Ancestral acquisitions, gene flow and multiple evolutionary trajectories of the type three secretion system and effectors in Xanthomonas plant pathogens

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英语. <p>Deciphering the evolutionary history and transmission patterns of virulence determinants<br />is necessary to understand the emergence of novel pathogens. The main virulence<br />determinant of most pathogenic proteobacteria is the type three secretion<br />system (T3SS). The Xanthomonas genus includes bacteria responsible for numerous<br />epidemics in agroecosystems worldwide and represents a major threat to plant<br />health. The main virulence factor of Xanthomonas is the Hrp2 family T3SS; however,<br />this system is not conserved in all strains and it has not been previously determined<br />whether the distribution of T3SS in this bacterial genus has resulted from losses or<br />independent acquisitions. Based on comparative genomics of 82 genome sequences<br />representing the diversity of the genus, we have inferred three ancestral acquisitions<br />of the Hrp2 cluster during Xanthomonas evolution followed by subsequent<br />losses in some commensal strains and re-acquisition in some species. While mutation<br />was the main force driving polymorphism at the gene level, interspecies homologous<br />recombination of large fragments expanding through several genes shaped<br />Hrp2 cluster polymorphism. Horizontal gene transfer of the entire Hrp2 cluster also<br />occurred. A reduced core effectome composed of xopF1, xopM, avrBs2 and xopR<br />was identified that may allow commensal strains overcoming plant basal immunity.<br />In contrast, stepwise accumulation of numerous type 3 effector genes was shown in<br />successful pathogens responsible for epidemics. Our data suggest that capacity to<br />intimately interact with plants through T3SS would be an ancestral trait of xanthomonads.<br />Since its acquisition, T3SS has experienced a highly dynamic evolutionary<br />history characterized by intense gene flux between species that may reflect its<br />role in host adaptation.</p>