Spatial inference of ancestor locations suggests northern refugia for canopy-forming kelps in the Pacific Northwest

Pockets of the formerly glaciated Pacific coastline of North America likely remained ice-free throughout the Last Glacial Maximum (LGM). These areas may have served as refugia for terrestrial species, but less is known about their role in the persistence of marine plants and other coastal species. We examined genetic diversity from > 1000 newly and previously sequenced whole genomes of canopy-forming kelps of the genera Nereocystis and Macrocystis and built simple ecological niche models. We then reconstructed ancestral recombination graphs and modeled the geographic locations of genetic ancestors through time. We detected high genetic diversity in both species in north-central British Columbia, in a region where suitable LGM habitat is plausible. Ancestor locations spatially converged backward in time toward this region, with multiple refugia inferred between northern Vancouver Island and southern Haida Gwaii. An expanded set of global samples for Macrocystis confirmed pre-LGM divergence with California but hinted at the possibility of subsequent gene flow. Nereocystis and Macrocystis survived glaciation in northern refugia. The northern persistence of these foundation species raises the possibility that biodiverse kelp forest ecosystems could have continuously occupied portions of the northern Pacific coastline since the LGM.