Increased clonality, decreased allele diversity and high genetic structure in tetraploid sea anemone Aulactinia stella populations from North Pacific to Atlantic across the Arctic Ocean

Reproductive modes, migration, genetic diversity and local adaptation are important evolutionary traits that limit species' ranges. Understanding the interactions between evolutionary and ecological processes at species' edges provides clues to how species adapt to rapid environmental changes. In the Arctic Ocean, melting sea ice and increasing human activity are bridging coastal populations that were once separated by extreme polar conditions. Aulactinia stella is a keystone tetraploid circumpolar sea anemone species that is witnessing these changes. Here, we investigated how reproduction and genetic diversity structured in A. stella populations and between adults and juveniles sampled across the Arctic Ocean. Outside Kamchatka, we only found females or individuals without gonads. Genetic indices and changes in genotype frequencies confirmed that this species reproduces partially by parthenogenesis. Populations on the Atlantic side were highly clonal, while those on the Pacific side reproduced more sexually. Allelic diversity was twice as high in Kamchatka and Kuril populations, suggesting North Pacific coasts being the main last glacial refugia of A. stella. Patterns of genetic differentiation between site pairs revealed gene flow routes extending from Kamchatka to Atlantic populations. These routes aligned with ocean currents and reduced sea ice, and not with the past maritime exchanges between Atlantic and Pacific Oceans. Within each population, only a subset of juvenile genetic diversity, mostly of local origin, was represented among established adults, suggesting local adaptation or kin recognition mechanisms promoting endemism. Juveniles showed less genetic differentiation across the Arctic Ocean, possibly indicating that global changes are favoring juvenile dispersal.