THE NUCLEOTIDE-SEQUENCE OF THE MITOCHONDRIAL-DNA MOLECULE OF THE GRAYSEAL, HALICHOERUS-GRYPUS, AND A COMPARISON WITH MITOCHONDRIAL SEQUENCES OF OTHER TRUE SEALS

The sequence of the mtDNA of the grey seal, Halichoerus grypus, was de termined. The length of the molecule was 16,797 base pairs. The organi zation of the molecule conformed with that of other eutherian mammals but the control region was unusually long due to the presence of two t ypes of repeated motifs. The grey seal and the previously reported har bor seal, Phoca vitulina, belong to different but closely related gene ra of family Phocidae, true (or earless) seals. In order to determine the degree of differences that may occur between mtDNAs of closely rel ated mammalian genera, the 2 rRNA genes, the 13 peptide coding genes, and the 22 tRNA genes of the 2 species were compared. Total nucleotide difference in the peptide coding genes was 2.0-6.1%. The range of con servative difference was 0.0-1.5%. In the inferred peptide sequences t he amino acid difference was 0.0-4.5%, and the difference with respect to chemical properties of amino acids was 0.0-3.0%. A gene that showe d a limited degree of difference in one mode of comparison did not nec essarily show a corresponding limited difference in another mode. The ratio for differences in codon positions 1, 2, and 3 was almost-equal- to 2.7:1:16. The corresponding ratio for conservative differences was almost-equal-to 1.8:1.1:1. The evolutionary separation of the two spec ies was calculated to have taken place 2-2.5 million years ago. This d ating gives the figure almost-equal-to 8 X 10(-9) as the mean rate of substitution per site and year in the entire mtDNA molecule. Compariso n with the cytochrome b gene of the Hawaiian monk seal and the Weddell seal suggested that the lineage of these two species and that of the grey and harbor seals separated almost-equal-to 8 million years ago.