European Culex pipiens Populations Carry Different Strains of Wolbachia pipientis

The mosquito Culex pipiens occurs in two ecotypes differing in their mating and overwintering behavior: pipiens mate in open environments and diapause, and molestus also mate in small spaces and is active throughout the year. Cx. pipiens carry Wolbachia endosymbionts of the wPip strain, but the frequency of infection differs between studied populations. Wolbachia infection affects the host reproductive success through cytoplasmic incompatibility. wPip Wolbachia is divided into five types, wPip I–V. The type of wPip carried varies among Cx. pipiens populations. In northern European locations different wPip types are found in the two ecotypes, whereas in southern locations, they often carry the same type, indicating differences in hybridization between ecotypes. In this study, Cx. pipiens specimens of both ecotypes were collected from Sweden and compared to specimens from Norway, England, Italy, and the Netherlands, as well as Cx. quinquefasciatus from Mali and Thailand. The abundance varied, but all specimens were infected by Wolbachia, while the tested specimens of other mosquito species were often uninfected. The wPip strains were determined through the sequence analysis of Wolbachia genes ank2 and pk1, showing that Cx. pipiens ecotypes in Scandinavia carry different wPip strains. The observed differences in wPip strains indicate that hybridization is not frequent and may contribute to barriers against hybridization of the ecotypes in Sweden and Norway.