Diversity and Evolution of Resistance Genes in the Compositae

Plants contain multiple phenotypic resistance (R) genes that provide resistance to diverse pathogens and pests. The majority of host recognition genes cloned to date encode nucleotide binding site–leucine rich repeat proteins (NLRs). Motif searches identified 1,141 genes in the lettuce reference genome of cv. Salinas that are involved in pathogen recognition, of which 413 were predicted to encode NLR proteins. Previously, 377 putative NLR-encoding genes were grouped into 40 families, including several large multi-gene families as well as 14 singletons. Members of four large NLR-encoding families, RGC1, RGC2, RGC21 and RGC12 were shown to be required for 16 disease resistance phenotypes in lettuce. We are currently studying the evolution of R genes in the Compositae family relative to other plant species. The NLR-encoding genes were extracted from genome assemblies of thirty species from the Compositae and families representing Angiosperm diversity and used for phylogenetic analysis by the using maximum-likelihood approach (RAxML). The resultant phylogenetic trees exhibited different patterns of evolution. There has been expansion of specific families.  In some cases, expansion happened prior to speciation, whereas in other cases, it happened afterwards. Further approaches are being undertaken in order to characterize the evolutionary pathway of NLR-encoding genes in the Compositae.