Plant & Animal Genome IX Conference Plant & Animal Genome IX Conference
Workshop: Compositae
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The major cluster of disease resistance genes in lettuce comprises of over 30 NBS-LRR encoding (RGC2) genes and spans at least 4 Mb. Only one member encodes Dm3 specificity. We have evidence for point mutations, recombination, gene conversions, and unequal crossing-over within and between homologs at this complex locus. However, relative importance of these genetic events to the evolution of new resistance specificities is not well understood. We have fully sequenced 22 RGC2 genes and partially sequenced an additional six from genomic clones of the cultivar Diana. Conserved primers were designed to amplify the RGC2 paralogs from cv. Diana. Using PCR with these primers RGC2 homologs were amplified from two additional genotypes each of L. sativa, L. serriola, and L. saligna as well as cv. Diana as the control. The 1.5-2.3kb products span two exons in 3' LRR region and an intron. PCR products from replicate amplifications were cloned and at least 100 clones were sequenced for each of the seven genotypes. Ten, 12 and 22 sequences were recovered for cultivars Mariska, Calmar, and Diana respectively; 15 and 38 sequences for the two L. serriola; and 9 and 12 sequences for the two L. saligna. Primary analysis detected many sequence exchanges among paralogs. However, obvious orthologous relationship remains between some members. These RGC2 sequences are not apparently evolving rapidly. Currently, we are analyzing the rates of mutation, frequencies of gene conversion and other genetic events to infer their relative significance to the evolution of resistance genes.