Osteological, Biological, and Genetic Consequences of Colonization of Fresh Waters by Marine Cottoid Fishes (Cottidae)

Мarine cottoid fishes of the genus Triglopsis colonized large lakes of glacial genesis in Northern Europe and North America. The fourhorn sculpin Triglopsis quadricornis dispersed into post-glacial European lakes (e.g., Ladoga and Onega) from the Baltic Sea. The Great Lakes are inhabited by a freshwater species, Triglopsis thompsonii. Lacustrine forms of Triglopsis from North American and European lakes are at different stages of evolution, making them a suitable model for testing hypotheses about evolutionary patterns and rates. An integrative analysis of morphological, genetic, and biological adaptations occurring in lacustrine forms of the genus Triglopsis was carried out. The obtained results demonstrate the presence of rapid adaptive changes in morphological features associated with skeletal lightening. The degree of morphological transformation varies from lake to lake, thus gradual morphological changes are observed. The diet of lake forms is dominated by crustaceans, as in marine Triglopsis, but the objects they consume gradually decrease in size. In contrast to morphological differences, populations are characterized by weak genetic differentiation from each other due to recent colonization (10-15 kya). However, genetic evidence of the adaptive nature of speciation processes occurring in the genus Triglopsis, which are not characteristic of Myoxocephalinae in general, was found. Methods for the detection of natural selection revealed an increase in episodic positive selection occurring in the cytb gene in species of the genus Triglopsis (as well as Megalocottus) that successfully adapted to low water salinity. Thus, the studied species undergo rapid adaptive changes associated with the colonization of a new habitat.