Phylogeography of Mazzaella species across the Cape Blanco to Cape Mendocino transition zone

The North East Pacific coastline is a diversity hotspot for many marine organisms, including seaweeds, which live in complex habitats influenced by tectonics, glaciation, and ocean circulation. This is the first study to thoroughly survey population genetic structure of seaweed encompassing Cape Blanco, OR and Cape Mendocino, CA. Two genetic markers (mtDNA: COI; rDNA: ITS-1) were used to determine if biogeographic and phylogeographic structure are present in the high intertidal red alga, Mazzaella parksii, along 800 kilometers of coastline. Haplotype networks revealed high diversity with a total of 20 haplotypes for both ITS-1 and COI. There was significant genetic differentiation at the individual, population and regional levels for ITS-1 (FST 0.45) and at the individual and population levels for COI (FST 0.43). All analyses demonstrated a deep biogeographic break at Cape Mendocino (40° N), establishing one haplogroup south of the cape. Increased haplotypic diversity north of Cape Mendocino is suggestive of substantial phylogeographic structure occurring where biogeographic boundaries do not. Mechanisms that may be driving this structure in M. parksii include vertical and discontinuous habitat, limited dispersal, as well as high irradiance, and desiccation. High haplotypic diversity and numerous species range limits that have been observed here, indicate that the two capes are part of an important transition zone for subtropical and temperate cold marine organisms. These results may be useful when considering future development of conservation strategies for the maintenance of genetic diversity in various marine species.