The P{Medea.myd88} construct acts as a maternal-effect selfish genetic element. It contains a maternal toxin (Myd88miRNA.bcd) which consists of a maternal germline-specific promoter that drives expression of a product that is toxic to the embryo, and it also contains a zygotic antidote (Myd88bnk.PC) which consists of a zygotic promoter driving expression of the antidote to the toxic maternal product.