P-256 - Gene flow strongly contributes to local adaptation to temperature in a plant pathogen

There is a pressing need to understand range shifts of plant pathogens under a changing climate in order to safeguard biodiversity and food security. Changing seasonal temperatures will have very strong effects on pathogen fitness, both directly and indirectly through modifying temperature-dependent plant responses. Bacterial pathogens can adapt to novel climatic conditions through mutation or gene flow. Xylella fastidiosa subsp. fastidiosa (Xff) is a naturally competent bacterium and the causal agent of Pierce’s disease of grapevine, among other plant diseases. In this study, we investigate the Northward spread of Xff in California using landscape genomics methods and >188 whole genomes across a latitudinal gradient. We found evidence of both winter and summer temperature-associated genetic variation, underscoring that distinct populations are locally adapted. Almost all (209 / 213) adaptive variants were the result of gene flow through homologous recombination. In addition, we identified that many regions of adaptive gene flow for summer temperatures were gained from a related Xylella subspecies (multiplex) with an overlapping range in California. This study provides an example of how intersubspecific gene flow can help bacterial pathogens adapt to seasonal temperatures. These findings have strong applications in understanding how co-circulating plant pathogens will adapt to future temperature shifts.