THE HIGHLY VARIABLE AND HIGHLY MUTABLE MITOCHONDRIAL-DNA MOLECULE OF THE DEEP-SEA SCALLOP PLACOPECTEN-MAGELLANICUS

Whereas in the vast majority of animals the mitochondrial DNA (mtDNA) molecule rarely exceeds 20 kb (kilobases) in length and is size-invari able among conspecific individuals, in several invertebrates, fish, am phibians and reptiles the length of this molecule can be much larger a nd may vary within and among individuals. An extreme case is presented by the deep-sea scallop Placopecten magellanicus, where the molecule is, on average, 36 kb and may vary from 31 to 42 kb. Most of this vari ation is due to arrays of tandemly repeated sequences whose copy numbe r varies among molecules. Such arrays (each consisting of a different repeated sequence) occur in more than one region of the mtDNA molecule . Once present, these arrays allow for frequent errors to occur during DNA replication, with the result that new size variants appear at a v ery high rate. The resulting mtDNA size polymorphism may be instrument al for long-term evolutionary phenomena (such as the rearrangement of genes around the molecule), but, because of its rapid turn-over, does not provide useful information for taxonomic studies.