Evolution of resistance against powdery mildew in winter wheat populationsconducted under dynamic management. I - Is specific seedling resistance selected?

Dynamic management has been proposed as a complementary strategy to gene ba nks for the conservation of genetic resources. The evolution of frequencies of genes for specific resistance towards powdery mildew (caused by Blumeri a graminis f, sp. tritici) in populations of a French network fur dynamic m anagement of bread wheat genetic resources was investigated after 10 years of multiplication without human selection. The objective was to determine w hether specific resistance gene diversity was maintained ill the population s and whether any changes could be attributed to selection due to pathogen pressure. Seven populations, originating from four of the network sites, we re characterized and compared to the initial population fur six specific re sistance gene frequencies detected by nine Blumeria graminis f. sp. tritici isolates. Diversity decreased at the population level, but because of a st rong differentiation between the populations, this diversity was maintained at the network: level. The comparison of Fst parameters estimated on neutr al markers (RFLP) and on resistance gene data revealed that in two of the p opulations specific resistance genes had bet II selected by pathogen pressu re, whereas evolution in two other populations seemed to be the result of g enetic drift. For the three last populations, conclusions were less clear, as one had probably experienced a strong bottleneck and the other two prese nted intermediate Fst values. A dynamic management network with sites contr asted for pathogen pressure, allowing genetic drift in some populations and selection in others, appeared, at least on the short term, to be a good to ol for maintaining the diversity of genes for specific resistance to powder y mildew.