Cetaceans of the Black Sea: where did they survive glacial?

Abstract

Three species of cetaceans, Phocoena phocoena, Delphinus delphis and Tursiops truncatus ponticus are found in the Black Sea. The Black Sea populations of all three species show morpho-ecological peculiarities that leaded to their subspecific status: P. p. relicta (PPR), D. d. ponticus (DDP), and T. t. ponticus (TTP). It is not clear how long-lasting was their isolation from the core conspecific populations that ensured the development of adaptive features of PPR, DDP, and TTP. The analysis of mitochondrial haplotypes of PPR suggests that the split time of the at least maternal lineage of the Black Sea population of harbour porpoise lasted for over 100 ky (i.e. they should survive at least the latest glacial maximum within the Black Sea). However, the analysis of multiple microsatellite genotypes leaded some authors to suggest that the isolation is much less long, since middle Holocene. We re-analysed published mitochondrial sequences of all three Black Sea cetaceans along with several tens of sequences obtained from the stranded cetaceans. Our analyses suggest that Black Sea populations of all three cetacean species have an important input of populations that survived the last (and maybe earlier) glacial maxima within the Black Sea, most likely in its south-eastern fragment, which did not freeze in winter time even during the glacial peaks. This analysis is supported by both molecular clock approach and simple population modelling based on the assumption on the effective population size range. Different from the PPR, whose Black Sea population is currently fully isolated, there is a limited migration between the Black Sea and Atlantic populations of T. truncatus and D. delphis, through the Mediterranean "bridge" population. However, the migration rates are not sufficient to overweight differential selection between the Black Sea and Mediterranean populations, and the local morpho-ecological specifics is successfully maintained.