| Research in the field of plant disease resistance has seen great progress in the last decade. Approximately twenty genes, which confer resistance against a variety of fungal, bacterial, viral and nematode pathogens, have been cloned from a variety of plant species, including maize, tomato, Arabidopsis, tobacco, flax, rice, and lettuce. The research that has been and is currently being conducted in lettuce reflects the progress in the field as a whole. This dissertation chronicles the genetic mapping of several resistance gene candidate sequences. These RGC sequences exemplified the genetic clustering behavior that resistance genes display in many plant species, and were important elements in the map-based cloning of the lettuce downy mildew resistance gene, Dm3. Also presented in this dissertation are genetic and molecular analyses of mutants in Dm1 , Dm3, Dm7, and Dm8 resistance, which were induced by the compound ethylmethanesulphonate. These EMS-induced dm3 mutants allowed the identification of the sequence that encoded the Dm3 specificity and thus were another component contributing to the cloning of this gene. The dm1, dm7 and dm8 EMS mutants represent important genetic stocks for the future cloning of these resistance genes. And finally, this thesis presents analyses of recombination and spontaneous mutation events at the Dm3 locus. These experiments were undertaken to examine the genetic forces which influence the evolution of this cluster of resistance genes. Recombination, unequal crossing-over, as well as gene conversion were found to affect the structure and stability of this locus. An increase in recombination activity was detected in association with two of the spontaneous mutation events. The picture that is emerging from these and other analyses is that the stability and recombination activity of resistance gene clusters are heavily dependent on the size and complexity of the locus, as well as the structural similarities between haplotypes in a particular meiotic pairing. |
