Distinct Patterns of Natural Selection Determine Sub-population Structure in the Fire Blight Pathogen, Erwinia Amylovora

The fire blight pathogen, Erwinia amylovora (EA), causes significant economic losses in rosaceae fruit crops. Recent genome sequencing efforts have explored genetic variation, population structure, and virulence levels in EA strains. However, the genomic aspects of population bottlenecks and selection pressure from geographical isolation, host range, and management practices are yet unexplored. We conducted a comprehensive analysis of whole genome sequences of 41 strains to study genetic diversity, population structure, and the nature of selection affecting sub-population differentiation in EA. We detected 72,741 SNPs and 2,500 Indels, representing about six-fold more diversity than previous reports. Moreover, nonsynonymous substitutions were identified across the effector regions, suggesting a role in defining virulence of specific strains. EA plasmids had more diversity than the chromosome sequence. Population structure analysis identified three distinct sub-groups in EA strains, with North American strains displaying highest genetic diversity. A five kilobase genomic window scan showed differences in genomic diversity and selection pressure between these three sub-groups. This analysis also highlighted the role of purifying and balancing selection in shaping EA genome structure. Our analysis provides novel insights into the genomic diversity and selection forces accompanying EA population differentiation.