Genetic variation of oaks (Quercus spp.) in Switzerland - 2. Genetic structures in "pure" and "mixed" forests of pedunculate oak (Q-robur L.) and sessile oak (Q-petraea (Matt.) Liebl.)

Sessile oak (Quercus petraea) and pedunculate oak (Q. robur) are two closel y related, interfertile taxa. They are the most frequent oak species in Swi tzerland. Allelic and genotypic structures at 17 isozyme gene loci were obs erved in 21 populations from Switzerland. Twelve populations of Q, petraea, six populations of Q. robur, and three "mixed" populations (Q. petraea and Q. robur) were investigated. The species status: of the populations was co nfirmed by Principal Component Analysis (PCA) based on leaf morphological t raits. All populations are highly variable at enzyme gene loci. Differentia tion among the taxa is reflected at allelic structures at several enzyme ge ne loci (ACP-C, GDH-A, IDH-B, NDH-A, PGM-A). An excess of homozygotes relat ive to corresponding HARDY-WEINBERG structures was observed in all populati ons. Moderate levels of inbreeding are likely to contribute to these genoty pic structures, but heterogeneity of inbreeding coefficients among loci sug gests that deviations from random mating are not the only cause of the homo zygote excess at particular loci (AAP-A, PGM-A). On average, expected heter ozygosity is highest in the "mixed" populations, but observed heterozygosit y of the "mixed" stands is in-between Q, petraea and Q. robur. A plausible explanation is partial reproductive isolation of both species in "mixed" fo rests resulting in a "Wahlund effect". Seed procurement in "mixed" oak fore sts is encouraged. The species status of populations is of prime importance for the selection of oak genetic resources in Switzerland.