Cytoplasmic Incompatibility Variations in Relation with Wolbachia cid Genes Divergence in Culex pipiens

Abstract

In arthropods, Wolbachia endosymbionts induce conditional sterility, called cytoplasmic incompatibility (CI), resulting from embryonic lethality. CI penetrance (i.e., embryonic death rate) varies depending on host species and Wolbachia strains involved. All Culex pipiens mosquitoes are infected by the endosymbiotic alphaproteobacteria Wolbachia wPip. CI in Culex, characterized as a binary “compatible/incompatible” phenomenon, revealed an unparalleled diversity of patterns linked to the amplification-diversification of cidA and cidB genes. Here, we accurately studied CI penetrance variations in the light of cid genes divergence by generating a C. pipiens compatibility matrix between 11 lines hosting different phylogenetic wPip groups and exhibiting distinct cid gene repertoires. We showed, as expected, that crosses involving wPip from the same group were mostly compatible. In contrast, only 22% of the crosses involving different wPip groups were compatible, while 54% were fully incompatible. For the remaining 24% of the crosses, “intermediate” compatibilities were reported, and a cytological observation of the first zygotic division confirmed the occurrence of “canonical” CI phenotypes in a fraction of the eggs. Backcross experiments demonstrated that intermediate compatibilities were not linked to host genetic background but to the Wolbachia strains involved. This previously unstudied intermediate penetrance CI was more severe and frequent in crosses involving wPip-IV strains exhibiting cid variants markedly divergent from other wPip groups. Our data demonstrate that CI is not always a binary compatible/incompatible phenomenon in C. pipiens but that intermediate compatibilities putatively resulting from partial mismatch due to Cid proteins divergence exist in this species complex. IMPORTANCE Culex pipiens mosquitoes are infected with wPip. These endosymbionts induce a conditional sterility called CI resulting from embryonic deaths, which constitutes a cornerstone for Wolbachia antivectorial methods. Recent studies revealed that (i) two genes, cidA and cidB, are central in Wolbachia-CI mechanisms, and (ii) compatibility versus incompatibility between mosquito lines depends on the wPip phylogenetic groups at play. Here, we studied CI variations in relation to wPip groups and cid genes divergence. We showed, as expected, that the crosses involving wPip from the same group were compatible. In contrast, 78% of the crosses involving different wPip groups were partially or fully incompatible. In such crosses, we reported defects during the first zygotic division, a hallmark of CI. We showed that CI was more severe and frequent in crosses involving wPip-IV strains exhibiting cid variants, which markedly diverge from those of other wPip groups.

Toward an accurate mechanistic understanding of Wolbachia‐induced cytoplasmic incompatibility

“…This may help explain the highly diverse CI phenotypes observed in crosses between C . pipiens , assuming binding is necessary for CI rescue (Namias et al, 2022; Sicard et al, 2021).…”

From Wolbachia genomics to phenotype: molecular models of cytoplasmic incompatibility must account for the multiplicity of compatibility types

“…Much more parsimoniously, within the TA framework, mutual incompatibilities can simply result from differences in CidA/CidB interaction sites. Intermediate affinities among Cid proteins and quantitative variations in their production could also readily explain the intermediate levels of rescue recently described [39].…”

“…Following the observation that, in contrast with D. simulans, C. pipiens only hosts one Wolbachia "strain" (named wPip) as far as standard molecular markers such as 16S rRNA can tell, it has been hypothesized that host genetic variation likely contributed to this complexity [37]. Yet, more discriminant molecular markers soon revealed that the wPip clade is in fact composed of several lineages [38] and more importantly, experiments controlling for the host genetic background repeatedly indicated that Wolbachia alone is responsible for the observed incompatibilities [35,36,39].…”

“…Yet, such quantitative variations would fail to explain the non-transitive CI relationships observed in Culex pipiens, unless one assumes host genetic variation is also involved, an hypothesis that, once again, was repeatedly ruled out in Culex pipiens, e.g. lastly in [39]. Furthermore, if changing the gene expression is indeed presumably "easier than evolving a new functional protein" [46], the existence of multiple polymorphic copies of cid genes is demonstrated in Culex [43].…”

pWCP is a widely distributed and highly conserved Wolbachia plasmid in Culex mosquitoes worldwide

“…Recently, the most common effect of host reproduction manipulation, Cytoplasmic Incompatibility (CI) was found to be associated with the cifs genes harbored by Wolbachia bacteriophage WO (LePage et al, 2017; Shropshire et al, 2018; Bonneau, Landmann, et al, 2018; Bonneau, Atyame, et al, 2018; Sicard et al, 2021). These genes are part of a so-called Eukaryotic Associated Module (EAM) that presumably aids phage particles to cope with both prokaryotic and eukaryotic cell membranes, as well as cytoplasmic and extracellular host environments (Bordenstein and Bordenstein, 2022, 2016).…”