COMPARISON OF GENETIC DIFFERENTIATION IN MARITIME PINE (PINUS-PINASTER AIT) ESTIMATED USING ISOZYME, TOTAL PROTEIN AND TERPENIC LOCI

Thirty genes belonging to three classes of biochemical markers (eight isozyme loci, 16 protein loci revealed by two-dimensional gel electrop horesis, and six terpene loci) were scored in six populations of marit ime pine. The purpose of the study was to compare the level of genetic differentiation (G(ST)) among the populations of this pine in order t o test whether differential selective pressures are acting on these ma rkers. However, each class of loci was found to display different leve ls of average diversity. Although this should not affect the compariso n of the level of differentiation (theoretically a diversity-independe nt genetic measure) we found here that single-locus values of differen tiation significantly depended on the values of diversity. This result was explained analytically by showing that the sampling of a limited number of populations results in G(ST) taking maximal possible values lower than unity, especially when the level of diversity is low. By re moving the less polymorphic loci, measures of differentiation independ ent of the level of diversity can be obtained. They turned out to be v ery close for each class of markers indicating the absence (or similar level) of selection acting on the three classes of loci and a high le vel of differentiation in this pine (G(ST) = 0.17) typical of a specie s having a highly fragmented range.