Genetic homogeneity in face of morphological heterogeneity in the harbor porpoises from the Black Sea and adjacent waters

Abstract

Isolated from North Atlantic populations, the Black Sea harbor porpoise (Phocoena phocoena relicta) is listed as Endangered due to the massive population decline triggered by historical hunting, and subsequently through fisheries bycatch, and other human activities. Of paramount importance for its conservation, is the characterization of the population structure. While morphological heterogeneity suggested population subdivision, previous genetic studies have failed to find any differences. Here, we investigated the population genetic structure of 144 harbor porpoises sampled opportunistically from across the entire subspecies range including the Aegean, Marmara, Black, and Azov Seas. Genetic variation of across one-fourth of the mitochondrial genome, in combination with the analysis of ten microsatellite loci revealed a nearly complete genetic homogeneity. While simulations show that this inability to reject panmixia does not stem from a lack of power (power to detect FST of 0.008). A genetic time-lag effect limiting our ability to detect population subdivision is also unlikely when effective population size is low, as is the case here. For now, genetic panmixia among porpoises of the Black Sea and adjacent waters cannot be rejected. Population subdivision may well exist, but conclusive evidence would require an improved sampling providing suitable contrasts (e.g., age, sex, season). Also, a genome scale study providing access to neutral and selected genetic variation may reveal cryptic differentiation indicative of ecologically subdivisions. As a precautionary approach, definition of management units should be based on evidence of population heterogeneity obtained from multidisciplinary approaches rather than just genetics.