Adaptive response of Pinus monticola driven by positive selection upon resistance gene analogs (RGAs) of the TIR-NBS-LRR subfamily

Abstract: Western white pine(Pinus monticola Dougl.) is an important forest tree species, which is intensively plagued by the fungus Cronartium ribicola. Resistance gene analogs (RGAs) are the most highly abundant class of potential resistance ( R) genessharing greatly conserved domains and structures. Hence RGAs are crucial components for disease resistance breeding programson P. monticola serving as useful functional markers. A total of 33 P. monticola RGAs gene homologues were mined from GenBank, encoding for R genemembers of the TIR-NBS-LRR subfamily. The existence of positive selection acting upon RGAs was determined using a series of maximum likelihood analyses. Robust evidence of positive selection was showed to be acting widely in three clades across RGA gene phylogeny, both on terminal and ancestral lineages. Furthermore, our analysis revealed that the majority of positively selected residues sites are localized widely across these RGAs sequences, putatively affecting the structures of their ligand-binding domains and offering novel specificities. These results may find immediate application in ongoing disease resistance breeding programs.