Long-term isolation at a low effective population size greatly reduced genetic diversity in Gulf of California fin whales

Abstract

The Gulf of California, Mexico is home to many cetacean species, including a presumed resident population of fin whales, Balaenoptera physalus . Past studies reported very low levels of genetic diversity among Gulf of California fin whales and a significant level of genetic differentiation from con-specifics in the eastern North Pacific. The aim of the present study was to assess the degree and timing of the isolation of Gulf of California fin whales in a population genetic analysis of 18 nuclear microsatellite genotypes from 402 samples and 565 mitochondrial control region DNA sequences (including mitochondrial sequences retrieved from NCBI). The analyses revealed that the Gulf of California fin whale population was founded ~2.3 thousand years ago and has since remained at a low effective population size (~360) and isolated from the eastern North Pacific ( N e m between 0.89–1.4). The low effective population size and high degree of isolation implied that Gulf of California fin whales are vulnerable to the negative effects of genetic drift, human-caused mortality and habitat change.

Population Biology of Fin Whales: Applying Demographic and Evolutionary Approaches to Studying Populations

“…Weir's estimates also reflected the highly divergent nature of the Gulf of California samples, even to the neighbouring fin whales in the North Pacific (Weir's = 0.22; Table 1), which was in line with results from previous genetic assessments of this isolated population (Bérubé et al, 1998(Bérubé et al, , 2002Rivera-León et al, 2019). In comparison, the divergence between the North Atlantic and the Mediterranean Sea sample was much lower (Weir's 0.005, Table 1), but significantly different from zero.…”

“…The level of genetic divergence between the North Atlantic and Mediterranean Sea at mitochondrial ( ST = 0.12) and nuclear (θ = 0.005) loci was markedly and significantly lower compared to the estimated divergence between ocean basins, such as the Gulf of California and the North Pacific Ocean (coastal California) which were estimated at 0.19 ( , mtCR) and between 0.17 -0.21 ( , nuclear loci; (Archer et al, 2019; , 2002Rivera-León et al, 2019). Despite low levels of genetic divergence among North Atlantic fin whale samples, some genetic sub-structuring was detected between sampling areas in the western (i.e.…”

“…6A). In comparison, the divergence between the Gulf of California and the North Pacific was relatively more recent, dated by Rivera-León et al (2019) to approximately 2.3 kya (95% HPD: 0-62.5 kya).…”

“…A population structure analysis of North Pacific and North Atlantic fin whale samples found evidence of two to three population clusters in the eastern North Pacific (Hatch, 2004). Fin whales in the Gulf of California constitute a highly isolated population from the eastern North Pacific fin whales based on population genetic analyses of multi-locus microsatellite genotypes and mitochondrial control region DNA sequences (Bérubé et al, 2002;Rivera-León et al, 2019). Significant deviation from panmixia was also detected among fin whales from the North Atlantic and Mediterranean Sea (Bérubé et al, 1998;Palsbøll et al, 2004).…”

“…Tissue samples were collected from 15 different areas in the North Atlantic, the Strait of Gibraltar, and the Mediterranean Sea from 1982 to 2018 (Figure 1). Samples from the resident Gulf of California fin whale population (Bérubé et al, 2002;Gilmore, 1957;López et al, 2019;Rivera-León et al, 2019) were included to serve as an out-group. Most samples were collected as skin biopsies from free-ranging animals, using a crossbow and bolt with a biopsy tip (Palsbøll et al, 1991).…”