Comparative Genomics Analysis in Grass Species Reveals Two Distinct Evolutionary Strategies Adopted by R Genes

Resistance genes play an important role in the defense of plants against the invasion of pathogens. In Setaria italica and closely related grass species, R genes have been identified through genetic mapping and genome-wide homologous/domain searching. However, there has been to date no systematic analysis of the evolutionary features of R genes across all sequenced grass genomes. Here, we determined and comprehensively compared R genes in all 12 assembled grass genomes and an outgroup species (Arabidopsis thaliana) through synteny and selection analyses of multiple genomes. We found that the two groups of nucleotide binding site (NBS) domains containing R genes-R tandem duplications (TD) and R singletons-adopted different strategies and showed different features in their evolution. Based on K_a/K_s analysis between syntenic R loci pairs of TDs or singletons, we conclude that R singletons are under stronger purifying selection to be conserved among different grass species than R TDs, while R genes located at TD arrays have evolved much faster through diversifying selection. Furthermore, using the variome datasets of S. italica populations, we scanned for selection signals on genes and observed that a part of R singleton genes have been under purifying selection in populations of S. italica, which is consistent with the pattern observed in syntenic R singletons among different grass species. Additionally, we checked the synteny relationships of reported R genes in grass species and found that the functionally mapped R genes for novel resistance traits are prone to appear in TDs and are heavily divergent from their syntenic orthologs in other grass species, such the black streak R gene Rxo1 in Z. mays and the blast R gene Pi37 in O. sativa. These findings indicate that the R genes from TDs adopted tandem duplications to evolve faster and accumulate more mutations to facilitate functional innovation to cope with variable threats from a fluctuating environment, while R singletons provide a way for R genes to maintain sequence stability and retain conservation of function.