Ochthebiini: a mitogenome approach to tempo and mode of salinity tolerance evolution

Abstract

Evolutionary novelties may influence the diversification of new lineages, allowing species to colonize new habitats or to exploit new resources. One of the most remarkable traits in primarily freshwater organisms is the possibility to tolerate saline or hypersaline habitats. Recent work in some water beetle families (Hydrophilidae and Dytiscidae) has led to the hypothesis that tolerance to salinity is an exaptation to drought resistance. However, whereas in Hydrophilidae there are fast and direct transitions from freshwater to hypersaline water, as expected under an exaptation scenario, in the studied Dytiscidae all transitions were gradual, from freshwater to saline and then hypersaline waters. In this work we study the third of the main lineages of water beetles with species in saline habitats, the genus Ochthebius (Hydraenidae). We build a comprehensive phylogeny of Ochthebius, with approximately 50% of the ca. 550 known species and a combination of three mitochondrial and two nuclear genes. Despite the general good support of most clades relationships among basal clades was still unresolved, so we obtained mitochondrial genomes of ca. 20 species with a mitochondrial metagenomics protocol with the aim to anchor some of the unsupported nodes in the tree. We compiled ecological data on the habitat of all species to reconstruct the tempo and mode of tolerance to salinity using Bayesian methods.