Cytoplasmic incompatibility
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. Symptoms. CI occurs when a "Wolbachia" infected male mates with a female that is infected by another "Wolbachia" strain (bidirectional CI) or is uninfected (unidirectional CI). Any other combination of un-/infected male/female crosses are compatible. An infected female is compatible with any uninfected male, or with any male infected with the same "Wolbachia" strain. On the other hand an uninfected female is only compatible with an uninfected male. In other words, if the male is infected by a CI-inducing strain of "Wolbachia" that is non-existent in its mate, it is  an incompatible cross. Turelli et al 2018 finds that CI can be resolved by infection of the females with the same strain that is affecting the males, which imposes a population level incentive in favour of CI-inducing strains of "Wolbachia". They also find that this propagates the WO phage. Hosts can be cured from "Wolbachia" infection by antibiotic use. In diploid organisms CI leads to embryonic mortality. In contrast, CI in haplodiploid hosts may lead to haploid (and thus male) offspring. The closely related species of the wasp "Nasonia" show embryonic mortality as well as male development among incompatible  crosses. In "N. vitripennis", however, the vast majority of the CI embryos are converted into males. Cellular mechanism. There are two distinguished events that lead to the CI inducing manipulation. The first occurs inside the "Wolbachia" infected male during spermatogenesis and is called modification. Because "Wolbachia" are absent from mature sperm and appear to be excluded during the individualization process, the modification must occur before the conclusion of spermatogenesis. The second event, called rescue, takes place inside the fertilized egg where "Wolbachia" presence prevents CI from occurring. As long as the "Wolbachia" strains in egg and sperm cells correspond, harmful  effects cannot be observed on a cellular level. A major consequence of CI is the delayed entry into mitosis of the male pronucleus. As a secondary consequence, stemming from this asynchrony, the paternal chromosomes do not properly condense and align on the metaphase plate during the first mitosis. As a consequence, only the maternal chromosome segregate normally, producing haploid embryos. The rescue of CI by infected eggs leads to the restoration of synchrony between the female and the male pronucleus. The exact mechanisms of how "Wolbachia" perform modification and rescue are unknown. In "Drosophila", the earliest effects caused by  CI can already be observed during the sperm chromatin remodeling of the paternal chromosomes. However, it was also observed that in other host species, the defects caused by CI only occur much later in development Evolutionary implications. CI, as described by Werren, results in selection pressure on uninfected males, as infected females can mate both with uninfected males and infected males, but uninfected females cannot mate with infected males. As "Wolbachia" are only transmitted by females, this mechanism promotes the spread of "Wolbachia" and therefore keeps "Wolbachia" from dying out because of incomplete transmission. This has led to discoveries  in control of disease transmission by using Wolbachia to control the reproduction of a population by introducing "Wolbachia"-infected males. This has been seen in the Aedes, mosquito, family, in the "Aedes albopictus" and "Aedes aegypti" species. Population genetics. The infection of a population with CI-inducing "Wolbachia" can have different effects on the population's dynamics. At low prevalence of "Wolbachia", the probability for an uninfected female to lose progeny because of mating with an infected male is fairly low. This loss of uninfected individuals is outweighed by the loss of infection due to the incomplete "Wolbachia" transmission. If the  prevalence of CI-inducing "Wolbachia" is high enough, the probability for an uninfected female to mate with an infected male is high and it is thus preferable for a female to be infected. Therefore, there is an "invasion threshold" for the spread of "Wolbachia" in an uninfected population, which depends on the transmission rate of the infection and the portion of incompatible progeny which survive until they can reproduce (the strength of CI), as well as on the fitness cost of "Wolbachia" infection to the female. Below this threshold the invaders will die out, above this threshold the prevalence will  reach a stable equilibrium. Typical thresholds in real populations are around 10% (See Fine and Stouthamer et al.). As described by Stouthamer, the invasion threshold may be crossed in two ways. Speciation. It is speculated that "CI" can lead to "rapid speciation". When two populations of the same species are infected by two "Wolbachia" strains A and B, they might be bidirectionally incompatible and crosses between the two populations do not lead to viable offspring. Thus gene flow between these two populations is interrupted, leading to constant segregation in development and, finally, to speciation. The populations develop to a point  where incompatibility would be maintained even in absence of "Wolbachia". Other pathogens. "Wolbachia" are not the only bacteria capable of inducing CI. For example, researchers have found that infection by bacteria of the genus "Cardinium" can also result in CI.