As an obligatory parasite of humans, the body louse ( Pediculus humanus humanus ) is an important vector for human diseases, including epidemic typhus, relapsing fever, and trench fever. Here, we present genome sequences of the body louse and its primary bacterial endosymbiont Candidatus Riesia pediculicola. The body louse has the smallest known insect genome, spanning 108 Mb. Despite its status as an obligate parasite, it retains a remarkably complete basal insect repertoire of 10,773 protein-coding genes and 57 microRNAs. Representing hemimetabolous insects, the genome of the body louse thus provides a reference for studies of holometabolous insects. Compared with other insect genomes, the body louse genome contains significantly fewer genes associated with environmental sensing and response, including odorant and gustatory receptors and detoxifying enzymes. The unique architecture of the 18 minicircular mitochondrial chromosomes of the body louse may be linked to the loss of the gene encoding the mitochondrial single-stranded DNA binding protein. The genome of the obligatory louse endosymbiont Candidatus Riesia pediculicola encodes less than 600 genes on a short, linear chromosome and a circular plasmid. The plasmid harbors a unique arrangement of genes required for the synthesis of pantothenate, an essential vitamin deficient in the louse diet. The human body louse, its primary endosymbiont, and the bacterial pathogens that it vectors all possess genomes reduced in size compared with their free-living close relatives. Thus, the body louse genome project offers unique information and tools to use in advancing understanding of coevolution among vectors, symbionts, and pathogens.

Wolbachia are intracellular microorganisms that form maternally-inherited infections within numerous arthropod species. These bacteria have drawn much attention, due in part to the reproductive alterations that they induce in their hosts including cytoplasmic incompatibility (CI), feminization and parthenogenesis. Although Wolbachia's presence within insect reproductive tissues has been well described, relatively few studies have examined the extent to which Wolbachia infects other tissues. We have examined Wolbachia tissue tropism in a number of representative insect hosts by western blot, dot blot hybridization and diagnostic PCR. Results from these studies indicate that Wolbachia are much more widely distributed in host tissues than previously appreciated. Furthermore, the distribution of Wolbachia in somatic tissues varied between different Wolbachia/host associations. Some associations showed Wolbachia disseminated throughout most tissues while others appeared to be much more restricted, being predominantly limited to the reproductive tissues. We discuss the relevance of these infection patterns to the evolution of Wolbachia/host symbioses and to potential applied uses of Wolbachia.

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 w Ri that induces very strong cytoplasmic incompatibility in its natural host Drosophila simulans . A comparison with the previously sequenced genome of W. pipientis strain w Mel from Drosophila melanogaster identified 35 breakpoints associated with mobile elements and repeated sequences that are stable in Drosophila lines transinfected with w Ri. Additionally, 450 genes with orthologs in w Ri and w Mel were sequenced from the W. pipientis strain w Uni, 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.

Strains of Drosophila simulans from Riverside, California (DSR) and Hawaii (DSH) harbour distinct strains of the cytoplasmic incompatibility microorganism Wolbachia, resulting in the expression of bidirectional incompatibility when crossed. D. simulans lines carrying both of these (superinfected) Wolbachia strains were generated by the transfer of infected DSH cytoplasm into DSR embryos by microinjection. The superinfected flies were unidirectionally incompatible with both DSR and DSH individuals. As a result of this pattern, the superinfected state was observed to replace single infections in laboratory populations. The ability of the superinfection to spread was modulated by the production of singly infected offspring from superinfected mothers: strain segregation was observed under crowded larval rearing conditions. An inverse correlation between the penetrance of the cytoplasmic incompatability phenotype and the degree of larval crowding was also observed. The findings have implications for the evolution of bidirectionally incompatible strains, and lead to the prediction that superinfections should be relatively common in field populations. Evidence for a natural superinfection in the mosquito Aedes albopictus is discussed. The results also have applied significance for the generation of insect lines capable of driving desirable genes into populations already infected with Wolbachia, thus allowing repeated opportunities for population replacement.

Wolbachia are obligate intracellular bacteria which commonly infect arthropods. They are maternally inherited and capable of altering host development, sex determination, and reproduction. Reproductive manipulations include feminization, male-killing, parthenogenesis, and cytoplasmic incompatibility. The mechanism by which Wolbachia avoid destruction by the host immune response is unknown. Generation of antimicrobial peptides (AMPs) and reactive oxygen species (ROS) by the host are among the first lines of traditional antimicrobial defense. Previous work shows no link between a Wolbachia infection and the induction of AMPs. Here we compare the expression of protein in a cell line naturally infected with Wolbachia and an identical cell line cured of the infection through the use of antibiotics. Protein extracts of each cell line were analyzed by two dimensional gel electrophoresis and LC/MS/MS. Our results show the upregulation of host antioxidant proteins, which are active against ROS generated by aerobic cell metabolism and during an immune response. Furthermore, flow cytometric and microscopic analysis demonstrates that ROS production is significantly greater in Wolbachia-infected mosquito cells and is associated with endosymbiont-containing vacuoles located in the host cell cytoplasm. This is the first empirical data supporting an association between Wolbachia and the insect antioxidant system.