%D 2011 %0 In-Process %T Adaptive evolution of Xa21 homologs in Gramineae . %J Genetica %V 139 %P 1465-75 %A Tan S %A Wang D %A Ding J %A Tian D %A Zhang X %A Yang S %M pub22451352 %X The XA21 protein has broad spectrum resistance against Xanthomonas oryzae pv . oryzae . Although Xa21-mediated immunity is well characterized , little is known about the origin and evolutionary history of this gene in grasses . Therefore , we analyzed all Xa21 gene homologs in eight whole-genome sequenced rice lines , as well as in four gramineous genomes , rice , Brachypodium , sorghum and maize ; using Arabidopsis Xa21 homologs as outgroups , 17 , 7 , 7 and 3 Xa21 homologs were detected in these four grasses , respectively . Synteny and phylogenetic analysis showed that frequent gene translocation , duplication and/or loss , have occurred at Xa21 homologous loci , suggesting that they have undergone or are undergoing rapid generation of copy number variations . Within the rice species , the high level of nucleotide diversity between Xa21-like orthologs showed a strong association with the presence/absence haplotypes , suggesting that the genetic structure of rice lines plays an important role in the variations between these Xa21-like orthologs . Strongly positive selection was detected in the core region of the leucine-rich repeat domains of the Xa21 subclade among the rice lines , indicating that the rapid gene diversification of Xa21 homologs may be a strategy for a given species to adapt to the changing spectrum of species-specific pathogens .