Wolbachia variability and host effects on crossing type in Culex mosquitoes [Letter]

Wolbachia is a common maternally inherited bacterial symbiont able to induce crossing sterilities known as cytoplasmic incompatibility (CI) in insects(1,2). Wolbachia-modified sperm are unable to complete fertilization of uninfected ova, but a rescue function allows infected eggs to develop normally. By providing a reproductive advantage to infected females, Wolbachia can rapidly invade uninfected populations(3), and this could provide a mechanism for driving transgenes through pest populations(4,5). CI can also occur between Wolbachia-infected populations and is usually associated with the presence of different Wolbachia strains(1). In the Culex pipiens mosquito group ( including the filariasis vector C. quinquefasciatus) a very unusual degree of complexity of Wolbachia-induced crossing-types has been reported, with partial or complete CI that can be unidirectional or bidirectional(6-11), yet no Wolbachia strain variation was found(11). Here we show variation between incompatible Culex strains in two Wolbachia ankyrin repeat-encoding genes associated with a prophage region, one of which is sex-specifically expressed in some strains, and also a direct effect of the host nuclear genome on CI rescue.