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

We examined the spatial distribution of maternally inherited chloroplast DNA markers over the French part of the range of Sorbus torminalis, a scattered temperate forest tree native to most of Europe. The survey by restriction analysis of polymerase-chain-reaction amplified fragments for 880 individuals distributed among 55 populations allowed the detection of 25 haplotypes. The coefficient of differentiation among populations computed on the basis of haplotype frequency (G(STc) = 0.34) was one of the lowest found in forest trees so far, and the mean within-population diversity was relatively high, indicating multiple-mother foundation events. A significant but slight geographical pattern was observed, up to distances of about 100 km. This pattern of differentiation was compared to the genetic structure of the same populations revealed by biparentally inherited markers (isoenzymes), and a new method to quantify the relative importance of seed and pollen dispersal was derived, based on isolation-by-distance models. Neither pollen- nor seed-mediated gene flow was predominant in S. torminalis, a finding that differs from those for the majority of tree species studied so far. This result was most likely due to an extinction-recolonization dynamics based on efficient seed dispersal strategies. The joint screening of 31 individuals of the related Sorbus aria and of 163 hybrid individuals shows that hybridization occurs predominantly in one direction and is rarely followed by cytoplasmic introgression. As a consequence, interspecific gene flow should not significantly affect the diversity dynamics within S. torminalis.