sex determination

(genetics) The mechanisms which determine whether the bipotential embryo will develop as male or female in a species.

The genetic mechanisms by which sex is determined in all living organisms. The nature of the genetic basis of sex determination varies a great deal among the various forms of life.

There are two aspects of sexuality: the primary form involves the gametes, and the secondary aspect is gender. In its broadest usage the term “sex” refers to the processes that enable species to exchange materials between homologous chromosomes, that is, to effect recombination. Generally, recombination is essential to their mechanism for reproduction. For most organisms this involves, either exclusively or as one stage in the life cycle, the formation of special cells, known as gametes, by meiosis. See also Gametogenesis.

Most sexually reproducing species produce two different kinds of gametes. The relatively large and sessile form, an ovum or egg, usually accumulates nutriments in its cytoplasm for the early development of the offspring. The relatively mobile form, a sperm (or pollen grain in many plants), contributes little beyond a haploid chromosome set. Thus the primary form of sex differentiation determines which kind of gamete will be produced. The formation of gametes usually involves the concomitant differentiation of specialized organs, the gonads, to produce each kind of gamete. The ova-producing gonad is usually known as an archegonium or ovary (in flowering plants it is part of a larger organ, the pistil or carpel); the gonad producing the more mobile gametes is usually known as a testis in animals and an antheridium or stamen in plants. See also Ovary; Ovum; Sperm cell.

In most animals and many plants, individuals become specialized to produce only one kind of gamete. These individuals usually differ not only in which kind of gonad they possess but also in a number of other morphological and physiological differences, or secondary sex characteristics. The latter may define a phenotypic sex when present, even if the typical gonad for that sex is absent or nonfunctional. The form that usually produces ova is known as female; the one that usually produces sperm or pollen is known as male. Since some sexual processes do not involve gametes, the more universal application of the term “gender” refers to any donor of genetic material as male and the recipient as female.

In plants, sexual reproduction is not always accompanied by the kinds of differentiation described above. The majority of plant species are monoecious, with both kinds of gonads on the same plant. Plants that bear male and female gonads on separate plants are dioecious. They occur in about 60 of the 400 or so families of flowering plants, 20 of which are thought to contain exclusively dioecious species. See also Reproduction (plant).

Although the sexes are distinct in most animals, many species are hermaphroditic; that is, the same individual is capable of producing both eggs and sperm. This condition is particularly common among sessile or sluggish, slowly moving forms. Some hermaphroditic and monoecious species are homothallic; that is, the eggs and sperm of the same individual can combine successfully; but most are heterothallic, the gametes being capable only of cross-fertilization, often evolving special mechanisms to ensure its occurrence.

Sex differentiations are often accompanied by consistent chromosomal dimorphisms, leading to the presumption that the chromosomal differences are related to, and possibly responsible for, the sex differences. Indeed, the chromosomes that are not alike in the two sexes were given the name sex chromosomes. Some workers use the term “heterosomes” to distinguish them from the autosomes, which are the chromosomes that are morphologically identical in the two sexes.

In most species, one of the sex chromosomes, the X chromosome, normally occurs as a pair in one gender but only singly in the other. The gender with two X chromosomes is known as the homogametic sex, because each of its gametes normally receives an X chromosome after meiosis. The gender with only one X chromosome generally also has a morphologically different sex chromosome, the Y chromosome. The X and Y chromosomes usually pair to some extent at meiosis, with the result that the XY is the heterogametic sex, with half its gametes containing an X and half containing a Y. Geneticists noted that the fundamental dimorphism of X and Y chromosomes lies in their genic contents: X chromosomes of the species share homologous loci, just as do pairs of autosomes, whereas the Y chromosome usually has few, if any, loci that are also represented on the X. Thus X and Y chromosomes are sometimes very similar in shape or size but are almost always very different in genetic materials.

The major factor in sex differentiation in humans is a locus on the short arm of the Y chromosome designated SRY or SrY (for sex-determining region of the Y). This comparatively small gene contains no introns and encodes for a protein with only 204 amino acids. The protein appears to be a deoxyribonucleic acid (DNA)-binding type that causes somatic cells of the developing gonad to become Sertoli cells that secrete a hormone, Müllerian inhibiting substance (MIS), that eliminates the Müllerian duct system (the part that would produce major female reproductive organs). The gonad is now a testis, and certain cells in it become the Leydig cells that produce testosterone, which causes the primordial Wolffian duct system of the embryo to develop the major male reproductive organs. If no MIS is produced, further development of the Müllerian duct structures occurs, and in the absence of testosterone the Wolffian ducts disappear, producing the normal female structures. Embryos lacking SRY or having mutated forms of it normally become females even if they are XY. This system of sex determination is called Y-dominant. It appears to be characteristic of almost all mammals, even marsupials, plus a few other forms. While SRY is the primary gene, many other genes, both autosomal and X-chromosomal, are involved in the course of developing the two sexes in mammals. See also Human genetics; Mutation.

Theories and myths about what might cause a child to be boy or girl, and what action might be taken to select one or the other, have no doubt been part of all human cultures. Of those that are recorded, we know that it was a common Hippocratic view, believed by Galen, that the right testicle and the right side of the womb produced male children, and the left counterparts, female. This was disputed by Aristotle, who contended that the male determined the sex of the offspring. And he proved to be correct: the sex of a child is determined by the father's sperm.

The germinal cell from which the sperm was formed contained, like all other cells in the man's body, 22 pairs of chromosomes plus another dissimilar pair of sex chromosomes — an X and a Y. The split that resulted in sperms meant that each sperm carried one of these two alternative chromosomes: either an X or a Y, and one copy of each of the other 22 chromosomes. The mother's germinal cells had 22 pairs plus paired X chromosomes, so that every ovum had an X. Fertilization may therefore result in an embryo carrying either two X chromosomes (one from the father, one from the mother), when development will be female (XX), or an X chromosome from the mother and a Y from the father chromosome, when development will be male (XY).

How does this happen? Sometimes the inheritance of the sex chromosomes is disturbed, and this provides clues to the normal mechanism. A person with only one X chromosome and no Y chromosome (XO) develops as a female, whereas an individual with one or more X chromosomes but also at least one Y chromosome develops as a male. An experiment by the French physiologist Jost provided the basis of our understanding of the processes underlying male or female development. He operated on fetal rabbits to remove their gonads at a stage before they had developed in either a male or female direction. All fetuses from whom the testis rudiment had been removed developed as though they were female, as also did those from whom the developing ovary was removed. Thus female development is the default. It is the presence of a testis which causes male development. The testis itself develops as a result of the presence of a Y chromosome. Thus the Y chromosome determines male development by the possession of some testis determining factor (TDF).

Analysis of individual people who have either inherited an altered Y chromosome and are still female, or who apparently have 2 XX chromosomes but are male, revealed that it was only a small region of the Y chromosome which was responsible as TDF. Pieces of the Y chromosome were either missing or located in another place, and it was therefore possible to identify the gene responsible, by a positional cloning approach. At the same time as this work was progressing a mouse was discovered with a Y chromosome with a mutation that made its TDF non-functional.

Other workers isolated a candidate human gene that fulfilled all the criteria for the TDF. The scientist who had isolated the mutant mice was able to show that the equivalent gene was mutated and that its normal expression was at a critical early stage of testis development. The final proof that this was the gene responsible for sex determination came when the two groups collaborated to make mice carrying an extra copy of the human gene. These mice developed as males.

This is therefore an example of a gene which causes a switch in developmental direction. We now need to know how this gene function is turned on at the critical point of gonad development and what downstream functions it itself controls to cause male development.

— Martin Evans

See also genetics, human; germ cells; gonads; heredity.

Sex determination refers to the mechanisms employed by organisms to produce offspring that are of two different sexes. First we present an overview of the sex determination mechanisms used by mammals. Then we discuss the great variety of mechanisms used by animals other than mammals.

Mammalian Mechanisms

A developing mammalian embryo's gender is determined by two sequential processes known as primary and secondary sex determination.

Primary Sex Determination

Early in an embryo's development (four weeks after fertilization, in humans), two groups of cells become organized into the gonad rudiment that will eventually develop into either the ovaries or testicles. These gonads will eventually be the source of gametes in the adult. However, at this early stage they are unstructured organs that lack sex-specific features but have the potential to develop into gonads.

The first visible indication of sex-specific development, occurring in week seven in humans, is in males, with the gonads restructuring into two distinct compartments: the testicular cords and the interstitial region. In females, the gonads appear to lack distinct structures until later in development. Primary sex determination, including the differentiation of an embryo's gonads, is dependent on genetic factors associated with the embryo's sex chromosomes.

Secondary Sex Determination

Secondary sex determination involves the development of additional sex-specific characteristics, such as the genitalia. This secondary pathway is controlled by sex-specific hormones that are secreted by the differentiated gonad. These hormones influence the sex differentiation of other parts of the body, including two pairs of ducts present in all developing embryos: the Müllerian ducts and the Wolffian ducts.

Testicles produce Müllerian inhibiting substance, a hormone that causes the Müllerian duct to degenerate. They also produce testosterone, which causes the Wolffian duct to develop into the internal male genitalia, such as the seminal vesicles and the vas deferens. Testosterone also promotes the development of the external male genitalia, including the penis, and it reduces the development of the breasts.

In females, where there are no testicles and where there is therefore no Müllerian inhibiting substance, the Müllerian duct differentiates into internal female genitalia: the fallopian tubes, uterus, and cervix.

Discovering the Testis-Determining Factor

The different effects of the primary and secondary sex determination pathways was demonstrated by embryological transplant experiments carried out by Alfred Jost in the 1940s at the Collége de France in Paris, France. When Jost placed an undifferentiated gonad from a male rabbit next to an undifferentiated gonad inside a female fetus, the gonad from the female developed into an ovary, and the gonad from the male developed into a testicle, as it would have done inside the male. Hence, the sex of the gonad was dependent upon its genotype (XX or XY) and is a result of primary sex determination. The genitalia of these experimental animals revealed the influence of secondary sex determination mechanisms. Under the conflicting signal of the two gonads, the Müllerian duct, which normally would have developed into female genitalia, degenerated partially, and the Wolffian duct, which normally would have degenerated, began to develop into male genitalia.

Jost's experiment indicated that the sex differentiation of a gonad is determined by its sex chromosomes, and that the sexual characteristics of other tissue are determined by the gonads, not by the chromosomes in the tissues themselves. Jost also showed that in the absence of either gonad, the fetus develops as a female. Female development, then, is apparently a "default" pathway that can be overridden to produce a male.

Studies by C. Ford, P. Jacobs, and co-workers in 1959 demonstrated the importance of genes on the X and Y chromosomes as sexual determinants by documenting the sexual phenotypes of humans with abnormal chromosomal constitutions. Errors in meiosis can produce sperm or egg cells that have abnormal numbers of sex chromosomes. Upon fertilization, these cells develop into embryos with their own aberrant sex chromosome dosage.

Cells in individuals with Turner's syndrome contain a single copy of the X chromosome and no copies of the Y chromosome. These "XO" individuals develop as females, although their ovaries are nonfunctional. This demonstrates that just a single copy of the X chromosome is sufficient to direct most female sex development.

The reciprocal condition, "YO," with no X chromosome present, has not been documented in humans, as X chromosomes contain genes necessary for an embryo's survival. Individuals with Klinefelter's syndrome (XXY), however, develop as phenotypic males, although they produce no sperm. This indicates that a single copy of the Y chromosome is sufficient to override the female developmental program and promote most male development.

Such observations led to speculation that the Y chromosome contains a "testis-determining factor" necessary to activate development of the male gonads. Several genes on the Y chromosome were suggested as possible testis-determining factors but ultimately rejected. In 1990 Peter Goodfellow and coworkers at the Human Genetics Laboratory in London, England, studied a group of sex-reversed XX males. Such individuals develop as phenotypic males despite being XX individuals.

The researchers discovered that the XX males had a small segment from a Y chromosome incorporated into one of their X chromosomes. This same segment was found to be missing from the Y chromosome of a group of sex-reversed XY individuals, who developed as phenotypic females. The segment acted as a testis-determining factor, as its presence was correlated with the activation of male development. DNA sequencing of the segment identified a gene that was named "SRY" from the description "sex-determining region of the Y chromosome."

Sry's Function

Studies in mice have supported SRY's role as a primary determinant of male development. The mouse homolog of SRY (Sry) is expressed in developing gonads in males but not females. It is present during but not after testis differentiation. Finally, experiments have been conducted where introducing the SRY gene into the genomic DNA of embryonic female (XX) mice causes some of them to develop as males.

Despite the clear causal relationship between this gene and male development, the specific mechanisms involved are unclear. The SRY protein is similar to "high-mobility group" proteins, which regulate the transcription of other genes. Its structure contains a domain that can bind to specific target DNA sequences. Mutations to this domain, which reduce SRY's ability to bind correctly to DNA, are frequently observed in XY individuals that develop as females.

SRY could conceivably function by activating genes involved in testicle differentiation, by repressing genes involved in ovary development, or by doing both of these things. To discriminate among the possibilities, it is necessary to know more about the next level of genes in the developmental cascade, SRY's targets. Several possible direct targets of SRY have been proposed. Most notable is the SOX9 gene, which also encodes a high-mobility group protein that is known to promote male differentiation and which is expressed in the developing male gonad immediately after SRY is first expressed.

Female Development As the Default

When Jost removed the gonads from embryonic rabbits, the embryos that were XX as well as those that were XY developed as females, though they lacked internal genitalia. This finding emphasized that gonads are critical to secondary sex determination and in the absence of male-specific hormones, female characteristics develop even in an XY individual.

As noted above, individuals with only a single copy of the X chromosome in each cell can survive, and they develop as females. Their ovaries develop normally at first but degenerate around the time of birth, resulting in a sterile adult. Hence a single X chromosome suffices for sex determination, but two copies are needed for ovary maintenance.

One explanation is that the female developmental program is the default, with embryos developing as females unless there are alternative instructions. SRY is a major switch gene required for male development, and only a gonad whose cells contain a copy of SRY will differentiate into a testicle.

Despite intensive searching, no major switch gene has been identified for the female developmental pathway. One candidate, DAX1, was proposed as the main ovarian differentiation gene principally because it was found to be duplicated on the X chromosomes of two XY siblings who developed as females. However, experimentally disrupting the mouse homolog of DAX1 had no effect on the sex determination, maturation, or fertility of female XX mice.

Instead of having a positive regulatory role, DAX1 appears to be antagonistic to SRY's function. When DAX1 is present in two copies, as in the XY sisters, it apparently disrupts SRY function sufficiently to prevent initiation of the male developmental program. These observations are consistent with the idea that the female sex determination pathway is the default option.

Nonmammalian Mechanisms

Sex determination and differentiation occur in virtually all complex organisms, but the mechanisms used by various animal classes, and even by various vertebrates, differ significantly. Birds, for instance, lack a clear homolog of SRY. In birds it is the female, rather than the male, that has two different sex chromosomes, with males being ZZ, and females ZW. In many reptiles, environmental conditions, rather than genetic factors, are the primary determinant of sex. The temperature at which eggs are incubated determines sex in some lizard, turtle, and alligator species.

In both the fruit fly Drosophila melanogaster and the nematodeCaenorhabditis elegans, the primary sex determination mechanisms and the molecular cascades controlling sexual differentiation have been studied extensively. Primary sex determination in these animals does not involve the Y chromosome but instead is determined by the ratio of the number of X chromosomes to autosomal (nonsex) chromosomes. By examining individuals with unusual numbers of various chromosomes it has been determined that in Drosophila those with one or fewer X chromosomes per diploid autosome set develop as males while those with two or more X chromosomes develop as females. Individuals with intermediate ratios such as those with two X chromosomes and a triploid set of autosomes develop as intersexes with both male and female characteristics. Although this ratio serves as the primary determinant of sex in both of these organisms, the specific gene products that influence this ratio assessment are different, demonstrating that different molecular mechanisms can be used for a similar purpose.

There is also significant variability in the strategies by which the outcome of sex determination is communicated to the various tissues that undergo sexual differentiation. In humans and most other mammals, the presence or absence of SRY protein in cells of the gonad specifies their sexual differentiation and which hormones are secreted by the gonad to direct the sexual differentiation of most other cells in the individual.

In Drosophilahormones have little effect on sexual differentiation. Instead, with only a few exceptions, each cell decides its sex independently of other cells and tissues. This "cell autonomous" mechanism is demonstrated in experimentally produced mosaic organisms called gynandromorphs ("male-female forms") in which some cells are XX (female) and others XO (male). Such individuals develop into adults with a mix of male and female cell types that match each cell's genotype. The lack of evolutionary conservation of sex-determining mechanisms among animals is particularly interesting because of the similarities that exist in other major switch genes for basic developmental processes.

Bibliography

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Cline, Thomas W., and Barbara J. Meyer. "Vive la Difference: Males vs. Females inFlies vs. Worms." Annual Reviews of Genetics 30 (1996): 637-702.

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Hodgkin, Jonathan. "Genetic Sex Determination Mechanisms and Evolution." BioEssays 4 (1992): 253-261.

Sinclair, Andrew H., et al. "A Gene from the Human Sex-Determining Region Encodes a Protein with Homology to a Conserved DNA-Binding Motif." Nature 346 (1990): 240-224.

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—Jeffrey T. Villinski and William Mattox

The determination of the sex of a fetus by identifying sex chromatin in amniotic fluid obtained by amniocentesis.