EFFECT OF GEOGRAPHICAL DISCONTINUITY ON GENETIC-VARIATION IN QUERCUS-ILEX L. (HOLM OAK) - EVIDENCE FROM ENZYME POLYMORPHISM

Allozymes at a maximum of eight loci were used to analyse the spatial patterns of genetic variation in 1942 helm oak trees (Quercus ilex L.) from 57 provenances distributed in the six main disjunct areas of the species distribution area. Polymorphism and genetic diversity were hi gh except in the six marginal populations growing in various locations under unfavourable climatic conditions. Ten per cent of the total gen etic diversity (H-t = 0.262) was accounted for by among-population var iation. In this long-lived species, the effect of geographical discont inuities on gene now restriction was shown by the occurrence of: (i) n umerous rare alleles limited to a single disjunct region; and (ii) all ele frequency variation among the disjunct regions for four alleles. T his result was obtained by comparing spatial autocorrelograms from the 57 populations of the entire distribution area and from the 31 popula tions of the Mediterranean continental region which constitutes the la rgest continuous area in that distribution. Life history traits (e.g. long life span and high outcrossing rate) and past changes in climate and geographical continuity are assumed to be the main factors respons ible for the present genetic variation patterns observed in the specie s.