CONTRASTING LEVELS OF VARIABILITY BETWEEN CYTOPLASMIC GENOMES AND INCOMPATIBILITY TYPES IN THE MOSQUITO CULEX-PIPIENS

Reproductive incompatibilities called cytoplasmic incompatibilities ar e known to affect a large number of arthropod species and are mediated by Wolbachia, a maternally transmitted microorganism. The crossing re lationships between strains of potential hosts define their compatibil ity types and it is generally assumed that differences between strains of Wolbachia induce different crossing types. Among all the described host species, the mosquito, Culex pipiens, displays the greatest vari ability of cytoplasmic incompatibility crossing types. We analysed mit ochondrial bacterial DNA variability in Culex pipiens in order to inve stigate some possible causes of incompatibility crossing type variabil ity. We sequenced fragments of the ftsZ gene, and the A + T-rich contr ol region of the mtDNA. We also sequenced the second subunit of the mi tochondrial cytochrome oxidase (COII) gene, in Culex pipiens and a clo sely related species, C. torrentium, in order to verify the usefulness of the A + T-rich region for the present purposes. No variability was found in the Wolbachia ftsZ gene fragment, and very limited variation of the mitochondrial marker whatever the compatibility type or the or igin of the host. A low variability was found in the A + T-rich region and comparison of divergence of the A + T-rich region and COII gene b etween C. pipiens and C. torrentium did not reveal any special constra ints affecting this region. In contrast to observations in other host species, variability of incompatibility crossing types is not due to m ultiple infections by distantly related Wolbachia strains.