Marked phylogeographic differentiation of sei whale based on mitochondrial DNA analyses from Northern and Southern Hemisphere populations
Maria Jose Perez-Alvarez, Francisca Rodriguez, Sebastian Kraft, Carlos Olavarria, C. Scott Baker, Debbie Steel, Naoka Funahashi, Verena Haussermann, Mauricio Ulloa, Camilo Naretto and Elie Poulin
The sei whale, Balaenoptera borealis, presents a disjointed geographic distribution, with populations that are separated either by continental landmass, as in the Northern Hemisphere, or by the Intertropical Convergence Zone between hemispheres. Such distribution, together with patterns of seasonal migration, could result in strong phylogeographic structure. Significant genetic divergence between sei whale from the North Atlantic and North Pacific Oceans has been reported, however, limited samples from the Southern Hemisphere precluded a global analysis. The largest whale mass mortality event ever recorded for this species occurred in southern Chile, with at least 340 dead whales. This has become the largest source of samples for the species in the SH. Here we evaluate the population structure between sei whale populations, comparing the North Pacific n = 27, the North Atlantic n = 86 and the Southern Hemisphere (n = 91, including 79 bones from the Chile mass mortality, n = 16 reported previously as â€œSouthern hemisphereâ€ and n= 2 reported previously for South Atlantic), and the South Atlantic (n = 2). At a local level, mitochondrial DNA control region analyses from South Pacific recovered 32 haplotypes, eighteen of which are shared by two or more individuals. High values of haplotype diversity (h = 0.97) and nucleotide diversity (Ï€ = 0,95%) were found. At a global scale, phylogeographic analyses showed a strong genetic differentiation between the Southern Hemisphere and both North Atlantic (ï†ST = 0.69, p = 0.001) and North Pacific (ï†ST = 0.32 p = 0.001), and a possible migration event from South Pacific to North Atlantic. Together with other recent studies, our results point to a marked phylogeographic differentiation among sei whale in the North Pacific, North Atlantic and Southern Hemisphere, which reflect the existence of three major population units. These results need to be complemented by more samples from other locations within the Southern Hemisphere and the use of nuclear markers.