Pollen- versus seed-mediated gene flow in a scattered forest tree species

We examined the spatial distribution of maternally inherited chloroplast DN A markers over the French part of the range of Sorbus torminalis, a scatter ed temperate forest tree native to most of Europe. The survey by restrictio n analysis of polymerase-chain-reaction amplified fragments for 880 individ uals distributed among 55 populations allowed the detection of 25 haplotype s. The coefficient of differentiation among populations computed on the bas is of haplotype frequency (G(STc) = 0.34) was one of the lowest found in fo rest trees so far, and the mean within-population diversity was relatively high, indicating multiple-mother foundation events. A significant but sligh t geographical pattern was observed, up to distances of about 100 km. This pattern of differentiation was compared to the genetic structure of the sam e populations revealed by biparentally inherited markers (isoenzymes), and a new method to quantify the relative importance of seed and pollen dispers al was derived, based on isolation-by-distance models. Neither pollen- nor seed-mediated gene flow was predominant in S. torminalis, a finding that di ffers from those for the majority of tree species studied so far. This resu lt was most likely due to an extinction-recolonization dynamics based on ef ficient seed dispersal strategies. The joint screening of 31 individuals of the related Sorbus aria and of 163 hybrid individuals shows that hybridiza tion occurs predominantly in one direction and is rarely followed by cytopl asmic introgression. As a consequence, interspecific gene flow should not s ignificantly affect the diversity dynamics within S. torminalis.