Cytoplasmic incompatibility
![Unidirectional CI: Mating of infected males and uninfected females results in CI. All other crosses are compatible.
Bidirectional CI: Mating of males infected with Wolbachia strain I and females infected by Wolbachia strain II (and vice versa) results in CI.](/Images/godic/202501/07/Uni_bidirectional_ci3216.png")
Cytoplasmic incompatibility (CI) is a phenomenon that results in sperm and eggs being unable to form viable offspring. The effect arises from changes in the gamete cells caused by intracellular parasites like Wolbachia, which infect a wide range of insect species. As the reproductive incompatibility is caused by bacteria that reside in the cytoplasm of the host cells, it is referred to as cytoplasmic incompatibility. In 1971, Janice Yen and A. Ralph Barr of UCLA demonstrated the etiologic relationship of Wolbachia infection and cytoplasmic incompatibility in Culex mosquitos when they found that eggs were killed when the sperm of Wolbachia-infected males fertilized infection-free eggs.