Geographical structure of gene diversity in Quercus petraea (Matt.) Liebl. II: Multilocus patterns of variation

Two-locus composite disequilibria were estimated for all 28 pairs of loci (from eight loci) in 81 populations of Quercus petraea spread over the natural distribution of the species. Significant disequilibria were found in almost all populations for two pairs (Aap-A/Lap-A, Mr-A/Dia-A) that have also shown strong linkage in cosegregation analysis. These pairs belong to the so-called complex loci producing nearly identical gene products and that are tightly linked. Significant disequilibria were also found for other pairs of loci that are linked to a smaller degree. The amount of disequilibrium followed a geographical pattern. The number of pairs showing significant disequilibria per population was higher on the edges of the natural range (Norway, Great Britain, Turkey). Disequilibria for the two pairs (Aap-A/Lap-A, Mr-A/Dia-A) were also higher at the edges of the distribution and were correlated with longitude. Population differentiation resulting from the associations of alleles at different loci was increased when the correlations of allele frequencies at the within- and between-population level were of different magnitude. It is suspected that correlations at the between-population level may arise from historical causes rather than gametic disequilibrium. Finally, the first canonical variate (multivariate score) including allelic frequencies of all eight loci follows a strong longitudinal pattern of variation.