Intrapopulation and interpopulation genetic variation of Quercus in Denmark

Native Danish oak stands are fragmented and decreasing because of the exten sive use of foreign seed sources. Therefore, the population structure of na tural Danish oak stands was analysed by means of six polymorphic enzyme loc i. A total of 17 stands of Quercus robur L. (including an "outgroup" sample from The Netherlands), seven Quercus petraea [Matt.] Liebl. and two putati ve hybrid stands were included in the study. The average genetic diversity was similar for the two species as well as for the putative hybrid stands: 0.25 for Q. robur, 0.27 for Q. petraea and 0.26 for the hybrid stands. The genotypic proportions at two (Pgm and Mnr) of the six loci showed many sign ificant deviations from Hardy-Weinberg expectations, always with an excess of homozygotes, whereas the remaining four loci accorded to Hardy-Weinberg proportions, suggesting a low level of inbreeding. The differentiation of t he Q. petraea and Q. robur populations was quantified with Wright's F-stati stics. The within-species component was low, 0.022, reflecting the wind-pol linated reproductive mode of the two species. The between species component was 10-fold higher, 0.235, indicating two separate groups. A phylogenetic tree estimated from allele frequencies also supported the presence of these two groups. Despite the two well-separated groups in the tree. it was not possible to assign all individuals to the species to which they supposedly belonged. In total, 10% of the Q. robur individuals and 14% of the Q. petra ea individuals were assigned to the other species. suggesting a limited amo unt of introgression between the two species.