A multilocus allozyme discontinuity in the mussel Mytilus galloprovincialis: the interaction of ecological and life-history factors

Electrophoretically detectable genetic variability of the Mediterranean mussel Mytilus galloprovincialis Lmk. was examined at 15 allozyme loci in 21 populations ranging from Santander (northern Spain) to Livorno (northwestern Italy).A major genetic break between Almeria and Alicante (southeastern Spain), as evidenced by 11 of 13 polymorphic loci examined, delimits 2 groups of populations with a high internal homogeneity.Roughly 75% of the total genetic differentiation was attributable to the divergence between these 2 groups of populations that displayed a genetic distance between them (D = 0.03) in the range of conspecific populations.This genetic break in M. galloprovincialis contrasts with earlier reports of genetic homogeneity among conspecific populations of the genus Mytilus over vast geographical distances, and represents an uncommon result in manne organisms with larval dispersal.The Zone of genetic divergence in M. galloprovincialis corresponds to a discontinuity in the distribution of this mussel, and to tbe position of the well-defined Almeria-Oran oceanographic front, with a distributional boundary between Atlantic and Mediterranean communities.In this region, other manne species exhibit similar patterns of intraspecific divergence, suggesting the action of common biogeographic processes.It is proposed that contemporary influences on gene flow related to an ecological barrier, perhaps in combination with selective pressures associated to water mass differences, maintain the abrupt change in southeastern Spain.