Hybridization between the blue mussels Mytilus galloprovincialis and M-trossulus along the Pacific coast of North America: evidence for limited introgression

Two species of blue mussel, Mytilus galloprovincialis and M. trossulus, co- occur and hybridize along the Pacific coast of North America. Using a set o f polymerase chain-reaction (PCR)-based genetic markers which diagnosticall y identify these species, we show that they are sympatric from the Cape Men docino region to the Monterey Peninsula in northern and central California, USA. Mussels with hybrid genotypes were detected in all populations sample d in the region of sympatry, and the frequency of hybrid genotypes in indiv idual hybrid populations ranged from 13 to 44%. Significant frequencies of first-generation backcross genotypes were detected in two individual hybrid zone populations (Berkeley and Monterey Marina) and in the hybrid zone as a whole, indicating that the potential exists for introgression between M. galloprovincialis and M. trossulus. Despite this potential, we found no evi dence of advanced introgression beyond first-generation backcrosses, sugges ting that gene flow between M. galloprovincialis and M. trossulus has been quite limited. The frequency of mussels with M. trossulus and hybrid genoty pes declined abruptly south of Monterey Peninsula, while the frequency of m ussels with M. galloprovincialis and hybrid genotypes declined precipitousl y north of Cape Mendocino. These abrupt genetic discontinuities indicate th at this blue mussel hybrid zone is presently positioned between two promine nt coastal features and there is little, if any, export of alleles from the hybrid zone into bordering parental populations.