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The mitochondrial genomes of seven species of scallops were examined for the presence of repeated sequences and within-species size variation. The mtDNA of one species, Argopecten irradians, shows a typical 16.2-kb invariant size and ap parently lacks repeated sequences. The mtDNA of another species, Placopecten mageilanicus, is known to be very large (>3 1 kb), to contain a tandemly repeated sequence, and to vary widely in size among individuals. The mtDNAs of five other species (Pecten maximus, Crassadoma gigantea, Aequipecten opercularis, Chlamys hastata, and Chlamys isfandica) represent intermediate cases: all are larger than typical animal mtDNA (>20 kb), contain repeated sequences, and exhibit abundant intraspecific size polymorphism. When the cloned repeated sequence of P. magellanicus was used as a probe, no sequence similarity was seen with fragments containing the repeated sequence of other species. The same results were obtained when those other fragments were used as probes, suggesting that the repeated sequences of the various scallop species do not share extensive sequence similarity. Our findings support the increasingly accepted view that the normally observed conservatism of animal mtDNA size and organization is not universal. Invertebrates and poikilotherm vertebrates may contain large-scale intraspecific size variation whose origin and fate is, most likely, determined by nonselective forces of repeatedsequence-mediated mutational or recombination mechanisms and by subsequent random drift in the resulting germ-cell lineages. Such size polymorphism can be so extensive that each individual may have a unique RFLP profile (mtDNA fingerprint), but the suspected high turnover rate of the number of repeated sequences may restrict the application of this polymorphism to population genetic studies.

Repeated Sequences and Large-Scale Size Variation of Mitochondrial DNA: A Common Feature among Scallops (Bivalvia: Pectinidae)