Resistance gene evolution.

Plant resistance genes are highly polymorphic and have diverse recognition specificities. These genes often occur as members of clustered gene families that have evolved through duplication and diversification. Regions of nucleotides conserved between family members and flanking sequences facilitate equal or unequal recombination events. Transposition contributes to allelic diversity. Resistance gene clusters appear to evolve more rapidly than other regions of the genome, and domains responsible for recognitional specificity, such as the leucine-rich repeat domain, are subject to adaptive selection.