Allozyme variation in cork oak ( Quercus suber L.): the role of phylogeography and genetic introgression by other Mediterranean oak species and 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 Medi- terranean Basin. Mean heterozygosity values over the polymorphic loci (Ho"0.283), the percentage of poly- morphic populations (M"0.76), and the total 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 holm oak (Q. ilex L.), another evergreen species in the same area, cork oak possessed a smaller allele pool and a lower average number of alleles per locus and per population (A"2.0). More particularly, very few low- frequency alleles were observed in cork oak except for eight populations in which allozyme polymorphism at locus Pgi 1, diagnostic between both species, indicates that these low-frequency alleles are introgressed from holm oak. On the basis of the genetic distance esti- mated from allozyme frequencies, 32 of the 40 cork oak populations studied were classiÞed into two very distinct sets which also corresponded to distinct geographic areas. One set gathered together the 18 populations from the Iberian peninsula and two adjac- ent areas in France, i.e. the centre of origin of cork oak, according to paleobotanical data. This set was charac- terized by a larger allele pool, a higher within-popula- tion genetic diversity and a lower di¤erentiation between populations than was observed in the other set, which comprised the populations from North Africa, Sicily, Sardinia, Corsica, continental Italy and the region of Provence (southeastern France). In these Communicated by P. M. A. Tigerstedt L. Toumi1 á R. Lumaret ( ) Centre dÕEcologie Fonctionnelle et Evolutive (C.E.F.E., C.N.R.S.), F-34293, Montpellier Cedex 05, France Present address: 1 Institut Sylvo-Pastoral, 8110 Tabarka, Tunisia more southern and eastern disjunct areas, cork oak migration from Iberia may have occurred at di¤erent periods since the end of the Tertiary. The possible e¤ect of human activity on cork oak genetic structure, i.e. the selection of good-quality cork, acorn over-use for ani- mal food, and even human nutrition, is discussed.