08 June 2010

Orca: maybe more than one species

In recent years differences in foraging techniques, prey preferences, behaviour and minor physical features within different killer whale populations drove researchers to question the existence of only a single species. A new study provided genetic evidence supporting the idea that there may be several kinds of Orca.

Researchers mapped the genome of 139 tissue samples from different areas and they found clear differences. Three types of killer whales may be different species. Several other types may also represent separate species or subspecies, but additional analyses are required.

Should species diversity be confirmed by future studies, we hope the animals will be given common names less stupid than 'killer whale'.

Silvia Bonizzoni

Photo: a killer whale spy hopping, by www.russianorca.com

For more information:

Morin P.A., Archer F.I., Foote A.D., Vilstrup J., Allen E.E., Wade P., Durban J., Parsons K., Pitman R., Li L., Bouffard P., Abel Nielsen S.C., Rasmussen M., Willerslev E., Gilbert M.T.P., Harkins T. 2010. Complete mitochondrial genome phylogeographic analysis of killer whales (Orcinus orca) indicates multiple species. Genome Res DOI: 10.1101/gr.102954.109

Abstract -- Killer whales (Orcinus orca) currently comprise a single, cosmopolitan species with a diverse diet. However, studies over the last 30 years have revealed populations of sympatric "ecotypes" with discrete prey preferences, morphology and behaviors. Although these ecotypes avoid social interactions and are not known to interbreed, genetic studies to date have found extremely low levels of diversity in the mitochondrial control region, and few clear phylogeographic patterns worldwide. This low level of diversity is likely due to low mitochondrial mutation rates that are common to cetaceans. Using killer whales as a case study, we have developed a method to readily sequence, assemble, and analyze complete mitochondrial genomes from large numbers of samples to more accurately assess phylogeography and estimate divergence times. This represents an important tool for wildlife management, not only for killer whales but for many marine taxa. We used high-throughput sequencing to survey whole mitochondrial genome variation of 139 samples from the North Pacific, North Atlantic and southern oceans. Phylogenetic analysis indicated that each of the known ecotypes represents a strongly supported clade with divergence times ranging from approximately 150,000 to 700,000 years ago. We recommend that three named ecotypes be elevated to full species, and that the remaining types be recognized as subspecies pending additional data. Establishing appropriate taxonomic designations will greatly aid in understanding the ecological impacts and conservation needs of these important marine predators. We predict that phylogeographic mitogenomics will become an important tool for improved statistical phylogeography and more precise estimates of divergence times.

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