Cytoplasmic bacteria of the genus Wolbachia are best known as the cause of cytoplasmic incompatibility (CI): many uninfected eggs fertilized by Wolbachia-modified sperm from infected males die as embryos. In contrast, eggs of infected females rescue modified sperm and develop normally. Although Wolbachia cause CI in at least five insect orders, the mechanism of CI remains poorly understood. Here I test whether the target of Wolbachia-induced sperm modification is the male pronucleus (e.g., DNA or pronuclear proteins) or some extranuclear factor from the sperm required for embryonic development (e.g., the paternal centrosome). I distinguish between these hypotheses by crossing gynogenetic Drosophila melanogaster females to infected males. Gynogenetic females produce diploid eggs whose normal development requires no male pronucleus but still depends on extranuclear paternal factors. I show that when gynogenetic females are crossed to infected males, uniparental progeny with maternally derived chromosomes result. This finding shows that Wolbachia impair the male pronucleus but no extranuclear component of the sperm.
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