Genetic relationships of Mytilus galloprovincialis Lmk. populations worldwide: evidence from nuclear -DNA markers

Allozyme surveys of genetic variation in Mytilus galloprovincialis Lmk. throughout the world have identified three groups within this species: a northeastern (NE) Atlantic group that also includes the M. galloprovincialis population of South Africa, a Mediterranean group that also includes the M. galloprovincialis populations from the eastern and the western coasts of the North Pacific, and an Australasian g roup. Hypotheses that have been proposed to account for the genetic differentiation patterns and disjunct, worldwide distribution of M. galloprovincialis include the recent introduction of this species into the Southern Hemisphere and the North Pacific through human agency, and an alternative hypothesis that each of the three groups is endemic. We used two nuclear-DNA markers (the polyphenolic adhesive protein gene Glu-5' and the first intron of the actin gene mac-1) to investigate in more depth the genetic relationships among M. galloprovincialis populations. Samples were taken between 1996 and 1999 from California, the NE Atlantic, the Mediterranean Sea, South Africa, Korea, Western Australia, Tasmania, and New Zealand. NE Atlantic M. edulis L. were used a s an outgroup. While all M. galloprovincialis samples were fixed, or nearly so, for the diagnostic G allele at locus Glu-5', correspondence analysis of mac-1 allele -frequency data highlighted the genetic distinctness of Australasian mussels relative to other M. galloprovincialis populations. The latter consisted of two differentiated groups (NE Atlantic and Mediterranean) as formerly reported at allozyme loci. Another sample, from Chile, was nearly identical to Mediterranean M. galloprovincialis. Nuclear-DNA data thus enforce the idea that M. galloprovincialis have probably been introduced from the Mediterranean to the North Pacific (and now Chile), and from the NE Atlantic to South Africa. We argue that Australasian mussels derive from a proto- M. galloprovincialis population introgressed by M. edulis-like genes, and should be considered as a regional subspecies of M. galloprovincialis