Wolbachia of the third kind

Poinsot, D. and Merçot, H.

Institut Jacques Monod, CNRS-Université Paris 7, 2 place Jussieu, F-75251 Paris cedex 05, France

Cytoplasmic incompatibility (CI) is an embryonic mortality induced by the presence of the endocytoplasmic bacterium Wolbachia in reproductive cells of arthropod species. The bacterium is harboured in males and females but is only transmitted maternally because spermatocytes shed their Wolbachia during maturation. CI appears when the Wolbachia variant present in the male is not also found in the egg. This occurs when the female is uninfected or when it bears another variant. Two types of Wolbachia have been described with respect to CI. The first type induces CI by modifying sperm and is also able to rescue its own CI phenotype in the egg (i.e. mod⁺, resc⁺ type). The other type (mod^-, resc^-) is unable both to induce CI and to rescue a CI phenotype. However, theory predicted the existence of a third type of variant (mod^-, resc⁺), which would not induce CI but which would be able to rescue a CI phenotype. We have discovered the first Wolbachia variant exhibiting a mod^- resc⁺ phenotype, in a Drosophila simulans population of Mount Kilimanjaro (Tanzania) [1]. The existence of such a Wolbachia demonstrates that the rescue capacity can act independently from the capacity to induce CI, and it strongly suggests therefore that at least two mechanisms are involved. This result has implications on the molecular models currently proposed to explain the phenomenon of cytoplasmic incompatibility and on the evolution of Wolbachia populations. Another team [2] has independently shown that the wMa variant, initially thought of as mod^- resc^-, is in fact mod^- resc⁺. It is most probable that more mod^- resc⁺ Wolbachia will be found. [1] Merçot, H. & Poinsot, D. (1998) Wolbachia of the third kind. Nature (in press) [2] Bourtzis K., Dobson, S.L., Braig, H. R., O'Neill, S.L. (1998) Wolbachia strains that do not modify Drosophila sperm can still protect eggs. Nature (in press)

LOCATION	DATE	TIME
Lecture Hall I	Thursday, April 9	09:50 am