Evolution of habitat transitions in aquatic Coleoptera

Understanding the origin and evolutionary consequences of novel traits are among the main questions in evolutionary biology. Using an integrated approach, we will combine physiological and ecological data with species-level molecular phylogenies to understand the evolution of two of the most common habitat transitions in aquatic macroinvertebrates: the shift between running and standing waters, and the development of tolerance to salinity. In Subproject 1 we will test the macroevolutionary consequences of these habitat transitions in both diversification rates and the size of the geographical ranges; while in Subproject 2 we will determine the origin and mechanism of salinity tolerance, its possible interaction with tolerance to aridity, and the trade-offs with other biological traits.
Under these general objectives we will test more specific hypotheses: 1) as a consequence of the different long-term stability of their habitats, lotic species should have a higher evolutionary turnover and narrower geographical ranges than lentic species; 2) if tolerance to high salinities is an extreme ecological specialisation, it should lead to diminished speciation and increased extinction rates, i.e. it should be an evolutionary dead-end, irreversible and evolutionary young; 3) tolerance to salinity maybe a by-product of tolerance to aridity, in which case all saline species should also be tolerant to desiccation, but there maybe species living in freshwater habitats in arid environments that can also tolerate high salinities, even when they are never found in them; 4) the mechanism of both tolerance to desiccation and salinity maybe related to a lower permeability of the cuticle, which may lead to different tolerances in the adult and the larvae; and 5) if tolerance to salinity has a high metabolic cost it should result in trade-offs with other biological traits, such as e.g. fecundity.
To test these hypotheses we will use as a model system aquatic Coleoptera of the genera Ochthebius (Hydraenidae), with 350 species and a mostly Holarctic distribution; Hydroporus (Dytiscidae), with 185 species and a Holarctic distribution; “Hygrotini sensu lato” (Dytiscidae), with 354 species and a Holarctic and Ethiopian distribution; and Enochrus (Lumetus) (Hydrophilidae), with 23 species and a mostly Mediterranean distribution. All have species exclusive of lotic or lentic water, and most, species living in saline or hypersaline environments. We will build comprehensive species-level molecular phylogenies using a combination of mitochondrial and nuclear markers including ca. 60% of the species of the respective groups, with a higher proportion in the lineages with saline species. We will estimate the speciation and extinction rates associated to each of the habitat types, as well as the rate and direction of change both of habitat preferences and size of the geographical range. We will combine these phylogenies with data on the tolerance to salinity, desiccation and high temperatures obtained experimentally, to determine the mechanisms by which saline tolerance was acquired, its evolutionary origin, and its macroevolutionary consequences. We expect that the integration of physiological, ecological and phylogenetic data will allow us to understand the processes that shaped the extant diversity of aquatic insects, and contribute to the preservation of the fragile and often neglected saline environments of the Mediterranean area.

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