Geographic Patterns of Genetic Differentiation among Killer Whales in the Northern North Pacific

Abstract

The difficulties associated with detecting population boundaries have long constrained the conservation and management of highly mobile, wide-ranging marine species, such as killer whales (Orcinus orca). In this study, we use data from 26 nuclear microsatellite loci and mitochondrial DNA sequences (988bp) to test a priori hypotheses about population subdivisions generated from a decade of killer whale surveys across the northern North Pacific. A total of 462 remote skin biopsies were collected from wild killer whales primarily between 2001 and 2010 from the northern Gulf of Alaska to the Sea of Okhotsk, representing both the piscivorous "resident" and the mammal-eating "transient" (or Bigg's) killer whales. Divergence of the 2 ecotypes was supported by both mtDNA and microsatellites. Geographic patterns of genetic differentiation were supported by significant regions of genetic discontinuity, providing evidence of population structuring within both ecotypes and corroborating direct observations of restricted movements of individual whales. In the Aleutian Islands (Alaska), subpopulations, or groups with significantly different mtDNA and microsatellite allele frequencies, were largely delimited by major oceanographic boundaries for resident killer whales. Although Amchitka Pass represented a major subdivision for transient killer whales between the central and western Aleutian Islands, several smaller subpopulations were evident throughout the eastern Aleutians and Bering Sea. Support for seasonally sympatric transient subpopulations around Unimak Island suggests isolating mechanisms other than geographic distance within this highly mobile top predator.

Phylogenomics of the killer whale indicates ecotype divergence in sympatry

“…Signal for selection was investigated using the F ST outlier method implemented in LOSITAN (Antao et al, 2008). Mean neutral F ST was calculated using the infinite alleles model, and assuming nine demes of size 10, following the different a prioridefined populations (based on the results obtained in Hoelzel et al (2007) and Parsons et al (2013)): Marion Island, North Atlantic, North Pacific offshores, Alaskan residents, Southern residents, Alaska transients, California transients, Bering Sea and Russia. Although some sample sizes were small per putative population, this is more likely to artificially inflate F ST , generating false outliers (which would be conservative in this case).…”

Unexpected patterns of global population structure in melon-headed whales Peponocephala electra

“…Haplotypic diversity in most strata (Table 2) was also higher than is typical for other blackfish (e.g. Amos et al 1993, Oremus et al 2009, Parsons et al 2013, Martien et al 2014. Haplotypic diversity was similar to that observed in smaller delphinids such as spinner dolphins (Oremus et al 2007, Andrews et al 2010, pantropical spotted dolphins Stenella attenuata (Escorza-Trevino et al 2005), and common bottlenose dolphins (Tezanos-Pinto et al 2009, Martien et al 2012, species that also do not exhibit deep mitochondrial divergences.…”

“…To determine whether the magnitude of mtDNA differentiation that we observed among MHW strata was comparable to that for other blackfish, we compared our Φ ST estimates to published estimates for resident killer whales (Parsons et al 2013), false killer whales (Martien et al 2014), long-finned pilot whales (Oremus et al 2009), and short-finned pilot whales (Oremus et al 2009, Van Cise et al 2016 ( Table S2 in the Supplement). We grouped the Φ ST values according to whether they were from comparisons of strata within the same ocean basin or between ocean basins and used Mann-Whitney U-tests to determine whether values were significantly lower in MHWs than in other blackfish.…”

“…Oremus et al 2009, Parsons et al 2013, Martien et al 2014, Van Cise et al 2016). However, our data did not reveal any such pattern.…”

“…For example, the cetacean subfamily Globicephalinae, commonly referred to as 'blackfish,' is comprised of several species of large, dark-colored dolphins, including killer whales Orcinus orca, false killer whales Pseudorca crassidens, pygmy killer whales Feresa attenuata, long-finned pilot whales Globicephala melas, and short-finned pilot whales G. macrorhynchus. Each of these species exhibits strong social structure and deep divergences within the mitochondrial genome, both within and between ocean basins (Bigg et al 1987, Amos et al 1993, Baird et al 2008, Oremus et al 2009, Morin et al 2010a, Parsons et al 2013, Martien et al 2014.…”