Lateral phage transfer in obligate intracellular bacteria (wolbachia): verification from natural populations

Abstract

Lateral transfer of mobile DNA is a hallmark of bacteria with a free-living replicative stage; however, its significance in obligate intracellular bacteria and other heritable endosymbionts remains controversial. Comparative sequence analyses from laboratory stocks infected with Wolbachia pipientis provide some of the most compelling evidence that bacteriophage WO-B transfers laterally between infections of the same insect host. Lateral transfer between coinfections, however, has been evaluated neither in natural populations nor between closely related Wolbachia strains. Here, we analyze bacterial and phage genes from two pairs of natural sympatric field isolates, of Gryllus pennsylvanicus field crickets and of Neochlamisus bebbianae leaf beetles, to demonstrate WO-B transfers between supergroup B Wolbachia. N. bebbianae revealed the highest number of phage haplotypes yet recorded, hinting that lab lines could underestimate phage haplotype variation and lateral transfer. Finally, using the approximate age of insect host species as the maximum available time for phage transfer between host-associated bacteria, we very conservatively estimate phage WO-B transfer to occur at least once every 0-5.4 My within a host species. Increasing discoveries of mobile elements, intragenic recombination, and bacterial coinfections in host-switching obligate intracellular bacteria specify that mobile element transfer is common in these species.

FIG. 1.

ML phylogeny based on a concatenated nucleotide sequence alignment of ftsZ, gatB, and hcpA Wolbachia MLST loci. The tree is midpoint rooted, and numbers denote ML bootstrap values and Bayesian posterior probabilities (in bold). Taxa are denoted by host arthropod species and Wolbachia strain. Field specimens analyzed in this study are shown in shaded boxes. Wolbachia supergroup A and B clades are indicated to the right of each tree.

FIG. 2.

ML phylogeny of the bacteriophage ORF7 nucleotide sequences. The tree is midpoint rooted, and numbers denote ML bootstrap values and Bayesian posterior probabilities (in bold). Taxa names denote host arthropod species, Wolbachia strain, and ORF7 haplotype. Pairs of haplotypes in shaded boxes illustrate putative lateral transfer events in field specimens collected in this study. Bold taxa represent previously published examples of lateral transfer through laboratory segregation experiments. The three group labels (I–III) were assigned according to the nomenclature in Bordenstein and Wernegreen (2004).