Reduced genetic variation occurs among genes of the highly clonal plant pathogen Xanthomonas axonopodis pv. vesicatoria, including the effector gene avrBs2

The bacterial plant pathogen Xanthomonas axonopodis pv. vesicatoria, also known as Xanthomonas campestris pv. vesicatoria group A, is the causal agent of bacterial spot in pepper and tomato. In order to test different models that may explain the coevolution of avrBs2 with its host plants, we sequenced avrBs2 and six chromosomal loci (total of 5.5 kb per strain) from a global sample of 55 X. axonopodis pv. vesicatoria strains collected from diseased peppers. We found an extreme lack of genetic variation among all X. axonopodis pv. vesicatoria genomic loci (average nucleotide diversity, π = 9.1 × 10^-5), including avrBs2. This lack of diversity is consistent with X. axonopodis pv. vesicatoria having undergone a recent population bottleneck and/or selective sweep followed by population expansion. Coalescent analysis determined that approximately 1.4 × 10⁴ to 7.16 × 10⁴ bacterial generations have passed since the most recent common ancestor (MRCA) of the current X. axonopodis pv. vesicatoria population. Assuming a range of 50 to 500 bacterial generations per year, only 28 to 1,432 years have passed since the MRCA. This time frame coincides with human, intervention with the pathogen's host plants, from domestication to modern agricultural practices. Examination of 19 mutated (loss-of-function) avrBs2 alleles detected nine classes of mutations. All mutations affected protein coding, while no synonymous changes were found. The nature of at least one of the avrBs2 mutations suggests that it may be possible to observe one stage of an evolutionary arms race as X. axonopodis pv. vesicatoria responds to selection pressure to alter avrBs2 to escape host plant resistance.