MAINTENANCE OF PARENTAL POPULATIONS BORDERING A BLUE MUSSEL HYBRID ZONE BY POST-SETTLEMENT SELECTION

A common question in studies of hybrid zones regards how nearby parental populations can remain distinct in the face of gene flow from the other species and hybrid populations. Environmentally dependent hybrid-zone models often invoke selection favouring each of the parental species in differ-ent habitats where immigrants will often fail to survive and reproduce. The present study tests whether a relatively pure population of Mytilus galloprovincialis, located on the edge of a hybrid zone with M. edulis, is being maintained by selection against hybrid immigrants. Comparisons of newly settled spat with members of the same cohort approximately 9 months later consistently show a reduction in M. edulis-specific alleles to levels observed in adult age classes at two diagnostic loci. These data provide evidence for selection against spat from the hybrid zone that settled at sites within the M. galloprovincialis population within the first year of life. The observed selection would result in removal of most M. edulis-specific alleles prior to reproduction and keep the M. galloprovincialis population distinct