GENETIC DIVERSITY AT THE MAJOR CLUSTER OF DISEASE RESISTANCE GENES IN CULTIVATED AND WILD Lactuca spp.

¹ Department of Vegetable Crops, University of California, Davis, CA 95616, USA
² Department of Plant Breeding, Wageningen Agricultural University, P.O. Box 386, 6700 AJ Wageningen, The Netherlands
³ Clemson University Genomics Institute, 100 Jordan Hall, Clemson University, Clemson, SC 29634-5780, USA
⁴ Departemento de Mejora Genetica y Biotechnologia, CIT-INIA, Ctra: De la Coruna, Km7, Madrid 28040, Spain
⁵ Institute of Evolution, University of Haifa, Haifa 31905, Israel

Diversity was analysed in wild and cultivated germplasm using molecular markers derived from resistance genes of the NBS-LRR type. Three molecular markers, one microsatellite marker and two SCAR markers that amplified LRR-encoding regions, were developed from sequences of resistance gene homologs at the major resistance gene cluster in lettuce. Variation for these markers was assessed in germplasm including accessions of cultivated lettuce, Lactuca sativa and three wild Lactuca spp., L. serriola, L. saligna and L. virosa. A large number of haplotypes were detected indicating the presence of numerous resistance genes in these wild species. The three markers were highly correlated with resistance phenotype in L. sativa. They were able to discriminate between wild accessions that had previously been shown to be resistant to all isolates of Bremia lactucae. Therefore, these markers will be highly informative for establishment of core collections and marker-aided selection. The diversity in haplotypes provided evidence for gene duplication and unequal crossing-over during the evolution of this cluster of resistance genes. However, there was no evidence for indels within the LRR-encoding regions studied. In order to study the influence of evolutionary forces in maintaining diversity at this cluster, both the three molecular markers at the major resistance gene cluster and random AFLP markers were used to detect diversity within and between natural populations of L. serriola. A total of nine and six populations of L. serriola were analyzed from Israel and California respectively; the former is a part of the center of diversity for Lactuca spp. while the latter is an area of recent colonization. A hierachical analysis of the population structure of L. serriola showed that countries as well as locations were significantly differentiated for both types of markers. The relative importance of drift and divergent selection in generating this differentiation is discussed.