MODELS OF POLYGENIC SEX DETERMINATION AND SEX RATIO CONTROL.

Abstract

A species' sex determining mechanism and its sex ratio are inextricably linked. In particular, if a pair of chromosomes determines sex, then the primary sex ratio is controlled by the segregation ratio of these chromosomes in the heterogametic sex. If instead an embryo's sex is determined in response to an environmental cue, then the primary sex ratio is controlled by the distribution of environmental effects in the population. Clearly, the sex determining mechanism constrains the way in which sex ratio modifications may occur. In considering the evolution of the sex ratio, it is therefore desirable to keep in mind the sex determining mechanism and its likely effect on the supply of genetic variation in the sex ratio. There are several ways in which genetic variability for the sex ratio can plausibly arise. (1) In species with sex chromosomes and female heterogamety, there may be differential segregation of the sex chromosomes to ova and polar bodies. (There is no analogous mechanism without loss of fertility in species with male heterogamety.) Alternatively, sex determining genes with major effect other than X and Y may occur, these being effectively sex ratio genes. (2) Some species of fish are thought to have a polygenic sex determining mechanism (Kosswig, 1964). In these, a zygote's sex is determined largely by the sum of the genetic effects over many loci. The sex determining genes are fundamentally the same as sex ratio modifiers acting in the zygote. (3) In some haplo-diploids the sex of each egg depends on whether the mother allowed it to be fertilized or not (Flanders, 1965). Genetic variation in female behavior to fertilize or not fertilize eggs therefore constitutes variation for sex ratio control. (4) In some invertebrates and reptiles, sex is not fixed at conception, but is determined in the embryo (larva) in response to its immediate environment (Charnov and Bull, 1977). In many turtles, for example, sex is determined by the incubation temperature of the egg, low temperatures producing males and high temperatures females (reviewed by Bull, 1980). In these the sex ratio is determined by two factors, (i) the mother's choice of a warm versus cool nest site, and (ii) the embryo's sex differentiation in response to temperature, and there may be genetic variation for both of these. The amount of genetic variability for sex ratio is likely to differ among these sex determining mechanisms. We may reasonably expect to observe genetic variation if sex ratio is controlled by maternal behavior or if sex is determined in the zygote by the environment or a polygenic mechanism. In these it is plausible that sex ratio is affected by many loci with individually weak effects. Species with sex chromosomes, however, often reveal little or no heritable sex ratio variation (cattle: Bar-Anan and Robertson, 1975; Drosophila: M. Toro, unpubl.; chickens: Foster and McSherry, 1980), except for major sex determining genes as in platyfish, flies, and some lemmings (Bull and Charnov, 1977; Fredga et al., 1977). Population genetics models of sex ratio evolution tra-