Genetic differentiation and relationship between genetic and geographical distance in Danish sea trout (Salmo trutta L.) populations

Twenty-four samples of anadromous brown trout from four river systems/geogr aphical regions were studied by PCR-RFLP analysis of the ND-1 and ND-5/6 re gions of the mitochondrial genome. A total of 14 haplotypes was observed, a nd these could be divided into three phylogenetic groups. Populations withi n river systems/regions tended to be more closely related to each other tha n to populations from other river systems/regions. Also, a significant corr elation was observed between geographical and genetic distances between pop ulations. These results contrast with results from other studies of mainly resident and landlocked populations, where no correspondence was observed b etween genetic relationships and geographical location of populations. Gene flow connecting anadromous populations is probably the reason for the obse rved isolation-by-distance patterns, whereas in isolated resident and landl ocked trout populations drift leads to random genetic divergence of populat ions. Tests for nonrandom geographical distribution of phylogenetic groups of haplotypes showed that drift: and gene flow are probably the predominant factors affecting the distribution of haplotypes. There were, however, als o some indications of dines in frequencies of phylogenetic groups of haplot ypes.