COLONIZATION WITH LONG-DISTANCE SEED DISPERSAL AND GENETIC-STRUCTURE OF MATERNALLY INHERITED GENES IN FOREST TREES - A SIMULATION STUDY

Since the last glacial period forest trees have expanded to their pres ent range very rapidly, with rates up to 500 m yr(-1) for oaks in Euro pe, which can be explained only by the dispersion of acorns over long distances. We used a stratified dispersal model, including both diffus ive and long-distance dispersal of seeds, to simulate the colonization of a 100 km x 300 km grid by populations of oak trees. An appropriate rate of spread is obtained with rare dispersal at distances of the or der of tens of kilometres. We simulated the effect of stratified versu s diffusive dispersal of seeds on the spatial genetic structure at a m aternally inherited locus. Founding events associated with stratified dispersal generate a high amount of genetic differentiation among popu lations, which is likely to persist for a long time after colonization . Using autocorrelation methods, we show that diffusive and stratified dispersals create quite different spatial patterns of variation for t he maternally inherited locus. Stratified dispersal creates patchy pat terns that are concordant with a previous experimental investigation o f chloroplast DNA variation at a regional scale in the oaks Quercus pe traea and Quercus robur. For plant populations that have passed throug h recent episodes of range expansion, long-distance dispersal events a re probably the most important factors of spatial genetic structuring of maternally inherited genes at small or medium geographic scales.