Identification of conservation units in the European Mergus merganser based on nuclear and mitochondrial DNA markers

The conservation status of small breeding areas of the Goosander (Mergus merganser merganser) in Central Europe is unclear. Geographic isolation of these areas suggests restricted gene flow to and from large North-European populations. On the other hand, migrating Goosanders from northern Europe join the Central European breeding population for wintering. To evaluate the conservation status of the small breeding areas we assessed the genetic structure of M. merganser populations in Europe by examining two nuclear marker systems (microsatellites and Single Nucleotide Polymorphisms, SNP) and mitochondrial (mtDNA) control region sequence variation for Goosanders in 11 sampling areas representing three of five distinct breeding areas and two subspecies (M. m. merganser and M. m. americanus). Overall population differentiation estimates including both subspecies were high, both based on mtDNA (Phi(ST) = 0.899; P < 0.0001) and nuclear markers (theta(ST) = 0.219; 95% CI 0.088-0.398, SNP and microsatellites combined). Within Europe, mtDNA revealed a strong overall (Phi(ST) = 0.426; P < 0.0001) and significant pairwise population differentiation between almost all comparisons. In contrast, both nuclear marker systems combined revealed only a small overall genetic differentiation (theta(ST) = 0.022; 95% CI 0.003-0.041). The strong genetic differentiation based on female-inherited mtDNA but not on biparentally inherited nuclear markers can be explained by sex-biased dispersal and strong female philopatry. Therefore, small breeding areas in Europe are endangered despite large male-mediated gene-flow, because when these populations decline, only males-but due to strong philopatry not females-can be efficiently supplemented by migration from the large North European populations. We therefore propose to manage the small breeding areas independently and to strengthen conservation efforts for this species in Central Europe.