Diversity and evolution of a non-TIR-NBS sequence family that clusters to a chromosomal "hotspot" for disease resistance genes in soybean

In soybean, genes controlling resistance to numerous diseases have been sho wn to cluster to regions on several chromosomes. One such vital chromosomal region is on the soybean molecular linkage group (MLG) F flanked by the RF LP markers K644 and B212. Here, genes controlling resistance to bacterial b light, Phytophthora root rot, and several viral diseases, as well as QTLs c onditioning resistance to corn earworm, root knot nematode, and white mold have been mapped. We have previously identified two classes (b and j) of di sease resistance-related nucleotide binding site (NBS) sequences that local ize to this cluster of genes. Using both cDNA and genomic analyses, we have studied one multi-gene family of sequences representing the previously rep orted class j NBS of soybean. This class of NBS resembles the RPS2-like NBS sequences. RPS2 and similar resistance genes are referred to as non-TIR be cause they do not encode motifs homologous to the Toll-Interleukin-1 region (TIR). By designing PCR primers that specifically target these non-TIR-NBS encoding sequences, we have amplified at least six class j sequence member s from soybean. In addition, we have conducted genomic and cDNA library scr eenings to identify additional class j members. In all, we have characteriz ed 12 class j NBS sequence members. These members have been mapped within a 2-cM region of the soybean F linkage group. We have also identified homoeo logous chromosomal regions on linkage groups A2 and E that contain class j NBS sequences. A BLAST search of the GenBank database has shown that non-TI R NBS sequences are present across the legume family. We have compared thes e non-TIR sequences from other legumes with the soybean clones to assess th e level of diversity within this class or disease resistance-related sequen ces.