The obligate intracellular bacterium Wolbachia pipientis infects around 20% of all insect species. It is maternally inherited and induces reproductive alterations of insect populations by male killing, feminization, parthenogenesis, or cytoplasmic incompatibility. Here, we present the 1,445,873-bp genome of W. pipientis strain wRi that induces very strong cytoplasmic incompatibility in its natural host Drosophila simulans. A comparison with the previously sequenced genome of W. pipientis strain wMel from Drosophila melanogaster identified 35 breakpoints associated with mobile elements and repeated sequences that are stable in Drosophila lines transinfected with wRi. Additionally, 450 genes with orthologs in wRi and wMel were sequenced from the W. pipientis strain wUni, responsible for the induction of parthenogenesis in the parasitoid wasp Muscidifurax uniraptor. The comparison of these A-group Wolbachia strains uncovered the most highly recombining intracellular bacterial genomes known to date. This was manifested in a 500-fold variation in sequence divergences at synonymous sites, with different genes and gene segments supporting different strain relationships. The substitution-frequency profile resembled that of Neisseria meningitidis, which is characterized by rampant intraspecies recombination, rather than that of Rickettsia, where genes mostly diverge by nucleotide substitutions. The data further revealed diversification of ankyrin repeat genes by short tandem duplications and provided examples of horizontal gene transfer across A-and B-group strains that infect D. simulans. These results suggest that the transmission dynamics of Wolbachia and the opportunity for coinfections have created a freely recombining intracellular bacterial community with mosaic genomes.horizontal transfer ͉ recombination ͉ ankyrin repeat gene ͉ genome evolution ͉ insect symbiosis Wolbachia pipientis are intracellular ␣-proteobacteria of the order Rickettsiales that infect insects as well as isopods, spiders, scorpions, mites, and filarial nematodes (1, 2). These bacteria represent a single species, with strains classified into supergroups, of which the most abundant are supergroups A and B.

Wolbachia strains are endosymbiotic bacteria typically found in the reproductive tracts of arthropods. These bacteria manipulate host reproduction to ensure maternal transmission. They are usually transmitted vertically, so it has been predicted that they have evolved a mechanism to target the host's germ cells during development. Through cytological analysis we found that Wolbachia strains display various affinities for the germ line of Drosophila. Different Wolbachia strains show posterior, anterior, or cortical localization in Drosophila embryos, and this localization is congruent with the classification of the organisms based on the wsp (Wolbachia surface protein) gene sequence. This embryonic distribution pattern is established during early oogenesis and does not change until late stages of embryogenesis. The posterior and anterior localization of Wolbachia resembles that of oskar and bicoid mRNAs, respectively, which define the anterior-posterior axis in the Drosophila oocyte. By comparing the properties of a single Wolbachia strain in different host backgrounds and the properties of different Wolbachia strains in the same host background, we concluded that bacterial factors determine distribution, while bacterial density seems to be limited by the host. Possible implications concerning cytoplasmic incompatibility and evolution of strains are discussed.

Wolbachia-induced cytoplasmic incompatibility (CI) is expressed when infected males are crossed with either uninfected females or females infected with Wolbachia of different CI specificity. In diploid insects, CI results in embryonic mortality, apparently due to the the loss of the paternal set of chromosomes, usually during the first mitotic division. The molecular basis of CI has not been determined yet; however, several lines of evidence suggest that Wolbachia exhibits two distinct sex-dependent functions: in males, Wolbachia somehow ''imprints'' the paternal chromosomes during spermatogenesis (mod function), whereas in females, the presence of the same Wolbachia strain(s) is able to restore embryonic viability (resc function). On the basis of the ability of Wolbachia to induce the modification and/or rescue functions in a given host, each bacterial strain can be classified as belonging in one of the four following categories:, and mod 1 resc À . A so-called ''suicide'' mod 1 resc À strain has not been found in nature yet. Here, a combination of embryonic cytoplasmic injections and introgression experiments was used to transfer nine evolutionary, distantly related Wolbachia strains (wYak, wTei, wSan, wRi, wMel, wHa, wAu, wNo, and wMa) into the same host background, that of Drosophila simulans (STCP strain), a highly permissive host for CI expression. We initially characterized the modification and rescue properties of the Wolbachia strains wYak, wTei, and wSan, naturally present in the yakuba complex, upon their transfer into D. simulans. Confocal microscopy and multilocus sequencing typing (MLST) analysis were also employed for the evaluation of the CI properties. We also tested the compatibility relationships of wYak, wTei, and wSan with all other Wolbachia infections. So far, the cytoplasmic incompatibility properties of different Wolbachia variants are explained assuming a single pair of modification and rescue factors specific to each variant. This study shows that a given Wolbachia variant can possess multiple rescue determinants corresponding to different CI systems. In addition, our results: (a) suggest that wTei appears to behave in D. simulans as a suicide mod 1 resc À strain, (b) unravel unique CI properties, and (c) provide a framework to understand the diversity and the evolution of new CI-compatibility types. WOLBACHIA is a group of maternally transmitted intracellular bacteria that infect numerous arthropod as well as filarial nematode species (Werren 1997;Bandi et al. 1998;Stouthamer et al. 1999). In arthropod hosts, Wolbachia mainly reside in ovaries and testes. In many cases, they manipulate host reproduction to ensure their own transmission by inducing feminization (Rigaud 1997), thelytokous parthenogenesis (Huigens and Stouthamer 2003), male killing (Hurst et al. 2003) and, most commonly, cytoplasmic incompatibility (CI) . In diploid species, CI is expressed as embryonic lethality of the progeny of a male infected by one (or more) Wolbachia strain(s) and a female that either is u...

The growth and distribution of the intracellular microbe Wolbachia pipientis during spermatogenesis in several different host/symbiont genetic combinations in Drosophila melanogaster and Drosophila simulans is described. Considerable intra- and inter-strain variation in Wolbachia density and tissue distribution was observed. Wolbachia were found inside spermatocytes and spermatids or within the somatic cyst cells surrounding the germ cells. Some strains displayed both tissue distributions. High rates of cytoplasmic incompatibility (CI) are correlated with high levels of Wolbachia only when spermatocytes and/or spermatids harbor the microbe. Wolbachia infection of somatic cyst cells, although sometimes present at high levels, did not result in significant CI expression. CI-inducing Wolbachia strains within D. simulans showed no distinguishable differences in distribution or density within infected spermatids. To dissect the relative contribution of host and symbiont to the expression of CI, Wolbachia from various host strains known to exhibit varying levels of CI were introgressed into new uninfected host genetic backgrounds. These introgression experiments confirm that the mod(+)/mod(-) phenotype is an intrinsic Wolbachia trait and is not determined by host factors. The level of sperm modification in those lines harboring Wolbachia capable of modifying sperm, however, is influenced by host genetic background. These results form the basis of the Wolbachia Infected Spermatocyte/Spermatid Hypothesis (WISSH). According to WISSH, Wolbachia infection in spermatocytes and then spermatids during sperm development is required for CI expression.

Wolbachia is a cytoplasmically inherited alpha-proteobacterium found in a wide range of host arthropod and nematode taxa. Wolbachia infection in Drosophila is closely associated with the expression of a unique form of post-fertilization lethality termed cytoplasmic incompatibility (CI). This form of incompatibility is only expressed by infected males suggesting that Wolbachia exerts its effect during spermatogenesis. The growth and distribution of Wolbachia throughout sperm development in individual spermatocysts and elongating sperm bundles is described. Wolbachia growth within a developing cyst seems to begin during the pre-meiotic spermatocyte growth phase with the majority of bacteria accumulating during cyst elongation. Wolbachia are predominantly localized in the proximal end of the immature cyst, opposite the spermatid nuclei, and throughout development there appears little movement of Wolbachia between spermatids via the connecting cytoplasmic bridges. The overall number of new cysts infected as well as the number of spermatids/cysts infected seems to decrease with age and corresponds to the previously documented drop in CI with age. In contrast, in one CI expressing line of Drosophila melanogaster, fewer cysts are infected and a much greater degree of variation in numbers is observed between spermatids. Furthermore, the initiation and extent of the fastest period of Wolbachia growth in the D. melanogaster strain lags behind that of Drosophila simulans. The possible implications on the as yet unexplained mechanism of CI are discussed.