Sex Determination

Sex determination is the process by which organisms develop as males or females. Some organisms reproduce only by asexual methods, and thus they may possess no system for sexual differentiation. For most species of plants and animals, however, sexual development is a basic element of the normal life cycle. In humans, sex is a fundamental characteristic that influences the development of many of the features of the body. This includes some obvious traits such as genital and breast development, but it also includes structures in the brain and other internal organs, the shape and mineral composition of bones, and a wide array of features observable at the cellular level.

The many clues, overt or subtle, that can be collected from careful examination of the bodies, organs, tissues, and even cells of the deceased remove much of the mystery of the sex of a victim whose remains are recovered from the scene of a crime, or the site of a fire, explosion or other disaster.

In humans, where there are two distinct sex chromosomes, the X and the Y chromosomes, it is the presence of the Y chromosome that specifies male development. More specifically, there is a gene on the Y chromosome called the Sex-determining Region of the Y chromosome (SRY) that causes male development. In fact, female development seems to be the default pathway, and in the absence of SRY, the urogenital tract develops as a female. The elementary structures for both male and female development are present in the early embryo, however, development of the female ductal system, called the Mullerian system, is inhibited by a substance produced by the early male embryo. Likewise, in females, the primordial male ductal system, called the Wolffian duct, degenerates as the Mullerian ductal system advances. The Mullerian ducts give rise to the fallopian tubes, uterus, and upper portion of the vagina. The Wolffian ducts give rise to the spermatic ducts and seminal vesicles which carry sperm from the mature testes during ejaculation. Although SRY, the primary sex-determining gene, is found on the Y chromosome, many of the genes responsible for development of both male and female reproductive structures and other sexual characteristics are found on the autosomes.

One of the earliest events in male development is the production of testis-determining factor within the sex cord cells. The sex cords begin to differentiate into Sertoli cells when SRY is present. The Sertoli cells secrete male-specific factors such as Mullerian Inhibitory Substance (MIS), which causes the female ductal system to degenerate. MIS also promotes the development of another male-specific cell population called Leydig cells, which produce testosterone. For female embryos, because of the absence of SRY, the sex cords develop along a different pathway to develop structures associated with the ovaries. As the embryo develops, hormones produced by the testes in males and the ovaries in females create a biochemical environment in which the more subtle elements of sexual development occur.

Sexual development is not always so straightforward in humans. Although people are usually considered either male or female, various disruptions can occur during sexual development and differentiation that give rise to atypical or mixed sexual development. These include sex chromosome abnormalities, where there are extra or missing copies of the sex chromosomes. This would include Turner syndrome, where females receive only a single X chromosome; Klinefelter syndrome, wherein males receive not only an X and Y chromosome but also an extra copy of the X chromosome; and a wide variety of other more rare numerical sex chromosome abnormalities where extra copies of the X and/or Y chromosomes are present. In addition, the SRY gene that is normally transmitted on the Y chromosome can become translocated to an X chromosome or an autosome, resulting in a reversal of sex. Also, when multiple cell lines are present, with different sex-chromosome allocations, individuals may develop both male and female characteristics.

True hermaphrodites have both testes and ovaries, and may have both intact male and female external genital structures. Pseudohermaphrodites have external genital structures that are opposite of what would be expected on the basis of having either testes or ovaries internally. In addition, the development of the external genital structures can be incomplete, and it may initially be difficult to determine sex at birth. Occasionally, some of the male or female structures fail to form altogether for reasons that are not usually clear. In cases where external genitalia are ambiguous, it was common practice for many years to assign a female gender, and to perform surgical alterations to make the external genitals look more completely feminine. In recent years, it has been recognized that the sexual identity of genetic males after puberty is typically male regardless of whether the child was reared as a male or female, and thus more consideration is given to sex assignment now than in previous years.

Sexual determination is not always as straightforward in other species as it is in humans, and there are many different basic mechanisms by which sex is determined. In fruit flies (Drosophila melanogaster ), for example, sex is determined by the ratio of X chromosomes to the number of sets of autosomes. Normal females have a ratio of 1:1, usually having two X chromosomes and two complete sets of autosomes. Males typically have one X chromosome and two sets of autosomes for a ratio of 1:2. Any ratio greater than 1.0 will result in female sex development, and any ratio below 0.5 results in male development. In between 0.5 and 1.0, the pattern of development is intermediate, bearing some aspects of both femaleness and maleness.

In most species, female development is associated with the presence of two X chromosomes, and male development with the presence of an X and a Y chromosome. Females are therefore typically the homoga-metic sex, meaning that their sex chromosomes are identical to one another. Males are said to be the heterogametic sex, have two different sex chromosomes. In some species, most notably in certain birds and butterflies, the male is homogametic, and the female is the heterogametic sex. In these species, the male sex chromosomes are referred to as Z chromosomes, and the females are said to have a W chromosome and a Z chromosome.

Sex determination in plants is also variable. The male-associated structures in flowering plants are the stamen and pollen. Female associated structures are the pistil and ovaries. Most plants exist as hermaphrodites, producing both male and female structures, often in the same flower. Other plants may exist as male or female individuals, producing only male or female flowers. The common sexual differentiation schemes among plants that produce seeds encased in ovaries are dioecy and gynodioecy. In dioecy, plants can be either male or female. In gynodioecy, plants are either female or hermaphroditic. Sex determination in plants is often less genetically deterministic than in humans. That is, genetic factors may not sufficiently specify the sex of the plant. This results in male, female, or hermaphroditic development being somewhat dependent on environmental conditions.

Sex determination in animals can also be heavily influenced by the environment in some species. For example, sex-determination in some species appears to be primarily dependent upon temperature at the time of development rather than on the presence or absence of specific genes or chromosomes. In certain species of fish, sexual development can change over time with individuals functioning as females for part of the life cycle and as males for other parts of the life cycle. This can be influenced by the relative abundance of individuals of the same or opposite sex in the environment, even when the other individuals are separated by an insuperable barrier such as a glass partition in an aquarium. Environmental pollutants can also influence sexual development in many species.

Bacteria are generally considered to be asexual reproducers, however, Escherichia coli sometimes contain a plasmid called the F-factor that contains 30 or so genes in a small plasmid. The presence of the F-factor permits a bacterium to conjugate with another bacterium lacking the F-factor. During conjugation, copies of the F-factor are transmitted to recipient cells, converting them from F^- to F⁺. This system is reminiscent of sexual systems in higher organisms.

There are many other unusual systems for sexual development and differentiation, and there seem to be as many exceptions as there are rules. For example, the parasitic wasp, Habrobracon juglandis can reproduce without a partner. This process is called parthenogenesis. Female wasps produce eggs at maturity and begin laying eggs regardless of whether there are males in the environment with which to mate. Both fertilized and unfertilized eggs hatch out and produce viable offspring. Eggs that are not fertilized contain only a single copy of each chromosome, a state that is called haploidy. Haploid offspring are male, and will produce sperm at maturity, and will mate with females to fertilize their eggs. The fertilized eggs receive two copies of each chromosome, one from each parent. This is called dip-loidy. Diploid offspring develop as females. Thus in this species, sex determination is dependent on the number of copies of each chromosome that are present at the time embryogenesis begins. When few males are present in the environment, most eggs will go unfertilized and the offspring will be haploid and thus male. When many males are present in the environment, most eggs are fertilized, giving rise to diploid offspring, which develop as females. While this system for sexual development is not common in nature, it illustrates one of the many innovative ways that sex can be determined in nature.

The benefits of sexual systems for reproduction are not very well understood in nature, but the presence of such elaborate and complex systems for development suggests that there must be benefits to sexual reproduction compared with asexual methods.

See also Chromosome; Developmental processes; Embryo and embryonic development; Parthenogenesis; Sexual dimorphism.

Robert Best