Cytoplasmic incompatibility in natural populations of a mosquito, Culex pipiens L.

When two strains of Culex pipiens (s.l.) of different geographical origin are cross-mated, the cross is frequently sterile in one or both directions. Such incompatibility is said to be cytoplasmic because the crossability of a strain is determined by its maternal lineage. The incompatibility is caused in some way by infection with a rickettsia-like bacterial symbiote, as removal of the symbiote abolishes the incompatibility. In compatibility has not been observed in crosses of American strains of C. pipiens. On the other hand, most workers in other parts of the world who have crossed C. pipiens strains have noticed incompatibility, although there are no reports of incompatible egg rafts being collected in the field. We now report incompatibility in crosses of sympatric American strains of C. pipiens and the collection of incompatible egg rafts in the field.