Population genetic structure and historical demography of North Pacific right whales
Luis A. Pastene, Mioko Taguchi, Aimee Lang, Mutsuo Goto and Koji Matsuoka
International Whaling Commission
The number and distribution of North Pacific right whale populations is uncertain. Previous studies based on catch, stranding and sighting records suggested that two populations occur on either side of the North Pacific. In this study this hypothesis was tested by analyzing new and published mitochondrial DNA (mtDNA) control region sequences (399bp) of right whale individuals from the western (n=29) and eastern (n=23) North Pacific. A sub-sample from the western side (n=18) was examined with 13 microsatellite loci to investigate the level of nuclear DNA diversity. Striking mtDNA differences were found between western and eastern North Pacific right whales. The FST between western and eastern North Pacific right whales was high (0.0929) and statistically significant (P=0.0002). This result is consistent with the hypothesis that separate populations inhabit the eastern and western North Pacific. Levels of nucleotide and haplotype diversities were high, 0.0174/0.8916 and 0.0165/0.8538 in the western and eastern populations, respectively. For the microsatellite data, the average expected heterozygosity in the western population was estimated at 0.595. The observed multimodal mtDNA mismatch distribution rejected a model of historical sudden expansion in both populations. Furthermore, Bayesian skyline plots (BSP) generated from the mtDNA data suggested a similar historical trend of female effective population size (Nef) for the two populations, with a stable Nef over time followed by a recent sharp decline. The timing of the decline ranged between 25,000 and 60,000 years ago (considering different populations and two assumptions of mutation rates), which coincide with the last glaciation period in the Pleistocene. Rapid climate changes during this period could have affected the habitat and prey resources of the North Pacific right whales, resulting in the sharp decline in their abundance. No signal of recent recovery was observed in the BSP analysis; however, this could be due to a lack of resolution for contemporary population size as shown in other studies.