Genome-wide Comparative Analysis Of NBS-encoding Genes In Rosaceae And Cucurbitaceae

Nucleotide binding site leucine-rich repeats(NBS-LRR)genes encode a large class of disease resistance(R)proteins in plants.Extensive studies have been carried out to identify and investigate NBS-encoding gene families in many important plant species.However,no comprehensive research into NBS-encoding genes in Rosaceae and Cucurbitaceae has been performed.In this study,five whole-genome sequenced Rosaceae species,including apple,pear,peach,mei,and strawberry,were analyzed to investigate the evolutionary pattern of NBS-encoding genes and to compare them to those of three Cucurbitaceae species,cucumber,melon,and watermelon.Considerable differences in the copy number of NBS-encoding genes were observed between Cucurbitaceae and Rosaceae species.In Rosaceae species,a large number and a high proportion of NBS-encoding genes were observed in peach(437,1.52%),mei(475,1.51%),strawberry(346,1.05%)and pear(617,1.44%),and apple contained a whopping 1303(2.05%)NBS-encoding genes,which might be the highest number of R-genes in all of these reported diploid plant.However,no more than 100 NBS-encoding genes were identified in Cucurbitaceae.Many more species-specific gene families were classified and detected with the signature of positive selection in Rosaceae species,especially in the apple genome.Taken together,our findings indicate that NBS-encoding genes in Rosaceae,especially in apple,have undergone extreme expansion and rapid adaptive evolution.Useful information was provided for further research on the evolutionary mode of disease resistance genes in Rosaceae crops.