The Highly Variable and Highly Mutable Mitochondrial DNA Molecule of the Deep Sea Scallop Placopecten magellanicus <B>The Highly Variable and Highly Mutable Mitochondrial DNA Molecule of the Deep Sea Scallop Placopecten magellanicus</B><P> Douglas I. Cook(1)</B><BR> (1)Marine Gene Probe Laboratory, Department of Biology, Dalhousie University, Halifax, N.S., Canada, B3H 4J1<B><P> Eleftherios Zouros(1,2)</B><BR> (2)Department of Biology, University of Crete and Institute of Marine Biology of Crete, 711 10 Iraklion, Crete, Greece<B><P> Abstract</B><P> Whereas in the vast majority of animals the mitochondrial DNA (mtDNA) molecule rarely exceeds 20 kb (kilobases) in length and is size-invariable among conspecific individuals, in several invertebrates, fish, amphibians and reptiles the length of this molecule can be much larger and may vary within and among individuals. An extreme case is presented by the deep-sea scallop<I> Placopecten magellanicus</I>, where the molecule is, on average, 36 kb and may vary from 31 to 42 kb. Most of this variation is due to arrays of tandemly repeated sequences whose copy number 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 very high rate. The resulting mtDNA size polymorphism may be instrumental 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.<B><P> Key Words:</B> mtDNA size variation, scallops, mtDNA evolution.<P> THE NAUTILUS, Supplement 2:85-90, 1994, Page 85<P>