Most filarial nematodes contain Wolbachia endobacteria that are essential for development and reproduction. An antibody against a Wolbachia surface protein was used to monitor the distribution of endobacteria during the B. malayi life cycle. In situ hybridization with probes binding to Wolbachia 16S rRNA were used to confirm results. Only a few cells contain Wolbachia in microfilariae and vector stage larvae; this suggests that the bacteria need to be maintained, but may have limited importance for these stages. Large numbers of Wolbachia were detected in the lateral chords of L4 larvae and of young adult worms, but not in the developing reproductive tissue. Confocal laser scanning and transmission electron microscopy showed that Wolbachia are aligned towards the developing germline. It can be hypothesized that Wolbachia invade developing ovaries from the lateral chords. In inseminated females, Wolbachia were detected in the ovaries and embryos. In young males, Wolbachia were found in parts of the testis and in the lateral chords in the vicinity of testicular tissue but never in mature spermatids or spermatozoa. The process of overcoming tissue boundaries to ensure transovarial transmission of Wolbachia could be an Achilles heel in the life cycle of B. malayi
