Molecular diversity at the major cluster of disease resistance genes in cultivated and wild Lactuca spp.

Diversity was analyzed in wild and cultivated Lactuca germplasm using molec ular markers derived from resistance genes of the NBS-LRR type. Three molec ular markers, one microsatellite marker and two SCAR markers that amplified LRR-encoding regions, were developed from sequences of resistance gene hom ologs at the main resistance gene cluster in lettuce. Variation for these m arkers were assessed in germplasm including accessions of cultivated lettuc e, Lactuca sativa L. and three wild Lactuca spp., L. serriola L., L. salign a and L. virosa L. Diversity was also studied within and between natural po pulations of L. serriola from Israel and California; the former is close to the center of diversity for Lactuca spp. while the latter is an area of mo re recent colonization. Large numbers of haplotypes were detected indicatin g the presence of numerous resistance genes in wild species. The diversity in haplotypes provided evidence for gene duplication and unequal crossing-o ver during the evolution of this cluster of resistance genes. However, ther e was no evidence for duplications and deletions within the LRR-encoding re gions studied. The three markers were highly correlated with resistance phe notypes in L. sativa. They were able to discriminate between accessions tha t had previously been shown to be resistant to all known isolates of Bremia lactucae. Therefore, these markers will be highly informative for the esta blishment of core collections and marker-aided selection. A hierarchical an alysis of the population structure of L. serriola showed that countries, as well as locations, were significantly differentiated. These differences ma y reflect local founder effects and/or divergent selection.