A maternal-effect selfish genetic element in .

Selfish genetic elements spread in natural populations and have an important role in genome evolution. We discovered a selfish element causing embryonic lethality in crosses between wild strains of the nematode The element is made up of , a maternal-effect toxin that kills developing embryos, and , its zygotically expressed antidote. has long been considered essential for pharynx development on the basis of its mutant phenotype, but this phenotype arises from a loss of suppression of toxicity. Inactive copies of the element show high sequence divergence from active copies, and phylogenetic reconstruction suggests that they represent ancestral stages in the evolution of the element. Our results suggest that other essential genes identified by genetic screens may turn out to be components of selfish elements.