RECOMBINATION AND SPONTANEOUS MUTATION AT THE MAJOR RESISTANCE GENE CLUSTER IN LETTUCE (Lactuca sativa)

¹ Department of Vegetable Crops, University of California, Davis, California 95616

² Dpto de Genetica Molecular de Plantas. CNB-CSIC. Campus UAM. Cantoblanco. 28049. Spain.

In order to investigate the genetic behavior of the major cluster of resistance genes in lettuce, two overlapping sets of experiments were conducted to examine recombination and spontaneous mutation events at the locus. A population of 2250 F₂ individuals derived from a cross between the cultivars Kordaat (Dm1, Dm3, Dm4) and Calmar (Dm7, Dm8, Dm13) was screened for recombinants in the Dm3 region. Recombinants and their progeny were screened with molecular markers to identify crossover breakpoints. Multiple generations were examined to determine the stability of recombination rates. In the second set of experiments, three populations of lettuce were screened for spontaneous mutations in downy mildew resistance: a Kordaat x Calmar F₁ population, an S₂ population from cv. Diana (Dm1, Dm3, Dm7, Dm8), and a population of recombinant F₃ families, derived from F₁ individuals in which Dm3 was homozygous but the flanking regions were heterozygous. Fifteen mutants have been identified from 24,500 screens. All fifteen mutants were single locus, recessive mutations at one of three Dm loci. Molecular analysis of the twelve spontaneous dm3 mutants showed that eleven were associated with chromosome deletions. In the four dm3 mutant families in which recombination could be analyzed, mutation events were also associated with exchange of flanking markers. One S₂ mutant family was the result of a gene conversion event between Dm3 and a closely related family member. Thus, the evolutionary forces which act upon the major resistance gene cluster in lettuce include recombination, unequal crossing-over as well as gene conversion.