Plant & Animal Genome VI Conference

P49

MANY DIVERGED RESISTANCE GENES OF ANCIENT ORIGIN EXIST IN LETTUCE

1. Department of Vegetable Crops, University of California, Davis, CA95616
2. Institute of Evolution, University of Haifa, Mt. Carmel, Haifa 31905, Israel

In lettuce, the majority of resistance genes to Bremia lactucae (Dm genes) cluster into three major linkage groups. Dm3 maps to the largest cluster which contains at least 11 other resistance specificities. A resistance gene candidate for Dm3 (RGC2B) consists of a nucleotide binding site (NBS), a large leucine rich region (LRR), and several introns. To investigate the genetic diversity of resistance gene haplotypes we developed a microsatellite marker in the third intron and two sets of primers designed to amplify the 5' end of the LRR and the small intron in the 3' end of the LRR from RGC2 homologs in the cluster but not from other RGC families. The amplified products were digested with a four-base cutter to generate fingerprints. In a broad range of germplasm collections including 75 cultivated (L. sativa) as well as 74 wild accessions (L. serriola, L. saligna, L. virosa, L. augustana, L. indica and L. perennis), the microsatellite marker and fingerprinting of the LRR detected many haplotypes suggesting numerous resistance genes available for introgression and little redundancy within the wild accessions; however, very little length variation was detected in the LRR. When L. serriola populations from 13 different locations in Israel and California were analyzed, the diversity of the resistance gene haplotypes and the concordance among the markers were detected. This study provides preliminary data on the mechanisms creating the molecular diversity and the influence of evolutional forces on the resistance gene complexes. Sequence comparison of the 5' end of the LRR from six haplotypes of three different species revealed that the polymorphisms within this major cluster are ancient, predating the separation of these species.