Factors influencing the extent of inbreeding depression: an example from scots pine

Detailed studies suggest that the level of inbreeding depression may vary between populations. In a study of Scots pine from Finland, the level of inbreeding depression was much lower in northern than in southern populations. We have examined theoretically whether population genetic factors, such as the level of selfing, intensity of selection against heterozygotes or homozygotes, level of mutation, a bottleneck, finite population size, or the level of polyembryony could account for this difference. Higher selfing or stronger selection against heterozygotes in the north, both at biologically reasonable levels, appear to produce changes consistent with the observed differences and we consider these to be the most likely explanations. In addition, the differences could have accumulated by these mechanisms over the age of the northern population, approximately 100 generations. Finally, the differences generated by these factors could still be maintained in the face of reasonable levels of gene flow from the south. Such a comprehensive theoretical investigation of this example has given some general insight into the potential influence of these evolutionary factors on the level of inbreeding depression and provides an approach that could be used to understand similar phenomena in other examples.