CYTOPLASMIC incompatibility (CI) describes the phenomenon whereby eggs fertilized by sperm from insects infected with a rickettsial endosymbiont fail to hatch. Unidirectional CI between conspecific populations of insects is a well documented phenomenonl-9. Bidirectional CI has, however, only been described in mosquito populations10, and recently between closely related species of parasitic wasps11, where it is of interest as both an unusual form of reproductive isolation1 and as a potential means of insect population suppression12-15. Here we report on the first known example of bidirectional CI between conspecific populations of Drosophila simulans. Further, we show that defects as early as the first cleavage division are associated with CI. This observation suggests that the cellular basis of CI involves disruption of processes before or during zygote formation and that CI arises from defects in the structure and/or function of the sperm during fertilization.
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