Past climate changes and strong oceanographic barriers structured low-latitude genetic relics for the golden kelp Laminaria ochroleuca

Aim Drivers of intraspecific biodiversity include past climate-driven range shifts and contemporary ecological conditions mediating connectivity, but these are rarely integrated in a common comprehensive approach. This is particularly relevant for marine organisms, as ocean currents strongly influence population isolation or connectivity, keeping or diluting the signatures left by past climates. Here we ask whether the coupling between past range shifts and contemporary connectivity explain the extant gene pools of Laminaria ochroleuca, a large brown alga structuring important marine forests from shallow to deep infralittoral grounds. Location Northeastern Atlantic Ocean. Taxon Laminaria ochroleuca. Methods We estimated population genetic diversity and structure of L. ochroleuca across its entire distribution range using 15 polymorphic microsatellite markers. This was compared with the outcomes of a palaeoclimatic model predicting latitudinal and depth range shifts from the Last Glacial Maximum (LGM) to the present. Genetic differentiation was further compared with potential connectivity inferred with a biophysical model developed with high-resolution data from HYCOM (Hybrid Coordinate Ocean Model). Results The biogeographical distribution of genetic variability showed overall agreement with the predictions from independently inferred past range shifts. Multiple regions of persistence were identified in deep and upwelling settings at the lowest latitudes of the current species distribution, where higher and unique genetic diversity was retained. The biophysical model revealed that despite the possibility of long-distance migration, contemporary oceanographic barriers strongly restrict connectivity of isolated genetic lineages. Main conclusions Integrating different processes at biogeographical scales explained the extant gene pools of marine forests of L. ochroleuca. Low-latitude genetic relics harbour a disproportional evolutionary significance, persisting as ancient populations in isolated deep and upwelling climate refugia. Their inferred rates of dispersal may be insufficient to accommodate anticipated climate warming.