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Evolution de l’incompatibilité cytoplasmique associée à la bactérie Wolbachia

Abstract : Cytoplasmic incompatibility (CI) is a phenomenon associated with the endocellular bacterium Wolbachia, which is very common in arthropods. CI is expressed by the early death of embryos resulting from crosses between infected males and uninfected females, while viability is normal if the female carries the same bacteria, or if the male himself is not infected. The molecular mechanisms are currently unknown, but this phenomenon can be described by a model with two functions, toxin / antidote model: the “toxin” (called mod, for “modification”) would be produced by Wolbachia and deposited in the maturing spermatozoa, inducing the death of the embryos, unless the bacteria are also present in the fertilized egg, producing an “antidote” (called resc, for “rescue”). Due to the mod function, uninfected females are sterilized by infected males present in the population. On the contrary, because of the resc function, infected females are not sterilized. Because Wolbachia is transmitted only by females, along with the cytoplasm of the egg, CI allows it to invade and persist in host populations. The phylogenetic diversity of Wolbachia is manifested by a phenotypic diversity of mod and resc functions. On the one hand, different variants can exhibit different "mod intensities", that is, kill a greater or lesser proportion of the progeny in crosses between infected males and uninfected females. On the other hand, different variants can be mutually incompatible, showing that the mod and resc functions interact in specific ways. This thesis focuses on the evolutionary processes underlying this diversity, studied by a combination of theoretical and experimental approaches. From the theoretical analysis, we show that compatibility types (mod / resc pairs) may evolve in a two-step process. The first involves neutral variations of the mod function, not subject to selection because mod is only expressed in males. The second step involves the selection of new resc functions. The experimental approach is based on the confrontation, after cytoplasmic injection into the model species Drosophila simulans, of bacterial variants that are closely related but evolving naturally in different hosts. Our results suggest that variants that are close but distinguishable by standard molecular markers may be partially or totally incompatible. We are also interested in the consequences of secondary loss of CI (phenotype [mod-]) on the stability of Wolbachia / host associations. Our results suggest that in Drosophila simulans and Drosophila yakuba species, the absence of CI does not necessarily induce loss of infection.
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Contributor : Sylvain Charlat <>
Submitted on : Wednesday, December 16, 2020 - 3:25:49 PM
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Sylvain Charlat. Evolution de l’incompatibilité cytoplasmique associée à la bactérie Wolbachia. Evolution [q-bio.PE]. Université Paris 6 - Pierre et Marie Curie, 2002. Français. ⟨tel-03077380⟩

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