Large-Scale Intersubspecific Recombination in the Plant-Pathogenic Bacterium Xylella fastidiosa Is Associated with the Host Shift to Mulberry

Homologous recombination plays an important role in the structuring of genetic variation of many bacteria; however, its importance in adaptive evolution is not well established. We investigated the association of intersubspecific homologous recombination (IHR) with the shift to a novel host (mulberry) by the plant-pathogenic bacteriumXylella fastidiosa . Mulberry leaf scorch was identified about 25 years ago in native red mulberry in the eastern United States and has spread to introduced white mulberry in California. Comparing a sequence of 8 genes (4,706 bp) from 21 mulberry-type isolates to published data (352 isolates representing all subspecies), we confirmed previous indications that the mulberry isolates define a group distinct from the 4 subspecies, and we propose naming the taxonX. fastidiosa subsp.morus . The ancestry of its gene sequences was mixed, with 4 derived fromX. fastidiosa subsp.fastidiosa (introduced from Central America), 3 fromX. fastidiosa subsp.multiplex (considered native to the United States), and 1 chimeric, demonstrating that this group originated by large-scale IHR. The very low within-type genetic variation (0.08% site polymorphism), plus the apparent inability of nativeX. fastidiosa subsp.multiplex to infect mulberry, suggests that this host shift was achieved after strong selection acted on genetic variants created by IHR. Sequence data indicate that a single ancestral IHR event gave rise not only toX. fastidiosa subsp.morus but also to theX. fastidiosa subsp.multiplex recombinant group which infects several hosts but is the only type naturally infecting blueberry, thus implicating this IHR in the invasion of at least two novel native hosts, mulberry and blueberry.