Hybridization of Southern Hemisphere blue whale subspecies and a sympatric area off Antarctica: impacts of whaling or climate change? CATHERINE R. M. ATTARD,* LUCIANO B. BEHEREGARAY, † K. CURT S. JENNER, ‡ PETER C. GILL, §¶ MICHELINE-NICOLE JENNER, ‡ MARGARET G. MORRICE, §¶ KELLY M. ROBERTSON** and LUCIANA M. MO ¨ LLER † *Department of Biological Sciences, Macquarie University, Sydney, NSW 2109, Australia, †School of Biological Sciences, Flinders University, GPO Box 2100, Adelaide, SA 5001, Australia, ‡Centre for Whale Research, PO Box 1622, Fremantle, WA 6959, Australia, §Blue Whale Study, C/- Post Office, Narrawong, VIC, 3285, Australia, ¶Deakin University, PO Box 423, Warrnambool, VIC, 3280, Australia, **Protected Resources Division, Southwest Fisheries Science Center, National Marine Fisheries Services, National Oceanic and Atmospheric Administration, 3333 N. Torrey Pines Ct, La Jolla, CA 92037, USA Abstract Understanding the degree of genetic exchange between subspecies and populations is vital for the appropriate management of endangered species. Blue whales (Balaenop- tera musculus) have two recognized Southern Hemisphere subspecies that show differ- ences in geographic distribution, morphology, vocalizations and genetics. During the austral summer feeding season, the Antarctic blue whale (B. m. intermedia) is found in polar waters and the pygmy blue whale (B. m. brevicauda) in temperate waters. Here, we genetically analyzed samples collected during the feeding season to report on several cases of hybridization between the two recognized blue whale Southern Hemisphere subspecies in a previously unconfirmed sympatric area off Antarctica. This means the pygmy blue whales using waters off Antarctica may migrate and then breed during the austral winter with the Antarctic subspecies. Alternatively, the subspecies may interbreed off Antarctica outside the expected austral winter breeding season. The genetically estimated recent migration rates from the pygmy to Antarctic subspecies were greater than estimates of evolutionary migration rates and previous estimates based on morphology of whaling catches. This discrepancy may be due to differences in the methods or an increase in the proportion of pygmy blue whales off Antarctica within the last four decades. Potential causes for the latter are whaling, anthropogenic climate change or a combination of these and may have led to hybrid- ization between the subspecies. Our findings challenge the current knowledge about the breeding behaviour of the world’s largest animal and provide key information that can be incorporated into management and conservation practices for this endangered species. Keywords: admixture, Balaenoptera musculus, biodiversity conservation, endangered species, microsatellite DNA, population genetics Received 21 February 2012; revision received 15 July 2012; accepted 25 July 2012 Introduction Subspecies are one of the human-defined phases in a continuum towards speciation that have been distin- guished by morphological, physiological, behavioural and genetic differences (Wilson & Brown 1953). Subspe- cies tend to be geographically isolated, but are pre- sumed to be capable of admixture when their distribution overlaps (Wilson & Brown 1953), which may lead to the formation of hybrid zones (Barton & Hewitt 1985). The dynamics of hybrid zones between Correspondence: Catherine Attard, Fax: +61 2 9850 8245; E-mail: catherine.r.attard@gmail.com © 2012 Blackwell Publishing Ltd Molecular Ecology (2012) 21, 5715–5727 doi: 10.1111/mec.12025