Molecular and cellular bases of the phenotypic diversity of Cytoplasmic Incompatibility caused by Wolbachia in insects – CIAWOL

The a-proteobacteria Wolbachia infect all major terrestrial arthropod groups (up to 60% of the arthropod species are suspected to be infected). This makes Wolbachia the agent of the most important pandemic in the animal world reported to date. This wide distribution of Wolbachia largely relies on a robust vertical transmission from mother to offspring due to a remarkably diverse array of strategies to manipulate their host reproduction. The most frequent strategy used by Wolbachia to spread through a host population is by causing cytoplamic incompatibility (CI) which confers an advantage to infected females compared to uninfected ones. CI occurs when males infected with a given Wolbachia are crossed with uninfected females or females infected with a different Wolbachia which is incompatible: such crosses result in non-viable embryos. This ability of Wolbachia to sterilize arthropod females has many possible applications in antivectorial and pest managements. It is thus of primordial importance to understand how Wolbachia induces CI. To date, almost nothing is known on the molecular bases of Wolbachia induced CI although cytological studies performed in Drosophila and Nasonia have provided the scientific community with some clues. Indeed, these works revealed an abnormal condensation and segregation of the paternal pronucleus in incompatible crosses. Thus a main hypothesis on CI functioning is the presence of a Wolbachia factor(s) located in the sperm that would modify the normal fate of embryogenesis and that could be rescued by a factor(s) present in compatible Wolbachia infected female.
All the mosquitoes from the Culex pipiens complex are infected with Wolbachia named wPip forming one single monophyletic group. In this system, a huge complexity of CI patterns has long been recognized all around the world. This intriguing complexity has discouraged most researchers to decipher CI mechanisms with help of C. pipiens-wPip interactions. The main reason was that until recently the high variability in CI was in discordance with the poor genetic variations observed between wPip samples. However, the recent discovery of a previously overlooked diversity in wPip based on hypervariable markers (transposons, phages, etc.) changed this point of view. Indeed, five phylogenetic groups (group I to V) have been defined in wPip and these groups are highly congruent with CI phenotypes. All this new knowledge accumulated to date on C. pipiens-wPip interactions provides our consortium with an important novel avenue to address the mechanisms of CI and the motor of its diversity. In the present project, we present five tasks that describe synergetic strategies sharing one goal: finding putative mechanism(s) responsible for Wolbachia induced CI in insects. We built a multidisciplinary project called CIAWOL including different approaches from evolutionary biology to functional genomics. We will track CI variations in natural populations to get access to a very fine scale of CI evolution. The genome of these closely related Wolbachia will then be sequenced and compared in order to examine genomic changes that may be responsible for CI variations. Further investigations on CI mechanisms using both Culex and Drosophila will combine (1) cytological analyses of the early embryogenesis coupled with transinfection experiments to study the conservation of the mechanisms responsible for CI across host species, (2) proteomics analyses of insect sperms seeking for secreted Wolbachia proteins responsible for CI and (3) functional analysis of candidates by transgenesis in Drosophila. The CIAWOL project will contribute to the comprehension and characterization of CI mechanism(s) from populations to molecules.