ALLOZYME VARIATION IN CORK OAK (QUERCUS-SUBER L.) - THE ROLE OF PHYLOGEOGRAPHY AND GENETIC INTROGRESSION BY OTHER MEDITERRANEAN OAK SPECIESAND HUMAN ACTIVITIES

Genetic variation in the cork oak (Quercus suber L.) was investigated using 11 loci from seven enzyme systems in 40 populations sampled over the entire distribution of this species in the western Mediterranean Basin. Mean heterozygosity values over the polymorphic loci (Ho = 0.28 3), the percentage of polymorphic populations (M = 0.76), and the tota l genetic diversity (Ht = 0.31) from which 11% was accounted for among -population variation, are among the highest recorded in oak species. In contrast to previous results in helm oak (Q. ilex L.), another ever green species in the same area, cork oak possessed a smaller allele po ol and a lower average number of alleles per locus and per population (A = 2.0). More particularly, very few low-frequency alleles were obse rved in cork oak except for eight populations in which allozyme polymo rphism at locus Pgi 1, diagnostic between both species, indicates that these low-frequency alleles are introgressed from helm oak. On the ba sis of the genetic distance estimated from allozyme frequencies, 32 of the 40 cork oak populations studied were classified into two very dis tinct sets which also corresponded to distinct geographic areas. One s et gathered together the 18 populations from the Iberian peninsula and two adjacent areas in France, i.e. the centre of origin of cork oak, according to paleobotanical data. This set was characterized by a larg er allele pool, a higher within-population genetic diversity and a low er differentiation between populations than was observed in the other set, which comprised the populations from North Africa, Sicily, Sardin ia, Corsica, continental Italy and the region of Provence (southeaster n France). In these more southern and eastern disjunct areas, cork oak migration from Iberia may have occurred at different periods since th e end of the Tertiary, The possible effect of human activity on cork o ak genetic structure, i.e. the selection of good-quality cork, acorn o ver-use for animal food, and even human nutrition, is discussed.