Evolution of environmental sex determination from genotypic sex determination

Abstract

Environmental sex determination is a mechanism in which an individual’s sex is decided after conception, according to its immediate environment. A previous theory proposed that environmental sex determination is adaptive in certain life histories by allowing control of sex in response to environmental effects on fitness. Although plausible, this theory did not explain how environmental sex determination evolves from an alternative mechanism. A model is presented here to describe the evolution of environmental sex determination from genotypic sex determination with male heterogamety (XX♀/XY♂). As environmental sex determination evolves, both sexes become XX and male heterogamety disappears. Except for certain types of X/Y sex chromosome effects, a pre-existing genotypic mechanism of sex determination does not intrinsically interfere with the evolution of environmental sex determination, and the adaptation theory gives the correct conditions for its evolution. The widespread occurrence of XX♀/X0♂ sex chromosome systems in nematodes suggests that genotypic sex determination is possibly ancestral to environmental sex determination in this group, but convincing evidence is lacking. © 1981 The Genetical Society of Great Britain.