Orthology-Based Estimate of the Contribution of Horizontal Gene Transfer from Distantly Related Bacteria to the Intraspecific Diversity and Differentiation of Xylella fastidiosa.

Giuseppe Firrao,Marco Scortichini,Laura Pagliari

doi:10.3390/pathogens10010046

Abstract

Xylella fastidiosa is a xylem-limited bacterium phylogenetically related to the xanthomonads, with an unusually large and diversified range of plant hosts. To ascertain the origin of its peculiarities, its pan-genome was scanned to identify the genes that are not coherent with its phylogenetic position within the order Xanthomonadales. The results of the analysis revealed that a large fraction of the genes of the Xylella pan-genome have no ortholog or close paralog in the order Xanthomonadales. For a significant part of the genes, the closest homologue was found in bacteria belonging to distantly related taxonomic groups, most frequently in the Betaproteobacteria. Other species, such as Xanthomonas vasicola and Xanthomonas albilineans which were investigated for comparison, did not show a similar genetic contribution from distant branches of the prokaryotic tree of life. This finding indicates that the process of acquisition of DNA from the environment is still a relevant component of Xylella fastidiosa evolution. Although the ability of Xylella fastidiosa strains to recombine among themselves is well known, the results of the pan-genome analyses stressed the additional relevance of environmental DNA in shaping their genomes, with potential consequences on their phytopathological features.

Highlights

The host range of Xylella fastidiosa is unusually large among plant pathogenic bacteria, and comprises 563 plant species grouped into 82 botanical families, including many of socioeconomic interest [1]
Each putative protein-encoding gene of the pan-genome of Xylella fastidiosa and, for comparison, of the pan-genomes of Xanthomonas vasicola and Xanthomonas albilineans, was investigated to ascertain whether or not it shares a common ancestry with genes of other members of the order Xanthomonadales, a taxon that includes all three named species
We confirmed the large size of the accessory genomes, but we showed that the gene reservoir accessible to Xanthomonas vasicola is primarily contained within the order Xanthomonadales

Summary

Introduction

Xylella fastidiosa is a Gram-stain-negative, xylem-limited, vector-borne, nutritionally fastidious phytopathogen that can colonize many wild and cultivated plant species. The host range of Xylella fastidiosa is unusually large among plant pathogenic bacteria, and comprises 563 plant species grouped into 82 botanical families, including many of socioeconomic interest (i.e., grapevine, citrus, coffee, and olive trees) [1]. Only a limited number of infected plant species shows the typical wilting of leaves, twigs and branches and the subsequent plant death, caused by the pathogen-induced blockage of xylem vessels [2]. The currently available data suggest that Xylella fastidiosa is associated with a large number of plant species as a commensalist, but only particular clades -or even genotypes- are responsible for a small number of plant diseases [3,4,5]

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Conclusion