Crataegus monogyna Jacq. and C-laevigata (Poir.) DC. (Rosaceae, Maloideae) display low level of genetic diversity assessed by chloroplast markers

Twenty one European populations of C. monogyna and nine of C. laevigata were examined to quantify genetic diversity at chloroplast DNA. PCR-RFLP and microsatellites (SSR) were analysed by PCR using conserved primers. The average within population gene diversity (h(S)), the total gene diversity (h(T)), and the differentiation for unordered alleles (G(ST)) and for ordered alleles (N-ST) were measured. For those populations where both species were present, an introgression ratio (IG) was calculated. Four different haplotypes were detected, one of them very common (85% in C. monogyna and 75% in C. laevigata populations) and distributed over the whole range. The value of the introgression ratio is close to one (IG = 0.83), indicating that the haplotypic variation is largely species-independent. The level of differentiation between C. monogyna (G(ST) = 0.241) and C. laevigata (G(ST) = 0.430) populations indicates a very effcient seed dispersal mechanism. Moreover, the difference between G(ST) and N-ST for both species is not significant, demonstrating that the phylogeographic structure is weak or absent.