Genomic approaches to natural and artificial evolution of plant disease resistance genes (abstract only)

We are interested in the evolution of specificity in plant-pathogen interactions. A significant proportion of genes in the genome potentially encode resistance related genes. A variety of genetic events have been demonstrated to occur at loci encoding disease resistance in a range of plant species. These include point mutations, insertion/deletions, intragenic and intergenic unequal crossing-over, and gene conversion. The relative frequencies and importance of each of these processes to the evolution of new resistance specificities is now beginning to be understood. We are currently refining a 'birth-and-death' model using data on the relative frequencies of genetic events in clusters of resistance genes in Arabidopsis, lettuce and tomato. Clusters of resistance genes exhibit a complex variety of patterns of evolution. Even within a cluster, different groups of genes may exhibit different evolutionary histories. In the major cluster of resistance genes in lettuce, some genes evolve slowly as distinct lineages with little sequence exchange between paralogs. Orthologs of these genes are readily detectable in diverse germplasm. Polymorphism is maintained by balancing selection. Deletion events have led to loss of certain lineages in some haplotypes. Other genes undergo frequent sequence exchange with other paralogs and close orthologs are rare in germplasm. Similar patterns are seen in Arabidopsis and tomato. We are also using in vitro DNA shuffling to determine the functional consequences of genetic rearrangements.