The mitochondrial DNA of most metazoan animals is highly conserved in size, averaging about 17 kilobase pairs (kbp). The mitochondrial DNA from the deep-sea scallop Placopecten magellanicus, in contrast, has been found to be approximately 34 kbp long. It is also highly variable in size from individual to individual and is unusual in the extent of its size variation. Mitochondrial DNAs from individuals collected at the same site differ by as much as 7 kbp. The size variation is due largely to differences in the number of copies of a tandemly repeated 1.2-kbp element.
There are significant genetic differences among populations of the blue mussel, Mytilus edulis, from six localities in St. Margaret's Bay, N.S., Canada, despite the presence of gene flow. The populations are differentiated into two groups, those at the head of the bay where ambient conditions fluctuate widely during the year, and those at the mouth of the bay where conditions (particularly salinity) fluctuate to a lesser degree. Three isoenzyme loci, i.e. leucine aminopeptidase 1, peptidase 2, and phosphoglucose mutase, show a clear shift in the predominant frequencies, from slow migrating alleles at the mouth of the bay to faster migrating alleles at the head of the bay. This shift is a microgeographic parallel of the pattern we observed in 1980 on a macrogeographic scale. A genetic comparison of these populations with those studied previously shows that allele "populations" cluster according to their environment and not according to geographical proximity. We conclude that differences in allele frequencies among localities index heterogeneity among environmental conditions, and that the sensitivity to environmental selective forces varies from locus to locus.
Genetic differences among Mytilus edulis populations from four localities in Atlantic Canada were examined using electrophoretic techniques. On the basis of allele frequency distributions at four loci (LAP-1, PEPTIDASE-2, PGI, PGM), the localities fell into two pairs, paralleling a similar pairing on the basis of values of specific environmental parameters and their variability. Gene flow between environments could be eliminated as an explanation of gene frequency similarity in one of the pairs. The large heterozygote deficiencies appear not to be satisfactorily explained by the Wahlund effect, inbreeding, or "silent alleles," without the added influence of environmental selection. We conclude that the allele frequency patterns reflect differences in the average environmental characteristics among localities.Key words: isoenzymes, blue mussels, genetic differences, environmental selection, Mytilus edulis
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