Molecular diversity in French bread wheat accessions related to temporal trends and breeding programmes.

A set of 41 wheat microsatellite markers (WMS), giving 42 polymorphic loci (two loci on each chromosome), was used to describe genetic diversity in a sample of 559 French bread wheat accessions (landraces and registered varieties) cultivated between 1800 and 2000. A total of 609 alleles were detected. Allele number per locus ranged from 3 to 28, with a mean allele number of 14.5. On the average, about 72% of the total number of alleles were observed with a frequency of less than 5% and were considered to be rare alleles. WMS markers used showed different levels of gene diversity: the highest PIC value occurred in the B genome (0.686) compared to 0.641 and 0.659 for the A and D genomes, respectively. When comparing landraces with registered varieties gathered in seven temporal groups, a cluster analysis based on an F(st) matrix provided a clear separation of landraces from the seven variety groups, while a shift was observed between varieties registered before and after 1970. There was a decrease of about 25% in allelic richness between landraces and varieties. In contrast, when considering only registered varieties, changes in diversity related to temporal trends appeared more qualitative than quantitative, except at the end of the 1960s, when a bottleneck might have occurred. New varieties appear to be increasingly similar to each other in relation to allelic composition, while differences between landraces are more and more pronounced over time. Finally, considering a sub-sample of 193 varieties representative of breeding material selected during the twentieth century by the six most important plant breeding companies, few differences in diversity were observed between the different breeding programmes. The observed structure of diversity in French bread wheat collections is discussed in terms of consequences, both for plant breeders and for managers of crop genetic resources.