea the taxon Ameronothridae n. gen. (nr. Aquanothrus) colonized fresh water independently from Selenoribatidae and Fortuyniidae (mainly marine Ameronothroidea). Reproductive mode was associated neither with marine nor with freshwater life; rather, in both habitats sexual and parthenogenetic taxa occur. However, the reproductive mode was related to the stability of the habitat. Species that live underwater permanently tend to be parthenogenetic whereas taxa whose life cycle is often interrupted by flooding, such as marine oribatid mites, or by desiccation, e.g., freshwater-living Ameronothridae n. gen. (nr. Aquanothrus) (Ameronothroidea) species, are mainly sexual, indicating that continuous access to resources indeed favours parthenogenetic reproduction. Findings of our study therefore suggest that parthenogenetic reproduction is not selected for by disturbances but by unlimited access to resources.