Plant, Animal & Microbe Genomes X Conference

January 12-16, 2002
Town & Country Convention Center
San Diego, CA

Session: Plant Session Lecture
            

NATURAL AND ARTIFICIAL EVOLUTION OF PLANT DISEASE RESISTANCE GENES

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

A variety of genetic events have now 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. However, the relative frequencies and importance of each of these processes to the evolution of new resistance specificities remains to be determined. We proposed a 'birth-and-death' process for the evolution of resistance genes in which clusters of resistance genes were proposed to evolve slowly through gene duplication events and subsequent divergent selection to create distinct lineages. Deletion events led to loss of certain lineages in some haplotypes. Polymorphism is maintained by balancing selection. We are now refining this model using data from Arabidopsis, lettuce and tomato and obtaining data on the relative frequencies of genetic events in clusters of resistance genes. Although some aspects of the birth-and-death model remain valid, current data indicates a more complex situation. Clusters of resistance genes exhibit a variety of patterns of evolution. Even within a cluster, different groups of genes may exhibit different behaviors. In the major cluster of resistance genes in lettuce, some genes are stable with little sequence exchange between paralogs. Orthologs of these genes are readily detectable in diverse germplasm. Other genes undergo frequent sequence exchange with other paralogs and close orthologs are rare in germplasm. We are also using in vitro DNA shuffling to determine the functional consequences of genetic rearrangements.