Unique Evolutionary Pattern Of Numbers Of Gramineous NBS-LRR Genes

Nucleotide binding site (NBS)-leucine-rich repeat (LRR) genes belong to the largest class of diseaseresistance gene super groups in plants, and their intra-or interspecies nucleotide variations have been studied extensively to understand their evolution and function. However, little is known about the evolutionary patterns of their copy numbers in related species. Here,129,245,239and508NBSs were identiWed in maize, sorghum, brachypodium and rice, respectively, suggesting considerable variations of these genes. Based on phylogenetic relationships from a total of496ancestral branches of grass NBS families, three gene number variation patterns were categorized:conserved, sharing two or more species, and species-speciWc. Notably, the species-speciWc NBS branches are dominant (71.6%), while there is only a small percentage (3.83%) of conserved families. In contrast, the conserved families are dominant in51randomly selected house-keeping genes (96.1%). The opposite patterns between NBS and the other gene groups suggest that natural selection is responsible for the drastic number variation of NBS genes. The rapid expansion and/or contraction may be a fundamentally important strategy for a species to adapt to the quickly changing species-speciWc pathogen spectrum. In addition, the small proportion of conserved NBSs suggests that the loss of NBSs may be a general tendency in grass species.