Analysis of the effect of geographic isolation on the genetic isolation of belugas

Analysis of the effect of geographic isolation on the genetic isolation of belugas

1 April 2013

Scientists have conducted research on the influence of geographical isolation on the degree of genetic isolation of populations of beluga whales in the Russian Far East.

The beluga whale is a species commonly occurring in the Sea of Okhotsk and the Bering Sea (Клейненберг и др. 1964). But beluga whales are rarely seen off southwestern coast of the Kamchatka Peninsula and are almost never seen off the eastern coast of Kamchatka, which indicates that the Sea of Okhotsk beluga whale stock is geographically separated from the Bering Sea stock and the Arctic stock (Мельников 2001). However, the possibility of existence of separate stocks within the Sea of Okhotsk has been disputed. Analysis of allelic distributions of microsatellite loci has shown that beluga whales that form the Sakhalin-Amur summer aggregation and the Shantar Islands summer aggregation belong to the same population (Языкова и др., this book). Of even greater interest is the issue whether beluga whales that aggregate in the above-mentioned sites are isolated from the stock summering off Kamchatka's west coast. Although the Sakhalin-Amur summer aggregation is separated from the Kamchatka's west coast summer aggregation by a distance of over 1000 km, the fact that Sakhalin-Amur stock spends the winter in the northern part of the Sea of Okhotsk (as was indicated by satellite tracking data – Шпак и др. 2010) leaves open a possibility of the encounters and mating between the Sakhalin-Amur stock and Kamchatka's west coast stock during the winter and spring months.


To assess the degree of genetic isolation both between different groups of the Sea of Okhotsk population and between the Sea of Okhotsk and the Bering Sea populations of beluga whales, we estimated allele frequencies at eight nuclear DNA microsatellite loci.


Samples were collected in the Sakhalin Gulf, on the Kamchatka's west coast, and in the Anadyr Estuary. Most of the samples were collected by the method of remote biopsy sampling. Some samples were collected from dead animals. Uniqueness of biopsy samples was identified by genotyping. Gender verification was performed by PCR, using the method described in (Rosel 2003).


Beluga whales from the Anadyr Estuary population had a greater number of alleles at most loci and somewhat higher mean diversity index values than beluga whales from the Sea of Okhotsk population. Comparison of different groups of the Sea of Okhotsk population showed a greater number of alleles in the Sakhalin stock, but mean heterozygosity values of both Sea of Okhotsk stocks were almost the same. The highly significant differences in allele frequency between three sample sets tested showed that three stocks belong to different populations (or reproductive groups).


Moreover, the genetic differences between the Sakhalin stock and the Kamchatka's west coast stock were even somewhat greater than the genetic differences between the Anadyr Estuary stock and the Kamchatka's west coast stock.


Using a clustering method, we confirmed the existence of obvious heterogeneity of the entire universal set: there was a higher probability that the samples being analysed were divided into three groups rather than two. The heterogeneity directly correlated with the sample collection locations. All the three sets of samples (samples from the Anadyr Estuary, Kamchatka's west coast, and Sakhalin Gulf) had a similarly high probability of belonging to their own stock. Thus, despite the absence of obvious geographical obstacles between two stocks inhabiting the same Sea of Okhotsk and despite the possible nearness of their wintering grounds and migration routes, these two stocks have a level of genetic isolation that is as high as the level of genetic isolation between the geographically separated the Sea of Okhotsk stock and the Bering Sea stock.


It is of importance that separate analyses for males and females still showed statistically significant differences between the three areas tested. Although the level of difference between the Kamchatka's west coast beluga males and Sakhalin beluga males was two times higher than the level of difference between the Kamchatka and Sakhalin females. Moreover, there was a significant difference in allele frequencies between males and females of the Kamchatka's west coast stock, while there were no genetic differences between males and females of the Sakhalin Gulf stock or between males and females of the Anadyr Estuary stock.


The clustering method did not reveal any genetic isolation between females of the Kamchatka's west coast stock and females of the Sakhalin Gulf stock (although we have already mentioned that there were statistically significant differences in the Fst values) – two groups (the Anadyr and the Sea of Okhotsk ones) had the mini-mum value of the log probability. However, the analysis of males by the clustering method showed isolation of all the three groups. To understand the possible causes of the isolation, further research and large sample sizes are required.


The research was done under the Beluga White Whale Program conducted by the Severtsov Institute of Ecology and Evolution of the Russian Academy of Sciences, with the support of the Russian Geographical Society. The program is part of the Permanent expedition of Russian Academy of Science studying Red Book animals and other focus species of Russia.


Borisova Ekaterina (1), Meschersky Ilya (2), Shpak Olga (2), Glazov Dmitry (2), Litovka Denis (3), Rozhnov Vyacheslav (2)


1. Lomonosov Moscow State University, Moscow, Russia


2. A.N. Severtsov Institute of Ecology and Evolution, RAS, Moscow, Russia


3. Chukotka branch of Pacific Research Fisheries Center, Anadyr, Russia



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