Mitochondrial DNA sequence data reveal the origins of postglacial marine macroalgal flora in the Northwest Atlantic

Following the Last Glacial Maximum, the marine macroalgal flora in the Northwest (NW) Atlantic reportedly recolonized from Northeast (NE) Atlantic refugia. Genetic evidence for the few species tested, however, has indicated that some species survived glaciation in the NW Atlantic. Owing to the significant amount of data currently available, we sought to determine if COI-5P (5' end of the cytochrome c oxidase subunit I gene) could distinguish between populations surviving glaciation on both sides of the Atlantic versus postglacial recolonization. COI-5P results were consistent with published findings using other markers in Chondrus crispus, Mastocarpus stellatus, Palmaria palmata, and Saccharina latissima. Having success, we then analyzed molecular data for several species of red and brown macroalgae to date isolation times between NE and NW Atlantic populations and determine what percentage of species survived in NW Atlantic refugia. We generated and gathered genetic data from COI-5P in 1560 specimens representing 20 amphi-Atlantic species, and estimated isolation times between NE and NW populations using calibrated red and brown COI-5P clocks in IMa2. Of the species surveyed, 60% had isolation time estimates between NE and NW Atlantic populations predating the Last Glacial Maximum. Recent shared ancestry was inferred in the remaining cases. Our results indicate that local refugia and/or trans-Arctic migration from the Pacific are the source populations for the majority of the NW Atlantic macroalgal flora. By shedding light on the phylogeographic history of the North Atlantic, we can better understand the nature of postglacial recolonization and forecast future changes to the NW Atlantic and Canadian Arctic.