Reproductive strategies and population genetic structure of Fucus spp . across a northeast Atlantic biogeographic transition

Geographical discontinuities in the composition of species assemblages are often mirrored by genetic clines and local adaptation in species that extend across such biogeographic disjunction. Species distributed across such biogeographic discontinuities are thus more prone to develop genetic clines and adaptations to environmental variation. The northwest coast of France encompasses a biogeographic boundary between cold- and warm-temperate marine ecosystems. Distinct hydrographic features characterize northern and southern shores, promoting physical oceanographic breaks that can affect dispersal and connectivity between populations. Furthermore, mosaics of contrasting conditions may affect local patterns of species assemblages and potential adaptive pressures. Along this stretch of coast, the intertidal habitat is largely dominated by brown algae of the genus Fucus, key structuring species common along North Atlantic shorelines. Fucus spiralis, F. guiryi and F. vesiculosus are sister species with different mating systems. They have extensive sympatric geographical distributions and distinct but overlapping vertical distributions in the intertidal zone. We examined genetic structure and diversity across this major environmental transition zone using neutral microsatellite markers to investigate patterns of differentiation between populations of each species inhabiting the two different biogeographic regions. We found contrasting results between the dioecious species (F. vesiculosus) and the two selfing hermaphroditic lineages (F. spiralis and F. guiryi). Genetic differentiation of northern and southern populations of F. vesiculosus followed the expectation from a hypothetical oceanographic discontinuity across this biogeographical transition zone, but this was not the case for the two hermaphroditic species. The former had higher genetic diversity and structure along the distribution range investigated whereas selfing hermaphrodites had less variability and were not geographically structured. Our results suggest that genetic patterns are correlated with this biogeographical transition zone but this effect is dependent on mating system, a determining factor affecting population structure. Additionally, other factors such as intertidal elevation may also play a role in observed difference in genetic structuring.