Founder effects and genetic population structure of brown trout (Salmo trutta) in a Danish river system

The influence of founder effects on the genetic population structure of brown trout (Salmo trutta) was studied in a small Danish river system. Samples of trout from seven locations were analysed by allozyme electrophoresis and mitochondrial DNA restriction fragment length polymorphism analysis. For comparison, allozyme data from other Danish trout populations and mtDNA data from two hatchery strains were included. Genetic differentiation among populations was found to be small but significant. Pairwise tests for homogeneity of allele and haplotype frequencies between samples showed that significance was predominantly due to a single population. In two of the locations studied, extinction-recolonization events had taken place. In one of the populations, founding had not resulted in divergence, while the other founded population was the one that had diverged significantly from the remaining populations. Computer simulations of the influence of founder effects on mitochondrial DNA differentiation and variability showed that the observed divergence could be due either to natural founder effects or to a genetic contribution by hatchery trout. However, the allozyme results pointed towards natural founder effects as the most likely explanation.