Population introgression and differentiation in the great cormorant Phalacrocorax carbo in Europe

Seven hypervariable microsatellite markers were used to genotype individuals from 21 European populations of great cormorant (Phalacrocorax carbo) with the aim of: (i) evaluating levels of pan-European population structuring; and (ii) determining the genetic provenance of birds present in newly formed colonies in southeast England. Significant population differentiation was detected overall, with populations of the 'North Atlantic' cormorant subspecies (P.c. carbo) showing greater levels of divergence than populations of the 'Eurasian' subspecies (P.c. sinensis). The populations from southeast England, which are of unknown racial origin but within the geographical distribution of the P.c. carbo subspecies, clustered with P.c. sinensis populations in phylogenetic topologies, highlighting the fact that P.c. sinensis individuals were present at these sites. Multivariate analyses (principal component analysis, principal coordinate analysis and discriminant function analysis) indicated that these inland colonies were not comprised solely of P.c. sinensis, but that both subspecies were living sympatrically and were probably hybridizing. The implications for cormorant management and conservation of P.c. sinensis breeding in the UK and of population introgression with P.c. carbo are considerable, and are discussed.