Plant Genome IV Conference Plant Genome IV Conference
Genes for resistance to several pathogens are clustered in the genome of lettuce. The majority of genes (Dm) for resistance to Bremia lactucae map to three major clusters. We have been studying the genetic and physical organization of these regions with emphasis on the largest cluster. The major cluster of downy mildew resistance genes in lettuce contains at least eleven resistance specificities and spans a recombination distance of at least 16 cM. We are screening a large F2 population from Kordaat (Dm1,3,4) x Calmar (Dm7,8,13) using multiplexed, codominant PCR-based markers to detect numerous recombinants in the region. Together with our mutant genetic stocks (see companion poster, R. Arroyo-Garcia et al.), these recombinants will help to localize the positions of resistance genes. In addition, to examine the stability and evolution of resistance specificities, molecular markers are being used to examine the underlying genetic changes occurring in the mutant and recombinant genetic stocks. The physical size of the major cluster, as determined by pulsed-field gel electrophoresis, is at least 6 Mb. Structural analysis of the Dm3 locus has revealed several classes of repeated genomic sequences. Some are unique to the Dm3 locus, whereas others are conserved between unlinked resistance gene clusters. we have generated and screened a Bacterial Artificial Chromosome (BAC) library of large genomic fragments of lettuce. BAC clones from the major cluster have been fingerprinted using AFLPs and hybridization probes and arranged into contigs. We have identified clones that potentially carry Dm3 and have initiated complementation experiments. We have also isolated sequences by PCR with degenerate oligonucleotide primers homologous to conserved domains in resistance genes cloned from other species. These are being characterized using the recombinant and mutant stocks.