The Cif proteins from Wolbachia prophage WO modify sperm genome integrity to establish cytoplasmic incompatibility

Inherited microorganisms can selfishly manipulate host reproduction to drive through populations. In Drosophila melanogaster, germline expression of the native Wolbachia prophage WO proteins CifA and CifB cause cytoplasmic incompatibility (CI) in which embryos from infected males and uninfected females suffer catastrophic mitotic defects and lethality; however, in infected females, CifA expression rescues the embryonic lethality and thus imparts a fitness advantage to the maternally transmitted Wolbachia. Despite widespread relevance to sex determination, evolution, and vector control, the mechanisms underlying when and how CI impairs male reproduction remain unknown and a topic of debate. Here, we use cytochemical, microscopic, and transgenic assays in D. melanogaster to demonstrate that CifA and CifB proteins of wMel localize to nuclear DNA throughout the process of spermatogenesis. Cif proteins cause abnormal histone retention in elongating spermatids and protamine deficiency in mature sperms that travel to the female reproductive tract with Cif proteins. Notably, protamine gene knockouts enhance wild-type CI. In ovaries, CifA localizes to germ cell nuclei and cytoplasm of early-stage egg chambers; however, Cifs are absent in late-stage oocytes and subsequently in fertilized embryos. Finally, CI and rescue are contingent upon a newly annotated CifA bipartite nuclear localization sequence. Together, our results strongly support the Host modification model of CI in which Cifs initially modify the paternal and maternal gametes to bestow CI-defining embryonic lethality and rescue.

Automated summary

Inherited microorganisms can manipulate host reproduction for their own benefit. In the fruit fly Drosophila melanogaster, the expression of certain proteins from the Wolbachia bacterium causes cytoplasmic incompatibility (CI), which leads to defects and death in embryos from infected males and uninfected females. However, in infected females, the same proteins can rescue the embryos and give an advantage to the Wolbachia. The mechanisms behind CI's effect on male reproduction have been unknown, but this study shows that the proteins localize to the DNA in the process of sperm production and cause abnormalities in the sperm. Additionally, these proteins are present in the early stages of egg development but disappear in late-stage embryos. The findings suggest that the proteins modify the paternal and maternal gametes to cause CI but also provide a rescue mechanism.