Genomic footprint of a shared Type 5 prophage in “Candidatus Liberibacter asiaticus” and “Candidatus Liberibacter africanus,” two destructive bacterial pathogens of citrus Huanglongbing

TL;DR

This study identifies a new shared prophage in two citrus disease-causing bacteria, expanding our understanding of their evolution and genomic diversity.

Contribution

The first evidence of a shared Type 5 prophage between CLas and CLaf, indicating horizontal gene transfer and positive selection.

Findings

A new CLas Type 5 prophage was identified and characterized in Réunion.

85.7% of CLas and all CLaf strains tested contained the Type 5-like prophage.

The prophage suggests horizontal gene transfer and co-evolution among HLB-associated bacteria.

Abstract

Huanglongbing (HLB) is a bacterial disease that affects citrus trees and is considered the most severe citrus disease in the world. The three HLB-associated “Ca. Liberibacter” species harbor prophage regions which have been described to play critical roles in their evolution and biology. In this study, using infected insect vectors, we assembled and characterized the accessory genome of the first circular de novo “Candidatus Liberibacter asiaticus” (CLas) assembly (V1R1) from Réunion, one of the sparse areas worldwide hosting CLas and “Ca. Liberibacter africanus” (CLaf). This 1.272 Mb-long whole-genome harbored 1,129 coding sequences and two complete prophages, including a 37,934 bp-long Type 1 prophage, frequently present in CLas genomes, and a 40,501 bp-long undescribed CLas prophage designated as P-V1R1-5. Comparative genomic approaches suggested that P-V1R1-5 has all the genetic elements to produce new viral particles and revealed that it likely belongs to a new CLas Type 5 prophage. While P-V1R1-5-like prophages had been identified previously in CLaf genomes, our study demonstrates the first lysogenic conversion of these prophages in CLas, suggesting a positive selection of prophage-like sequences among HLB-associated “Ca. Liberibacter” species. Using PCR amplifications targeting P-V1R1-5, we also showed that 85.7% of the tested CLas strains from Réunion and all tested CLaf strains from Madagascar and Réunion harbored a Type 5-like prophage. The identification of CLas Type 5-like prophages not only expanded our knowledge of CLas genomic diversity in Réunion but also provided new insights into the role of horizontally transferred elements in the evolution of the sympatric HLB-associated bacteria.

Huanglongbing (HLB) is the most severe citrus disease worldwide. The disease is associated with three “Candidatus Liberibacter” species harboring prophage regions that have been described to play critical roles in their biology. In this study, using infected insect vectors, we assembled and characterized the accessory genome of the first circular de novo “Ca. Liberibacter asiaticus” (CLas) assembly from Réunion, one of the very few areas in the world where both of CLas and “Ca. Liberibacter africanus” (CLaf) coexist. Comparative genomic approaches demonstrated that this genome harbored two complete prophages, including a new CLas Type 5 prophage that was previously identified in CLaf but was reported for the first time in a CLas strain. This first evidence of shared prophage-like sequences among HLB-associated species expands our knowledge of CLas genomic diversity but also provides new insights into the role of the accessory genome in the evolution of these bacteria.

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