The genetic architecture of resistance

Plant resistance genes (R genes), especially the nucleotide binding site le ucine-rich repeat (NBS-LRR) family of sequences, have been extensively stud ied in terms of structural organization, sequence evolution and genome dist ribution. These studies indicate that NBS-LRR sequences can be split into t wo related groups that have distinct amino-acid motif organizations, evolut ionary histories and signal transduction pathways. One NBS-LRR group, chara cterized by the presence of a Toll/interleukin receptor domain at the amino terminal end, seems to be absent from the Poaceae. Phylogenetic analysis su ggests that a small number of NBS-LRR sequences existed among ancient Angio sperms and that these ancestral sequences diversified after the separation into distinct taxonomic families. There are probably hundreds, perhaps thou sands, of NBS-LRR sequences and other types of R gene-like sequences within a typical plant genome. These sequences frequently reside in 'mega-cluster s' consisting of smaller clusters with several members each, all localized within a few million base pairs of one another. The organization of R-gene clusters highlights a tension between diversifying and conservative selecti on that may be relevant to gene families that are unrelated to disease resi stance.