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 o f brown trout (Salmo trutta) was studied in a small Danish river syste m. Samples of trout from seven locations were analysed by allozyme ele ctrophoresis and mitochondrial DNA restriction fragment length polymor phism analysis. For comparison, allozyme data from other Danish trout populations and mtDNA data from two hatchery strains were included. Ge netic differentiation among populations was found to be small but sign ificant. Pairwise tests for homogeneity of allele and haplotype freque ncies between samples showed that significance was predominantly due t o a single population. In two of the locations studied, extinction-rec olonization events had taken place. In one of the populations, foundin g had not resulted in divergence, while the other founded population w as the one that had diverged significantly from the remaining populati ons. Computer simulations of the influence of founder effects on mitoc hondrial DNA differentiation and variability showed that the observed divergence could be due either to natural founder effects or to a gene tic contribution by hatchery trout. However, the allozyme results poin ted towards natural founder effects as the most likely explanation.