Molecular markers to study genetic drift and selection in wheat populations

Studying the heterogeneity in variation of gene frequency among populations or between generations may be a possible way to detect genomic regions exp eriencing selection. In order to evaluate this approach, RFLP markers were used to compare the allelic frequencies in wheat populations that had been submitted to natural selection. In 1984, samples of two composite cross pop ulations were distributed in the French network for dynamic management of g enetic resources. Since then, all the sub-populations have been cultivated in the same sites with no human selection. The strong differentiation betwe en populations found for agro-morphological traits (earliness, resistance t o pathogens,...) provided evidence of their adaptation to local conditions. The two initial populations and six derived sub-populations cultivated for 10 years in four contrasted sites were studied with RFLP markers. Differen tiation between sub-populations based on RFLP diversity was highly signific ant. Variations of allelic frequencies of the 30 loci scored were found to be much greater than expected under genetic drift only. This led us to conc lude that selection greatly influenced the evolution of the populations. So me of the loci clearly presented a higher differentiation than the others. This might indicate that they were genetically linked to other loci polymor phic in the populations and involved in adaptation. However, the effect of one selected gene on a marker, even located very close to the gene, could n ot be predicted with certainty. Hence, though the populations were predomin antly selfing, it seems that initial linkage disequilibriums between marker s and selected genes were not strong enough to control closely the evolutio n of allelic fequencies at the markers.