COMPARISON BETWEEN THE COMPLETE MTDNA SEQUENCES OF THE BLUE AND THE FIN WHALE, 2 SPECIES THAT CAN HYBRIDIZE IN NATURE

The sequence of the mitochondrial DNA (mtDNA) molecule of the blue wha le (Balaenoptera musculus) was determined. The molecule is 16,402 bp l ong and its organization conforms with that of other eutherian mammals . The molecule was compared with the mtDNA of the congeneric fin whale (B. physalus). It was recently documented that the two species can hy bridize and that male offspring are infertile whereas female offspring may be fertile. The present comparison made it possible to determine the degree of mtDNA difference that occurs between two species that ar e not completely separated by hybridization incompatibility. The diffe rence between the complete mtDNA sequences was 7.4%. Lengths of peptid e coding genes were the same in both species. Except for a small porti on of the control region, disruption in alignment was usually limited to insertion/deletion of a single nucleotide. Nucleotide differences b etween peptide coding genes ranged from 7.1 to 10.5%, and difference a t the inferred amino acid level was 0.0-7.9%. In the rRNA genes the me an transition difference was 3.8%. This figure is similar in degree to the difference (3.4%) between the 12S rRNA gene of humans and the chi mpanzee. The mtDNA differences between the two whale species, involvin g both peptide coding and rRNA genes, suggest an evolutionary separati on of greater-than-or-equal-to 5 million years. Although hybridization between more distantly related mammalian species may not be excluded, it is probable that the blue and fin whales are nearly as different i n their mtDNA sequences as hybridizing mammal species may be.