All-female broods in the polymorphic butterfly Danaus chrysippus L. and their ecological significance.

Many female Danaus chrysippus produce all-female broods. Of the two common colour forms at Dar-es-Salaam, Tanzania, form dorippus usually givs normal broods while more than 50 per cent of form chrysippus tested produced all-female broods. It is suggested that all-female broods are the result of meiotic drive governed by a Y-linked gene and that a dominant autosomal suppressor, closely linked with the forewing colour locus, must be present in the population. As selection for the colour morphs changes with season, the sex ratio also alternates between an excess of females at the start of the rains when population density increases rapidly, and an excess of males, probably through superiour survival, in the driest months. Changes in sex ratio allow the population to alter its numbers very quickly in response to prevailing conditions.