Analysis of the speciation process suggests a dual lifestyle in the plant pathogen Ralstonia solanacearum species complex

Ralstonia solanacearum species complex (RSSC) is the causal agent of bacterial wilt disease in many plant species, including important crops. It is composed of a heterogeneous group of strains whose phylogeny indicates four main groups (phylotypes) which have been re-arranged into three different species: R. pseudosolanacearum, R. solanacearum, and R. syzygii. Strains belonging to the four phylotypes were isolated from different continents indicating that geographic isolation and spatial distance have structured the populations and shaped the evolving process to become different species (allopatric speciation). RSSC strains can also live in soil and the surrounding water. Soil bacteria adapt to new environmental conditions through genome changes which are facilitated by the acquisition of genetic material through horizontal gene transfer. These genetic changes and ecological fit may drive bacterial evolution following sympatric speciation. Here, comparative genomic tools were used to investigate the speciation process of RSSC by studying the essential and flexible gene content and the openness of the pangenome of 314 genomic sequences. The results indicate that the vast majority of genes in RSSC pangenome are dispensable with only 228 essential genes. The pangenome of RSSC is open, which is characteristic of sympatric species. The R. pseudosolanacearum, R. solanacearum, and R. syzygii pangenomes are also open, however, the pangenome of R. syzygii is near to turn closed. Phylogenetic and statistical evidence reported in the literature and the results obtained in this work suggest that both allopatric and sympatric lifestyles have shaped the speciation process of RSSC.