9 Sexual Dimorphism and Sex Determination

Abstract

I. INTRODUCTION
Sexuality is a phenomenon that occurs throughout the animal kingdom and is therefore of general biological interest. From an evolutionary point of view, sexuality raises the questions of how separate sexes evolved at all and why so many different sexual systems have arisen. From the viewpoint of developmental biology, sexuality is important because the different sexes of an animal result from different developmental pathways, which must be separately established and maintained. Understanding how the cells of a whole animal develop as male rather than female is fundamentally the same problem as understanding how the cells in one part of the animal develop into an intestine as opposed to a muscle, or a wing as opposed to a leg. From this standpoint, the study of sexual differentiation is essentially a problem in developmental biology.

It is a special problem for several reasons. First, sexual specialization is possibly the most far-reaching of the determinative decisions that must occur in development, because correct sexual development requires specialized behavior on the part of a great variety of cells in many different parts of the animal. This is particularly true of Caenorhabditis elegans, in which at least 30% of the somatic cells and all of the germ cells are sexually specialized. Second, it is possible to identify the primary sex-determining signal (usually chromosomal, as in C. elegans, but environmental in some organisms). This defines the first of the events that eventually lead to different sexual phenotypes, and it is therefore possible to work...