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GENETIC VARIATION OF OAKS (QUERCUS SPP.) IN SWITZERLAND. 1. ALLELIC DIVERSITY AND DIFFERENTIATION AT ISOZYME GENE LOCI

Genetic variation of 28 oak populations (Quercus cerris, Q. petraea, Q. pubescens, Q. robur) in Switzerland has been assessed at 17 isozyme gene loci. Individual Q. cerris trees were identified by the morphology of their buds; the species status of the other taxa was confirmed by Principal Component Analysis (PCA) of leaf morphological data. All populations proved to be highly variable (expected heterozygosity H, in-between 0.180 and 0.262). On average, Q. robur and Q. petraea populations show similar levels of genetic diversity (He = 0.241 and He= 0.249 respectively). Diversity is slightly lower for Q. pubescens (He= 0.215) and Q. cerris ( H e = 0.221). The average number of alleles per locus is highest for Q. pzibescens populations. The two Q, cerris collectives are strongly differentiated from all other populations (D, > 0.5) indicating wide systematic separation and complete reproductive isolation from the other species. Differentiation among Q. petraea, Q. pubescens, and Q, robur is much lower and in close correspondence to differentiation patterns of leaf morphological traits. A hierarchical division of the total genetic diversity for the three species reveals that most of the variation is within populations (93.8 %), and a comparatively high amount of the variation is between species (4.3 %). Differentiation among populations within species is low (1.9 %).Results support the close systematic relationship between Q. petraea, Q. pubescem, and Q, robur. The significance of the results for the systematics of the "white oaks" and for the conservation of genetic resources of oaks in Switzerland is discussed.