Marine landscape shapes hybrid zone in a broadcast spawning bivalve: introgression and genetic structure in Canadian west coast Mytilus

Adult marine mussels are sessile, but their highly dispersive planktonic larval stage plays a critical role in shaping population structure. However, shoreline geography and oceanographic currents can modify the dispersal pattern of pelagic larvae. On Vancouver Island (VI), British Columbia, 3 species of blue mussels (native Mytilus trossulus and introduced M galloprovincialis and M edulis) form a localized hybrid zone. Here we genetically mapped the distribution of Mytilus species and populations along VI and the surrounding islands. Using diagnostic species markers and microsatellite loci, we estimated the extent of the Mytilus hybrid zone on VI and measured population differentiation among the observed sites in 2005 and 2006. We predicted that the distribution of non-native genotypes would be mirrored by the microsatellite allelic patterns, which correspond to oceanographic features that reflect barriers to gene flow in the Strait of Georgia. Generally, non-native genotypes were restricted to southern VI and strong microsatellite population structure was detected. The distribution of non-native genotypes reflected patterns of microsatellite allele frequency in the Strait of Georgia. Using a landscape genetics approach, we identified 2 genetic discontinuities, which correspond to oceanographic and hydrographic features of the Strait of Georgia. Thus, physical dispersal barriers likely limit the spread of the VI Mytilus hybrid zone; however, additional biological barriers to dispersal must also exist. The VI Mytilus hybrid zone provides an excellent example of complex dispersal patterns in a non-equilibrium system