EVOLUTION OF SEX DETERMINING MECHANISMS: A REVIEW OF FACT AND THEORY.

Abstract

Over the last 10 years evolutionary biologists have devoted considerable attention to the evolution and ecology of sex and sex ratio. The underlying theme in most of this work is that differences in the frequency and mode of sexual reproduction and variation in sex ratio are the products of natural selection and therefore represent adaptations of species to their environments. However, this variation may also reflect evolutionary constraints imposed on species by their genetic architecture and mechamism of sex determination. The importance of such constraints becomes strikingly obvious when we examine the evidence for adaptive variation in sex ratio. Species with haplo-diploid sex determination can control progeny sex ratio directly, and recent evidence indicates that these species adaptively alter sex ratio in response to changes in resource availability and population structure (Chamov, 1982). In contrast, species with well developed X-Y sex chromosomes show no adaptive variation in sex ratio. Primary sex ratio in these species is typically binomially distributed as expected for any neutral Mendelian unit character (Williams, 1979). Clearly, an understanding of the evolution of sex determining mechanisms would provide a deeper understanding of those forces affecting the evolution of sex and sex ratio. In Evolution of Sex Determining Mechanisms, J. J. Bull examines the diversity of sex determining mechanisms from a modem theoretical perspective. The book begins with a review of terminology and proceeds to discuss 2-factor (classical malefemale heterogamety) and multiple-factor systems. Bull uses analytical and numerical models to draw conclusions about the evolution of these two types of sex determining mechanisms. His most important conclusion is that the evolution of sex factors is a natural outcome of the nearly universal selection for a sex ratio of 1/2. Another conclusion is that multiple-factor sex determination is rarer than male-female heterogamety because it is stable only under special circumstances. I found that many of these models are not presented in enough detail to permit critical evaluation. Next, Bull reviews examples and theory of polyfactorial and environmental sex determination. According to theory largely developed by Bull, envi-