The Wolbachia CinB Nuclease is Sufficient for Induction of Cytoplasmic Incompatibility

Wolbachia are obligate intracellular bacteria that can alter reproduction of their arthropod hosts, often through a mechanism called cytoplasmic incompatibility (CI). In CI, uninfected females fertilized by infected males yield few offspring, but if both are similarly infected, normal embryo viability results (called rescue). CI factors (Cifs) responsible for CI are pairs of proteins encoded by linked genes. The downstream gene in each pair encodes either a deubiquitylase (CidB) or a nuclease (CinB). The upstream gene products, CidA and CinA, bind their cognate enzymes with high specificity. Expression of CidB or CinB in yeast inhibits growth, but growth is rescued by expression of the cognate CifA protein. By contrast, transgenic Drosophila male germline expression of both cifA and cifB was reported to be necessary to induce CI-like embryonic arrest; cifA expression alone in females is sufficient for rescue. This pattern, seen with genes from several Wolbachia strains, has been called the 2-by-1 model. Here we show male germline expression of the cinB gene alone, from a distinct clade of cif genes from wNo Wolbachia, is sufficient to induce nearly complete loss of embryo viability. This male sterility is fully rescued by cognate cinAwNo expression in the female germline. The proteins behave similarly in yeast. CinBwNo toxicity depends on its nuclease active site. These results demonstrate that highly divergent CinB nucleases can induce CI, that rescue by cognate CifA factors is a general feature of Wolbachia CI systems, and that CifA is not required in males for CI induction. ImportanceWolbachia are bacteria that live within the cells of many insects. Like mitochondria, they are only inherited from females. Wolbachia often increase the number of infected females to promote spread of infection using a type of male sterility called cytoplasmic incompatibility (CI): when uninfected females mate with infected males, most embryos die; if both are similarly infected, embryos develop normally, giving infected females an advantage in producing offspring. CI is being used against disease-carrying mosquitoes and agricultural pests. Wolbachia proteins called CifA and CifB, which bind one other, cause CI, but how they work has been unclear. Here we show that a CifB protein singly produced in fruit fly males causes sterility in crosses to normal females, but this is rescued if the females produce the CifA partner. These findings clarify a broad range of observations on CI and will allow more rational approaches to using it for insect control.