The northern pike Esox lucius L. is a freshwater fish exhibiting pronounced population subdivision and low genetic variability. However, there is limited knowledge on phylogeographical patterns within the species, and it is not known whether the low genetic variability reflects primarily current low effective population sizes or historical bottlenecks. We analysed six microsatellite loci in ten populations from Europe and North America. Genetic variation was low, with the average number of alleles within populations ranging from 2.3 to 4.0 per locus. Genetic differentiation among populations was high (overall theta(ST) = 0.51; overall rho(ST) = 0.50). Multidimensional scaling analysis of genetic distances between populations and spatial analysis of molecular variance suggested a single phylogeographical race within the sampled populations from northern Europe, whereas North American and southern European populations were highly distinct. A population from Ireland was monomorphic at all loci, presumably reflecting founder events associated with introduction of the species to the island in the sixteenth century. Bayesian analysis of demographic parameters showed differences in theta (a product of effective population size and mutation rate) among populations from large and small water bodies, but the relative differences in theta were smaller than expected, which could reflect population subdivision within the larger water bodies. Finally, the analyses showed drastic population declines on a time scale of several thousand years within European populations, which we ascribe to either glacial bottlenecks or postglacial founder events. (C) 2005 The Linnean Society of London.
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