THE MYTILUS-EDULIS SPECIES COMPLEX IN SOUTHWEST ENGLAND - EFFECTS OF HYBRIDIZATION AND INTROGRESSION UPON INTERLOCUS ASSOCIATIONS AND MORPHOMETRIC VARIATION

Loglinear analysis of electrophoretic data from two hybrid Mytilus edu lis x galloprovincialis populations in southwest England revealed non- significant associations between genotypes at four allozyme loci, each of which is partially diagnostic for differences between the two taxa . Significant non-random genotypic associations within the context of the non-significant model involved all four assayed loci equally, cons istent with their occurrence in a relatively tight linkage group. Mult ivariate analyses were used to examine electrophoretic variation from the two hybrid populations, and morphometric variation in the hybrid p opulations and in four allopatric (two M. edulis and two M. galloprovi ncialis) populations from western Europe. While the number of hybrid m ussels is high at both sites (22% at Croyde, 53% at Whitsand) the two taxa have largely maintained the genetic differences which exist betwe en them in allopatry. However, morphological differences between the t axa have been eroded for mussels within the hybrid zone, whereas these differences are quite pronounced for mussels from allopatric populati ons. It is proposed that each taxon within the genus maintains its gen etic identity, despite high dispersal potential, widespread hybridizat ion, and high levels of introgression, as a result of adaptation to di fferent environments. The worldwide occurrence of all four Mytilus hyb rid zones at ecotones between recognized biogeographical provinces whi ch are characterized by differences in temperature and salinity is con sistent with such an interpretation.