Evidence of concurrentlocal adaptation and high phenotypic plasticityin a polar microeukaryote.

Here we investigated whether there is evidence of local adaptation in strains of an ancestrallymarine dinoflagellate to the lacustrine environment they now inhabit(optimal genotypes) and/orif they have evolved phenotypic plasticity (a range of phenotypes). Eleven strains of Polarella glacialis were isolated and cultured from three different environments: the polar seas, ahyposaline,and a hypersaline Antarctic lake. Local adaptation was tested by comparing growth rates of lacustrine and marine strains at their own and reciprocal site conditions. To determine phenotypic plasticity, we measured thereaction norm for salinity.We found evidence of both, limitedlocal adaptation andhigherphenotypic plasticity in lacustrine strains when compared tomarine ancestors. At extreme high salinities,local lake strains outperformed otherstrains, and at extreme low salinities, strains from the hyposaline lake outperformed all other strains. The data suggest that lake populations may have evolved higher phenotypic plasticity in the lake habitats compared to the sea, presumably due to the high temporal variability in salinity in the lacustrine systems. Moreover, the interval of salinity tolerance differed between strains from the hyposalineand hypersaline lakes, indicatinglocal adaptation promoted by different salinity.