Complete Genome Sequences of Three Xanthomonas citri Strains from Texas

A network approach to decipher the dynamics of Lysobacteraceae plasmid gene sharing

Plasmids provide an efficient vehicle for gene sharing among bacterial populations, playing a key role in bacterial evolution. Network approaches are particularly suitable to represent multipartite relationships and are useful tools to characterize plasmid-mediated gene sharing events. The Lysobacteraceae bacterial family gathers plant commensal, plant pathogenic and opportunistic human pathogens for which plasmid mediated adaptation was reported. We searched for homologues of plasmid gene sequences from this family in all the diversity of available bacterial genome sequences and built a network of plasmid gene sharing from the results. While plasmid genes are openly shared between the bacteria of the Lysobacteraceae family, taxonomy strongly defined the boundaries of these exchanges, that only barely reached other families. Most inferred plasmid gene sharing events involved a few genes only, and evidence of full plasmid transfers were restricted to taxonomically close taxon. We detected multiple plasmid-chromosome gene transfers, among which the otherwise known sharing of a heavy metal resistance transposon. In the network, bacterial lifestyles shaped sub-structures of isolates colonizing specific ecological niches and harboring specific types of resistance genes. Genes associated to pathogenicity or antibiotic and metal resistance were among those that most importantly structured the network, highlighting the imprints of human-mediated selective pressure on pathogenic populations. A massive sequencing effort on environmental Lysobacteraceae is therefore required to refine our understanding on how this reservoir fuels the emergence and the spread of genes amongst this family and its potential impact on plant, animal and human health.

Draft Genome Sequences of 284 Xanthomonas citri pv. citri Strains Causing Asiatic Citrus Canker

High-quality Illumina assemblies were produced from 284 Xanthomonas citri pv. citri pathotype A strains mostly originating from the Southwest Indian Ocean region, a subset of which was also sequenced using MinION technology. Some strains hosted chromosomally encoded transcription activator-like effector (TALE) genes, an atypical feature for this bacterium.

Time-calibrated genomic evolution of a monomorphic bacterium during its establishment as an endemic crop pathogen

Horizontal gene transfer is of major evolutionary importance as it allows for the redistribution of phenotypically important genes among lineages. Such genes with essential functions include those involved in resistance to antimicrobial compounds and virulence factors in pathogenic bacteria. Understanding gene turnover at microevolutionary scales is critical to assess the pace of this evolutionary process. Here, we characterized and quantified gene turnover for the epidemic lineage of a bacterial plant pathogen of major agricultural importance worldwide. Relying on a dense geographic sampling spanning 39 years of evolution, we estimated both the dynamics of single nucleotide polymorphism accumulation and gene content turnover. We identified extensive gene content variation among lineages even at the smallest phylogenetic and geographic scales. Gene turnover rate exceeded nucleotide substitution rate by three orders of magnitude. Accessory genes were found preferentially located on plasmids, but we identified a highly plastic chromosomal region hosting ecologically important genes such as transcription activator-like effectors. Whereas most changes in the gene content are probably transient, the rapid spread of a mobile element conferring resistance to copper compounds widely used for the management of plant bacterial pathogens illustrates how some accessory genes can become ubiquitous within a population over short timeframes.

Origin and diversification of Xanthomonas citri subsp. citri pathotypes revealed by inclusive phylogenomic, dating, and biogeographic analyses

BACKGROUND

Xanthomonas citri subsp. citri pathotypes cause bacterial citrus canker, being responsible for severe agricultural losses worldwide. The A pathotype has a broad host spectrum, while A* and Aw are more restricted both in hosts and in geography. Two previous phylogenomic studies led to contrasting well-supported clades for sequenced genomes of these pathotypes. No extensive biogeographical or divergence dating analytic approaches have been so far applied to available genomes.

RESULTS

Based on a larger sampling of genomes than in previous studies (including six new genomes sequenced by our group, adding to a total of 95 genomes), phylogenomic analyses resulted in different resolutions, though overall indicating that A + AW is the most likely true clade. Our results suggest the high degree of recombination at some branches and the fast diversification of lineages are probable causes for this phylogenetic blurring effect. One of the genomes analyzed, X. campestris pv. durantae, was shown to be an A* strain; this strain has been reported to infect a plant of the family Verbenaceae, though there are no reports of any X. citri subsp. citri pathotypes infecting any plant outside the Citrus genus. Host reconstruction indicated the pathotype ancestor likely had plant hosts in the family Fabaceae, implying an ancient jump to the current Rutaceae hosts. Extensive dating analyses indicated that the origin of X. citri subsp. citri occurred more recently than the main phylogenetic splits of Citrus plants, suggesting dispersion rather than host-directed vicariance as the main driver of geographic expansion. An analysis of 120 pathogenic-related genes revealed pathotype-associated patterns of presence/absence.

CONCLUSIONS

Our results provide novel insights into the evolutionary history of X. citri subsp. citri as well as a sound phylogenetic foundation for future evolutionary and genomic studies of its pathotypes.