Genotypic and phenotypic differences between Baltic and North Sea populations of Mytilus edulis evaluated through reciprocal transplantations. II Genetic variation

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Abstract
There are large differences in allozyme frequencies between Baltic and North Sea populations of Mytilus edulis. These differences have recently been ascribed to the presence of 2 species, viz. M. edulis L. in the North Sea and M. trossulus Gould in the Baltic. Our study supports the earlier findings of an extensive differentiation between North Sea and Baltic mussels. However, in reciprocal transplantations of mussels between a North Sea (20 to 30.permill.S) and a Baltic (6 to 7.permill.S) site, 96% of the North Sea mussels transferred to the Baltic site died immediately. Baltic mussels transferred to the North Sea site survived the first year but 99% suddenly died after 16 mo. The deaths altered the allele frequencies of the transplanted populations drastically at the 2 loci Pgm (phosphoglucomutase) and Pqi (phosphoglucose isomerase). The surviving Pgi genotypes were very similar to those of mussels native to each site, and this was true also for surviving Pgm genotypes of Baltic mussels transplanted to the North Sea. The genotype distribution of the surviving mussels also suggests that Pgi and Pgm are in linkage-disequilibrium. Allele frequencies at the Ap (aminopeptidase) locus did not differ between Baltic and North Sea populations, possibly as a result of a gene flow between Baltic and North Sea mussels. Rare alleles of ''wrong'' types being present in both North Sea and Baltic populations at the loci Lap (leucine aminopeptidase, also called Lap-2), Pgm and Pgi supports the lack of a reproductive barrier. This study shows that substantial genetic differentiation may be maintained by selection, and we suggest that Baltic and North Sea mussels might well be of the same evolutionary lineage.