Heads or tails: host-parasite interactions in the Drosophila-Wolbachia system.

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.

Organization of Wolbachia pipientis in the Drosophila fertilized egg and embryo revealed by an anti-Wolbachia monoclonal antibody.

Cytoplasmic incompatibility (CI) in Drosophila is related to the presence of Wolbachia, an intracellular microorganism found in many species of insects. In order to study the intracellular localization of Wolbachia in eggs and embryos, we have purified the bacteria from fly embryos and subsequently generated a monoclonal antibody (Mab Wol-1) specific for Wolbachia. Indirect immunofluorescence staining using Wol-1 reveals that during mitosis, Wolbachia are localized near spindle poles and centrosomes. Double label immunofluorescence experiments using anti-tubulin and anti-Wolbachia antibodies show that Wolbachia co-localize with centrosomal microtubules throughout the cell cycle. Direct interactions between the bacteria and centrosome-organized microtubules are implied from seven observations: (1) throughout the mitotic cycle, the position and movement of Wolbachia precisely mimic the behavior of the centrosome and apparently associated with centrosome-organized microtubules; (2) Wolbachia segregate equally to each spindle pole during mitosis; (3) Wolbachia do not associate with spindle microtubules during mitosis; (4) Wolbachia located in the egg cortex localize to the domains of cytoplasm organized by microtubules during blastoderm formation; (5) polar body nuclei that lack centrosomes but contain associated microtubules do not contain Wolbachia; (6) Wolbachia no longer associated with yolk nuclei, following differentiation and loss of centrosomes; (7) during pole cell formation, Wolbachia co-localize with the centrosome on the apical side of the nucleus as pole cells form. Quantitative data indicates that no Wolbachia growth occurs during the preblastoderm period even though rapid nuclear, and subsequent cellular, proliferation takes place during this same period. This indicates that Wolbachia are under strict growth regulation by the host suggesting that host factors play a role in regulating growth of Wolbachia in the egg. Further cellular and molecular studies of the extensive, global interactions between host and symbiont observed in this egg should provide important new insights into the evolution of host/symbiosis and the cell biology of cytoplasmic incompatibility.

Expression of Rous sarcoma virus-derived retroviral vectors in the avian blastoderm: potential as stable genetic markers.

Retroviruses are valuable tools in studies of embryonic development, both as gene expression vectors and as cell lineage markers. In this study early chicken blastoderm cells are shown to be permissive for infection by Rous sarcoma virus and derivative replication-defective vectors, and, in contrast to previously published data, these cells will readily express viral genes. In cultured blastoderm cells, Rous sarcoma virus stably integrates and is transcribed efficiently, producing infectious virus particles. Using replication-defective vectors encoding the bacterial lacZ gene, we further show that blastoderms can be infected in culture and in ovo. In ovo, lacZ expression is seen within 24 hr of virus inoculation, and by 96 hr stably expressing clones of cells are observed in diverse tissues throughout the embryo, including epidermis, somites, and heart, as well as in extraembryonic membranes. Given the rapid onset of vector expression and the broad range of permissive cell types, it should be feasible to use Rous sarcoma virus-derived retroviruses as early lineage markers and expression vectors beginning at the blastoderm stage of avian embryogenesis.

Characterization of the hemopoietic target cells for the avian leukemia virus E26.

The dual leukemogenic response, involving both the erythroid and myeloid hemopoietic systems in chickens infected with E26 virus, has previously been described (C. Moscovici, J. Samarut, L. Gazzolo, and M. G. Moscovici, 1981. Virology 113, 765-768; K. Radke, H. Beug, S. Kornfeld, and T. Graf, 1982. Cell 31, 643-653). Similarly, the in vitro response of the two lineages resulted in the concomitant transformation and proliferation of erythroblast and myeloblast leukemic cells. The present study, using embryonic tissues at very early stages of development, was valuable in implying that E26 target cells are recruited among uncommitted erythroid-myeloid stem cells as well as myeloid- or erythroid-committed progenitor cells. Therefore, E26 may be the first avian retrovirus capable of interacting with uncommitted hemopoietic precursor cells.

Transforming ability of avian defective leukemia viruses in early embryogenesis.

The response to infection of chicken hemopoietic cells derived from the early stages of embryogenesis by avian myeloblastosis virus (AMV) and avian erythroblastosis virus (AEV) was investigated. It was found that erythroid progenitor cells were present in the blastoderm at a higher frequency than that of myeloid progenitor cells. These results correlate with the observation that target cells for AEV were found to be more numerous than those for AMV. Therefore, blastoderm cells are of potential value in understanding the mechanisms of oncogenesis at the level of the target cells.