* Department of Vegetable Crops, University of California, Davis, California, 95616
† Current address: Dpto de Genética Molecular de Plantas, CNB-CSIC, Campus UAM, Cantoblanco, 28049 Madrid, Spain
‡ Department of Biology, University of Massachusetts, Boston, Massachusetts 02125
Two sets of overlapping experiments were conducted to examine recombination and spontaneous mutation events within clusters of resistance genes in lettuce, with emphasis on the region containing Dm3. A population of ~2,220 F2 individuals was derived from a cross between the cultivars Kordaat (Dm1, Dm3, Dm4) and Calmar (Dm7, Dm8, Dm13) and screened for recombinants in the Dm3 region using flanking PCR-based markers. Crossover points were characterized in recombinants and their progeny. The Dm3 region is not highly recombinagenic, exhibiting a recombination frequency 18-fold lower than the genome average. Multiple generations were examined to determine the variation in recombination rates. Recombinants were identified only rarely within the cluster of resistance gene homologs and no crossovers within genes were detected. Recombination rates were stable over three generations except in two spontaneous mutant families. In the second set of experiments, three populations were screened for spontaneous mutations in downy mildew resistance: a Kordaat x Calmar F1KC population; an S2 population from cv. Diana (Dm1, Dm3, Dm7, Dm8); and a population of recombinant F3 families, derived from crossing F2 sibs of the first experiment to produce F1R progeny that were homozygous for Dm3 but heterozygous for flanking markers. Sixteen Dm mutants were identified from 27,500 screens corresponding to spontaneous mutation rates of 10-3 to 10-4 per generation for Dm1, Dm3, and Dm7. All sixteen mutants were single locus, recessive mutations at the corresponding Dm locus. Twelve mutants were detected at Dm3; eleven of these mutants were associated with large chromosome deletions in the region immediately surrounding Dm3. In the four dm3 mutant families for which recombination could be analyzed, deletion events were associated with exchange of flanking markers, suggesting the occurrence of unequal crossing-over. Although the number of resistance gene homologs was changed, no novel chimeric genes were detected as the result of the unequal crossing-over. In two of these four mutant families, spontaneous mutation was correlated with elevated levels of recombination. One S2 mutant was the result of a gene conversion event between Dm3 and a closely related homolog, generating a novel chimeric gene. The short-term evolution of the major cluster of resistance genes in lettuce involves several genetic mechanisms including unequal crossing-over and gene conversion. Data from lettuce and other species indicate that the size and complexity of a locus as well as the structural and sequence similarities in a particular pairing of haplotypes profoundly influence the stability and recombination activity of resistance gene clusters.