Mitochondrial DNA sequence variation in North Atlantic long-finned pilot whales, Globicephala melas

Abstract

I sequenced mitochondrial DNA (mtDNA) from 59 long-finned pilot whales (Globicephala melas) that stranded on the coasts of Cape Cod, Newfoundland, Nova Scotia, Scotland, and England or were caught by commercial fisheries operating in the western North Atlantic, to determine if there is more than one genetic stock in the North Atlantic. Samples from 11 Atlantic and 2 Pacific short-finned pilot whales (G. macrorhynchus) and 11 bottlenose dolphins (Tursiops truncatus) were also analyzed. Sequences were obtained from 400-bp of the D-loop, a non-coding region involved in replication, and from 303-bp of the protein gene coding for cytochrome b. The D-loop sequences determined from 55 of the long-finned pilot whales were completely identical. Only the 2 sequences from Canadian whales showed some variability, differing from the other sequence by 0.25 - 0.50% (pairwise sequence divergence). All of the Atlantic short-finned pilot whales had identical D-loop sequences, and this sequence differed from the long-finned pilot whale sequences by 3.25 - 3.75%. The two Pacific short-finned pilot whale sequences differed from each other by 0.25%, from the Atlantic short-finned pilot whale sequence by 0.25 - 0.50%, and from the long-finned pilot whale sequences by 2.75 - 3.50%. D-loop nucleotide diversity in long-finned pilot whales was 0.03% and in short-finned pilot whales was 0.05%. The cytochrome b gene sequences determined for 16 long-finned pilot whales from all sampled locations, 4 Atlantic short-finned pilot whales, and two Pacific short finned pilot whales were all identical within each group, and differed from each other by 0.33 - 0.99%. Finally, D-loop sequences were also determined from 11 bottlenosed dolphins. All of the individuals had distinct D-loop sequences that differed by 0.25 - 4.25%, and the nucleotide diversity was 1.25%. Two dolphins were caught together, and the sequence divergence within this pair was 3.50%. These results suggest that long-finned pilot whales from the eastern and western North Atlantic are not genetically isolated from each other and that mtDNA variability in pilot whales may be unusually low. This might be a result of a slow rate of sequence evolution or metapopulation dynamics resulting from the social system of pilot whales. To examine the effect of social structure, I used an individual-based model designed to study the effect of sub-population extinction on mitochondrial genetic diversity in a pilot whale population which is subdivided as a result of this species' social system. MtDNA diversity was monitored in a population of pilot whales when extinction rates, mutation rates, and pod dynamics were altered. The results of the simulations indicate that if a pilot whale population experienced a moderate level of pod extinctions, it could undergo large fluctuations of mtDNA heterozygosity over time and frequently have the low heterozygosity observed in the North Atlantic pilot whale population.