Colonizing the Wild West: Low Diversity of Complete Mitochondrial Genomes in Western North Pacific Killer Whales Suggests a Founder Effect

Abstract

In the North Pacific, fish-eating R-type "resident" and mammal-eating T-type "transient" killer whales do not interbreed and differ in ecology and behavior. Full-length mitochondrial genomes (about 16.4 kbp) were sequenced and assembled for 12 R-type and 14 T-type killer whale samples from different areas of the western North Pacific. All R-type individuals had the same haplotype, previously described for R-type killer whales from both eastern and western North Pacific. However, haplotype diversity of R-type killer whales was much lower in the western North Pacific than in the Aleutian Islands and the eastern North Pacific. T-type whales had 3 different haplotypes, including one previously undescribed. Haplotype diversity of T-type killer whales in the Okhotsk Sea was also much lower than in the Aleutian Islands and the eastern North Pacific. The highest haplotype diversity for both R- and T-type killer whales was observed in the Aleutian Islands. We discuss how the environmental conditions during the last glacial period might have shaped the history of killer whale populations in the North Pacific. Our results suggest the recent colonization or re-colonization of the western North Pacific by small groups of killer whales originating from the central or eastern North Pacific, possibly due to favorable environmental changes after the Last Glacial Maximum.

Postglacial Colonization of Northern Coastal Habitat by Bottlenose Dolphins: A Marine Leading-Edge Expansion?

“…The demographic contraction and subsequent expansion observed in the coastal populations of this study, as well as the significantly reduced nucleotide and haplotype diversities, suggest that bottlenose dolphins indeed expanded their range northwards to coastal areas via founder events during leading-edge expansion. A similar founder event has recently been suggested to give rise to the present-day resident-type killer whale communities in the Russian Pacific (Filatova et al, 2018), and rapid demographic expansions after the LGM have been documented across other marine taxa (e.g. Klimova et al, 2014;Jenkins et al, 2018).…”

Genetic and cultural evidence suggests a refugium for killer whales off Japan during the Last Glacial Maximum

“…During periods of ice formation, killer whales may have responded by retreating east to the Aleutian Islands that remained ice-free, or south to Japan. Results of complete mitogenome sequencing in Morin et al (2015) and Filatova et al (2018) support the hypothesis of the central Aleutian refugium.…”

“…This unusually low haplotype diversity encouraged the further analysis of complete mitochondrial DNA (Morin et al, 2010, 2015), which revealed higher diversity in the Aleutian Islands than off the mainland. Filatova et al (2018) showed that all R‐type killer whales in the western North Pacific from the northern Kuril Islands to Karaginsky Gulf, a distance of 1,100 km, shared the same complete mtDNA haplotype, which is unusual for animal populations. Mitogenome diversity was slightly greater in the eastern North Pacific (3 complete mitogenome haplotypes), and the greatest diversity (11 complete mitogenome haplotypes) was observed in the central Aleutian Islands.…”

Killer whale genomes reveal a complex history of recurrent admixture and vicariance

“…Our “resident” samples originate from across the ecotype's North Pacific range, from Washington State, USA to the Sea of Okhotsk off Eastern Russia. Therefore, the variation segregating in the “resident” ecotype and driving PC2 in Figure b must predate the separation into the several “resident” subpopulations which have subsequently colonised much of the Pacific rim (Filatova et al, ). If this drift in allele frequencies shared among “residents” occurred postadmixture with the “transient” ecotype, it would increase genetic differentiation between the two currently sympatric North Pacific ecotypes.…”