SEX CHROMOSOMES AND THE EVOLUTION OF SEXUAL DIMORPHISM

The evolutionary significance of sex chromosomes has generally been associated with their effect on sex determination (see Mittwoch, 1967; Ohno 1967, 1979 for review). Here I will evaluate the idea that sex chromosomes also play an important role in the evolution of sexually dimorphic traits. As a premise I will assume that sexual dimorphism results from natural selection that favors different phenotypic characteristics in the two sexes. For example, in bighorn sheep (Ovis canadensis) horn size and shape are sexually dimorphic. The massive recurved horns of males are presumably an adaptive compromise between the need to blunt head-to-head collisions between fighting males and to act as weapons against predators. The smaller dagger-like horns of females are favored in this' sex since their sole function is defense against predators (Geist, 1971). Many less obvious characteristics are also likely to differ in their selective value between the sexes. In an extensive review of sexual dimorphism in mammals, Glucksmann (1981) suggested that traits such as growth rate, thermoregulation, metabolic rate, biorhythms, sensory modality, and a wide variety of other traits differ between the sexes in their optimal value. Characteristics that are selectively favored in one sex but selected against in the other will be referred to as "sexually-antagonistic" traits. Consider the evolution of a sexually dimorphic trait from a monomorphic state. For example, Geist (1971) used paleontological evidence and taxonomy to argue that the sexually dimorphic horns of bighorn sheep evolved from a sexually monomorphic ancestor. Because an exact record of this evolutionary change is unavailable, I will arbitrarily assume for the purpose of illustration that the size of female horns remained unchanged while the size of male horns increased. The evolution of sexual dimorphism in horn size could have proceeded in at least two ways: 1) The increase in frequency of genes that enhanced horn size in males but not in females, and 2) The increase in frequency of genes that enhanced horn size in both sexes followed by the evolution of modifier genes that restricted the expression of increased horn size to males. The first way will be referred to as the "pleiotropy-mechanism. " It requires genetic variability that simultaneously produces the sexually-antagonistic trait (increased horn size) and is sex-limited in its expression. The second way will be referred to as the "modifermechanism. " It requires genetic variability for both the sexually-antagonistic trait (increased horn size) and sex-limited expression of this trait. A similar classification was previously proposed by Turner (1978) for the evolution of sexlimited traits in butterflies. Most mutations that have been studied carefully in the laboratory have not been found to be completely sex-limited in their expression. Thus it seems reasonable to assume that most of the genetic variability available for the evolution of sexual dimorphism would be initially expressed in both sexes. This assumption may be unreasonable when considering the enhancement of an established sexually dimorphic trait since developmental canalization (Waddington, 1962) may facilitate sex-limited gene expression. However, during the initial evolution of sexual dimorphism from a monomorphic state, a feasible sequence of events would be the "modifier-mechanism" described above. In the following model I will suggest how the sex chromosomes can facilitate