▪ Abstract  Most metazoans engage in recombination every generation. In theory this is associated with considerable cost, such as the production of males, so that asexual organisms, which do not pay this cost, should be able to invade populations of sexuals. Some asexuals depend on sperm of sexual males to trigger embryogenesis, a reproductive mode called gynogenesis. The genetic information of males is typically not used. Theory predicts that such mating complexes are short-lived and highly unstable. Sperm dependency is not only the defining feature of the biology of gynogenetic metazoans, it is also a major puzzle in evolutionary biology. Organisms that apparently combine disadvantages of both sexuality and asexuality are a serious challenge to theory. A number of questions about these systems are still unresolved.