POPULATION-STRUCTURE OF NUCLEAR AND MITOCHONDRIAL-DNA VARIATION AMONGHUMPBACK WHALES IN THE NORTH PACIFIC

The population structure of variation in a nuclear actin intron and th e control region of mitochondrial DNA is described for humpback whales from eight regions in the North Pacific Ocean: central California, Ba ja Peninsula, nearshore Mexico (Bahia Banderas), offshore Mexico (Soco rro Island), southeastern Alaska, central Alaska (Prince Williams Soun d), Hawaii and Japan (Ogasawara Islands). Primary mtDNA haplotypes and intron alleles were identified using selected restriction fragment le ngth polymorphisms of target sequences amplified by the polymerase cha in reaction (PCR-RFLP). There was little evidence of heterogeneity in the frequencies of mtDNA haplotypes or actin intron alleles due to the year or sex composition of the sample. However, frequencies of four m tlDNA haplotypes showed marked regional differences in their distribut ions (Phi(ST) = 0.277; P < 0.001; n = 205 individuals) while the two a lleles showed significant, but less marked, regional differences (Phi( ST) = 0.033; P < 0.013; n = 400 chromosomes). An hierarchical analysis of variance in frequencies of haplotypes and alleles supported the gr ouping of six regions into a central and eastern stock with further pa rtitioning of variance among regions within stocks for haplotypes but not for alleles. Based on available genetic and demographic evidence, the southeastern Alaska and central California feeding grounds were se lected for additional analyses of nuclear differentiation using alleli c variation at four microsatellite loci. All four loci showed signific ant differences in allele frequencies (overall F-ST = 0.043; P < 0.001 ; average n = 139 chromosomes per locus), indicating at least partial reproductive isolation between the two regions as well as the segregat ion of mtDNA lineages. Although the two feeding grounds were not panmi ctic for nuclear or mitochondrial loci, estimates of long-term migrati on rates suggested that male-mediated gene flow was several-fold great er than female gene flow. These results include and extend the range a nd sample size of previously published work, providing additional evid ence for the significance of genetic management units within oceanic p opulations of humpback whales.