thelytoky Obligatory parthenogenesis, such that populations consist entirely of females, with occasional functionless males. It is the only genetic system in which fertilization (the union of egg and sperm) is eliminated completely. For example, in some Aphididae the sexual stage of the life cycle has disappeared and populations consist exclusively of females; Eriosoma lanigerum (woolly apple aphid), a N. American species, was introduced to Europe, where it became thelytokous on apples because it has no access to its primary host, the American elm, which presumably supplies a chemical stimulus necessary for sexual reproduction. Some species of parasitic wasps are also thelytokous. Evolution in thelytokous species can therefore take place only by favourable mutations occurring in a single individual, and persisting in the line descending from that individual.

thelytoky Obligatory parthenogenesis, such that populations consist entirely of females, with occasional functionless males. It is the only genetic system in which fertilization (the union of egg and sperm) is eliminated completely. For example, in some aphids the sexual stage of the life cycle has disappeared and populations consist exclusively of females; Eriosoma lanigerum (woolly apple aphid), a North American species, was introduced to Europe, where it became thelytokous on apples because it has no access to its primary host, the American elm, which presumably supplies a chemical stimulus necessary for sexual reproduction. Some species of parasitic wasps are also thelytokous. Evolution in thelytokous species can therefore take place only by favourable mutations occurring in a single individual, and persisting in the line descending from that individual.

thelytoky Obligatory parthenogenesis, such that populations consist entirely of females, with occasional functionless males. It is the only genetic system in which fertilization (the union of egg and sperm) is eliminated completely. Evolution in thelytokous species can therefore take place only by favourable mutations occurring in a single individual, and persisting in the line descending from that individual.

thelytoky The phenomenon occurring in the reproduction of certain animals in which fertilized eggs give rise to males and unfertilized eggs to females. It is found among aphids and certain other insects, and in some mites. The males are diploid, whereas females are haploid and transmit only the maternal genome – their production represents thelytokous parthenogenesis. Compare arrhenotoky.