Irreversible habitat specialization does not constrain diversification in hypersaline water beetles

Specialization to extreme environments is often considered an evolutionary dead-end, leading to irreversible adaptations and reduced evolvability. There is, however, mixed evidence of this macroevolutionary pattern, and limited data from speciose lineages. Here, we tested the effect of habitat specialization to hypersaline waters in the diversification rates of aquatic beetles of the genus Ochthebius (Coleoptera, Hydraenidae), using a molecular phylogeny with more than 50% of the 546 recognized species, including representatives of all subgenus and species groups but one. Phylogenies were built combining mitochondrial and nuclear genes, with the addition of 42 mitochondrial genomes. Using Bayesian methods of character reconstruction, we show that hypersaline tolerance is an irreversible ecological specialization that arose multiple times independently. Two lineages of Ochthebius experienced a significant increase in diversification rates, one of them inhabiting hypersaline waters, but there was no overall correlation with habitat or any significant decrease in diversification rates despite the irreversibility of the hypersaline tolerance. Our study tested for the first time the impact of hypersaline habitat specialization on diversification rates, without support of being an evolutionary dead-end. On the contrary, multiple and ancient lineages fully adapted to these extreme osmotic conditions seem able to diversify over long evolutionary periods.