Exploring the phenotypic and genetic diversity of a bacterial plant pathogen, the Ralstonia solanacearum species complex (RSSC)

The Ralstonia solanacearum species complex (RSSC) is a globally distributed bacterial plant pathogen and the causative agent of bacterial wilt disease. This pathogen can infect over 175 plant species, including many important crops, having huge impacts on agriculture. While the genetic and phenotypic diversity of this pathogen species has been explored, especially when concerning pathogenicity, knowledge of the phenotypic diversity using a broad range of ecologically relevant phenotypes is lacking. This thesis uses a combination of comparative analysis, high throughput phenotyping, and experimental evolution to explore the ecological diversity among the RSSC, progressing our understanding at the global (chapter 2), population (chapter 3), and isolate level (chapter 4). Potential causes of this trait variation were also explored, and phenotypic variation was linked with genomic data to reveal genetic mechanisms underpinning traits. Collecting 46 ecologically relevant traits across a collection of RSSC strains revealed that the local environment selects for similar ecological differences within two RSSC species (R. pseudosolanacearum and R. solanacearum) in chapter 2. In contrast, a lack of genetic variation was observed among a UK population of R. solanacearum despite significant phenotypic differences between isolates in chapter 3. Evolving one UK R. solanacearum isolate to different abiotic environmental stress conditions within the laboratory (chapter 4) revealed that exposure to stresses increases RSSC diversity, driven by the negative trait correlations observed between the different stresses and metabolic capacity. Furthermore, insertion sequence (IS) movement was found to cause adaptation to environmental stress conditions within the lab, potentially explaining the lack of genetic variation observed within the UK population. A genome-wide association study (GWAS) also revealed genes associated with cold tolerance and rifampicin resistance within the RSSC. Overall, this thesis provides a better understanding of RSSC ecological diversity which can improve our knowledge of the epidemiology of this plant pathogen.