Widespread recombination throughout Wolbachia genomes

Evidence is growing that homologous recombination is a powerful source of genetic variability among closely related free-living bacteria. Here we investigate the extent of recombination among housekeeping genes of the endosymbiotic bacteria Wolbachia. Four housekeeping genes, gltA, dnaA, ftsZ, and groEL, were sequenced from a sample of 22 strains belonging to supergroups A and B. Sequence alignments were searched for recombination within and between genes using phylogenetic inference, analysis of genetic variation, and four recombination detection programs (MaxChi, Chimera, RDP, and Geneconv). Independent analyses indicate no or weak intragenic recombination in ftsZ, dnaA, and groEL. Intragenic recombination affects gltA, with a clear evidence of horizontal DNA transfers within and between divergent Wolbachia supergroups. Intergenic recombination was detected between all pairs of genes, suggesting either a horizontal exchange of a genome portion encompassing several genes or multiple recombination events involving smaller tracts along the genome. Overall, the observed pattern is compatible with pervasive recombination. Such results, combined with previous evidence of recombination in a surface protein, phage, and IS elements, support an unexpected chimeric origin of Wolbachia strains, with important implications for Wolbachia phylogeny and adaptation of these obligate intracellular bacteria in arthropods.