The zone of sympatry and hybridization of Mytilus edulis and M-galloprovincialis, as described by intron length polymorphism at locus mac-1

Intron-size variation at the actin gene locus mac-1 was used to characteriz e mussel, Mytilus spp., populations in the similar to 2000-km wide zone of contact and hybridization ('hybrid zone') between M. edulis and M. gallopro vincialis in western Europe. Twenty-five samples were collected in 1995-99 in locations within the hybrid zone and from reference populations of each species. We used correspondence analysis on the matrix of allelic frequenci es to determine which alleles are characteristic of each species, and to ch aracterize samples along the genetic gradient between M. edulis and M. gall oprovincialis. In the hybrid zone, some samples exhibited mac-1 allele freq uencies that were typical of M. edulis; other samples were distributed alon g the M. edulis/M. galloprovincialis gradient and displayed variable levels of intergradation that were not correlated with geography. Some of the lat ter samples exhibited significant heterozygote deficiencies. The simple adm ixture hypothesis (Wahlund effect) could not be rejected for two-fifths of the samples. The hybrid zone thus appeared as a mosaic of populations which are either pure M. edulis. or hybrid between M. galloprovincialis and M. e dulis, or a mixture of the foregoing with Al. galloprovincialis individuals . These results were consistent with published allozyme data, suggesting th at they can be extended to the entire nuclear genome. M. edulis mac-1 allel es were present at moderate frequency in Atlantic M. galloprovincialis, and at significantly lower frequency in some Mediterranean samples. This patte rn was homogeneous over a broad geographical range within each basin. It wa s not evident that introgression of M. edulis into M. galloprovincialis pre sently occurs south or the zone of contact. We propose that the distinctnes s of the Atlantic M. galloprovincialis population results from past introgr ession by M. edulis alleles.