RAPID REORGANIZATION OF RESISTANCE GENE HOMOLOGS IN CEREAL GENOMES

We used conserved domains in the major class (nucleotide binding site plus leucine-rich repeat) of dicot resistance (R) genes to isolate rel ated gene fragments via PCR from the monocot species rice and barley, Peptide sequence comparison of dicot R genes and monocot R-like genes revealed shared motifs but provided no evidence for a monocot-specific signature, Mapping of these genes in rice and barley showed linkage t o genetically characterized R genes and revealed the existence of mixe d clusters, each harboring at least two highly dissimilar R-like genes , Diversity was detected intraspecifically with wide variation in copy number between varieties of a particular species, Interspecific analy ses of R-like genes frequently revealed nonsyntenic map locations betw een the cereal species rice, barley, and foxtail millet although tight collinear gene order is a hallmark of monocot genomes, Our data sugge st a dramatic rearrangement of R gene loci between related species and implies a different mechanism for nucleotide binding site plus leucin e-rich repeat gene evolution compared with the rest of the monocot gen ome.