Homologous recombination and the invasion of a new plant host by the pathogenic bacterium, Xylella fastidiosa
L. NUNNEY (1)
(1) University of California Riverside, Riverside, CA, U.S.A.
Phytopathology 101:S130

Homologous recombination plays an important role in the structuring of genetic variation of many bacteria; however its importance in pathogen evolution has yet to be established. We investigated the involvement of recombination in the shift to a novel host (mulberry) by the plant pathogenic bacterium Xylella fastidiosa. X. fastidiosa infects xylem and causes leaf scorch diseases in many economically important plant species, including Pierce’s disease of grapevines. Mulberry leaf scorch was identified about 25 years ago in the eastern U.S. and since that time it has spread to California. Previous genetic analysis separated the mulberry isolates from the 4 recognized subspecies. Comparison of a newly sequenced genome of a mulberry strain with pre-existing genome data showed that this form originated by massive recombination between two of the subspecies, Xylella fastidiosa subsp. fastidiosa and Xylella fastidiosa subsp. multiplex, resulting in a genome consisting of roughly an equal mix of the two subspecies. The extensive recombination involved in the origin of the mulberry type, combined with a very low level of within-type genetic variation, suggests the host shift was achieved after strong selection acted on genetic variants created by inter-subspecific homologous recombination. These data show that the invasion of mulberry by X. fastidiosa provides a compelling example of the importance of recombination in the shift of a pathogen to a new plant host.