Sex-Determining Mechanisms Help (page 3)

The Importance of Sex

We typically think of sex in terms of males and females . Not all organisms, however, possess only two sexes. Some of the simplest forms of plant and animal life may have several sexes. For example, in one variety of the ciliated protozoan Paramecium bursaria there are eight sexes, or "mating types," all morphologically identical. Each mating type is physiologically incapable of conjugating with its own type, but may exchange genetic material with any of the seven other types within the same variety. However, most complex organisms have only two sexes. These sexes may reside in different individuals or within the same individual. An animal with both male and female reproductive organs is usually referred to as a hermaphrodite. In plants where staminate (male) and pistillate (female) flowers occur on the same plant, the term monoecious is used. Most flowering plants have both male and female parts within the same flower. A few angiosperms are dioecious, i.e., having the male and female elements in different individuals. Among the common cultivated crops known to be dioecious are asparagus, date palm, hemp, hops, and spinach.

Whether there are two or more sexes, or whether these sexes reside in the same or different individuals, sex is still important: it is a mechanism that provides for the great amount of genetic variability that characterizes most natural populations. The evolutionary process of natural selection depends upon this genetic variability to supply the raw material from which better adapted phenotypes usually survive to reproduce. Many subsidiary mechanisms have evolved to ensure cross-fertilization in most species as a means for generating new genetic combinations in each generation.

Sex Chromosome Mechanisms

Most mechanisms for the determination of sex are under genetic control and may be classified into one of the following categories.

Heterogametic Males

In most mammals, there are two different, or heteromorphic, sex chromosomes, the X and the Y chromosomes. The presence of the Y chromosome determines maleness. Normal human males have 22 pairs of autosomes and an X and a Y sex chromosome; females also have 22 pairs of autosomes, but have two X chromosomes. Since the male produces two kinds of gametes as far as the sex chromosomes are concerned, he is said to be the heterogametic sex. The female, producing only one kind of gamete, is the homogametic sex. Thus, assuming unbiased segregation and equal success of each type of gamete during fertilization, an equal number of each sex should be produced in each generation. The proportion of males to females is referred to as the sex ratio. This mode of sex determination is commonly referred to as the XY method.

A gene called SRY, for sex-determining region Y, on the short arm of the Y chromosome encodes a gene product often referred to as the testis-determining factor (TDF). SRY seems to be highly conserved in mammals. This gene, in combination with several other autosomal genes, encodes a DNA-binding protein that appears to control the expression of one or more other genes in a hierarchy or cascade of gene activation involved in testicular development and sperm production. In the absence of SRY embryonic, gonadal tissue would normally develop into an ovary. The location of SRY was aided by the discovery of rare exceptions to the rule that XX results in femaleness and XY results in maleness. Normal-appearing but sterile XX human males have at least some of the SRY gene attached to one of their X chromosomes and normal-appearing human XY females have a Y chromosome that has lost a crucial part of the SRY.

In some insects, especially those of the orders Hemiptera (true insects) and Orthoptera (grasshoppers and roaches), males are also heterogametic, but produce either X-bearing sperm or gametes without a sex chromosome. In males of these species, the X chromosome has no homologous pairing partner because there is no Y chromosome present. Thus, males exhibit an odd number in their chromosome complement. The one-X and two-X condition determines maleness and femaleness, respectively. If the single X chromosome of the male is always included in one of the two types of gametes formed, then a 1 : 1 sex ratio is predicted in the progeny. This mode of sex determination is commonly referred to as the XO method where the O symbolizes the lack of a chromosome analogous to the Y of the XY system.

Heterogametic Females

This method of sex determination is found in a comparatively large group of insects including the butterflies, moths, caddis flies, and silkworms, and in some birds and fishes. The 1-X and 2-X condition in these species determines femaleness and maleness, respectively. The females of some species (e.g., domestic chickens) have a chromosome similar to that of the Y in humans. In these cases, the chromosomes are sometimes labeled Z and W instead of X and Y, respectively, in order to call attention to the fact that the female (ZW) is the heterogametic sex and the male (ZZ) is the homogametic sex. The females of other species have no homologue to the single sex chromosome as in the case of the XO mechanism discussed previously. To point out this difference, the symbols ZZ and ZO may be used to designate males and females, respectively. A 1 : 1 sex ratio is expected in either case.

Genic Balance

The presence of the Y chromosome in Drosophila, although essential for male fertility, is not involved in the determination of sex. Instead, the factors for maleness residing in all of the autosomes are "weighed" against the factors for femaleness residing on the X chromosome(s). In fact, it is the ratio of X chromosomes to haploid sets of autosomes that determines sex in Drosophila. Using the letter A to represent a haploid set of chromosomes, a normal female (2X : 2A) has an X : A ratio of 2 : 2, or 1.0, and therefore the balance is in favor of femaleness. When only one X chromosome is present in a normal male (XY : 2A), the ratio is 1 : 2 or 0.5. Several abnormal combinations of chromosomes have confirmed this hypothesis. For example, an individual with three sets of autosomes and two X chromosomes (2X: 3A) has a ratio of 2 : 3 or 0.67, in between the ratios for normal maleness and femaleness. This kind of fly, called intersex, is sterile and has sexual characteristics intermediate between the male and female. Ratios above 1.0 produce sterile metafemales (previously called super-females) and ratios below 0.5 produce sterile metamales.

The ratio determines sex by activating sex-specific gene expression of several genes, such as Sex-lethal (Sxl), transformer (tra), and doublesex (dsx). Simply stated, in females, the Sxl gene is active and leads to the production of an active tra gene product. This results in the further production of a female-specific DSX protein and development of female flies. The X:A chromosome ratio in normal males does not result in production of the SXL protein; thus, the tra protein is not produced and the default, or male developmental pathway, is followed. A recessive mutation in the tra⁺ gene, when homozygous, can transform a diploid female into a sterile male, since the absense of tra⁺ leads to maleness. The X/X, tra/tra individuals resemble normal males in external and internal morphology with the exception that the testes are reduced in size. This mutation has no effect in normal males. The presence of this mutation can considerably alter the sex ratio.