In this essay we will discuss about the process of sex determination in different species.
Process of Sex Determination
- Essay on the Introduction to the Process of Sex Determination
- Essay on the Chromosome Theory of Sex Determination
- Essay on Animals with Heterogametic Females
- Essay on the Process of Sex Determination in Human Beings
- Essay on Genic Balance Theory of Sex Determination in Drosophila
- Essay on Haplodiploidy and Sex Determination in Hymenoptera
- Essay on the Process of Sex Determination in Coenorhabditis Elegans
- Essay on Environmental Factors and Sex Determination
Essay # 1. Introduction to the Process of Sex Determination:
In nature a large number of diverse mechanisms exist for determination of sex in different species. The fruit fly Drosophila melanogaster and human beings are very important in the development of genetic concepts because in these two organisms, and in many others, individuals normally occur in one of two sex phenotypes, male or female.
In these species males produce male gametes, sperm, pollen or microspores while females produce female gametes namely, eggs, ovules or macrospores. In many species the two sexes are phenotypically indistinguishable except for the reproductive organs. Sex determination is aimed at identifying the factors responsible to make an organism a male or female or in some cases a hermaphrodite. So far the mechanism of sex determination has been related to the presence of sex chromosomes whose composition differs in male and female sexes.
However, in recent years sex determination has been differentiated from sex differentiation, and sex determination mechanism is explained more on the basis of the specific genes located on sex chromosomes and autosomes. Sex determination is recognized as a process in which signals are initiated for male or female developmental patterns.
During sex differentiation, events occur in definite pathways leading to the development of male and female phenotypes and secondary sexual characters. Significant progress has been made in understanding the mechanism of sex in human beings and other mammals and new genes have been identified.
Sex determination in higher animals is controlled by the action of one or more genes. The testis determining factor (TDF) gene is the dominant sex determining factor in human beings. Hemking a German biologist identified a particular nuclear structure throughout the spermatogenesis in some insects. He named it as “X-body” and showed that sperm differed by its presence or absence. The X body was later found to be a chromosome that determined sex. It was identified in several insects and is known as the sex or X chromosome.
Thus, the chromosome theory of sex determination states that female and male individuals differ in their chromosomes. Chromosomes can be differentiated into two types, autosomes and sex chromosomes. Sex chromosomes carry genes for sex. In some animals, females have one more chromosome than males, thus they have two X chromosomes and males have only one.
Females are therefore cytologically XX and males are XO, where ‘O’ denotes the absence of X chromosome. During meiosis in the female the 2X chromosome pairs and separates producing eggs that contain a single X chromosome. Thus all eggs are of the same type containing only one X chromosome.
During meiosis in the male, the single X chromosome moves independently of all the other chromosomes and is incorporated into half of the sperm, the other half do not have any X chromosome. Thus, two types of sperms are produced, one with X chromosome and the other without the X chromosome or designated as ‘O’.
When the sperm and eggs unite, two types of zygotes are produced; XX that develop into females and XO that develop into males. Because both of these types are equal in number, the reproductive mechanism preserves a 1:1 ratio of males to females.
In many animals, including human beings, males and females have the same number of chromosomes. This numerical equality is due to the presence of a chromosome in the male called the ‘Y’ chromosome, which pairs with the X. During meiosis in the male, the X and Y-chromosomes separate from each other producing two types of sperm, one type with X chromosome and the other type having Y chromosome.
The frequencies of the two types are approximately equal. Females with XX chromosomes produce only one type of eggs, all with X chromosome. In random fertilization, approximately half of the zygotes are with XX chromosomes and the other half with XY chromosomes leading to a sex ratio of 1:1.This mechanism is called XX – XY type of sex determination.
The XY mechanism is more prevalent than the XO mechanism. The XY type is considered characteristic in higher animals and occurs in some plants. This mechanism is operative in Drosophila melanogaster and human beings. Both species exhibit the same pattern of transmission of X and Y chromosomes in normal individuals in– natural populations. In human beings, the X chromosome is considerably longer than the Y chromosome.
The total complement of human chromosomes includes 44 autosomes: XX in the female and XY in the male. Eggs produced by the female in oogenesis have a complement of 22 autosomes plus an X chromosome. Sperm from the male have the same autosomal number and either an X or a Y chromosome. Eggs fertilized with sperm containing a Y chromosome result in zygotes that develop into males; those fertilized with sperm containing an X chromosome develop into females.
In animals with XX-XY mechanism of sex determination, females (XX) produce gametes that have the same chromosome composition (one X plus one set of autosomes). These females are homogametic sex as all the gametes are the same. The males of these animals are heterogametic as they produce two types of gametes, one half containing one X chromosome plus one set of autosomal chromosomes and the other one half contain one Y chromosome plus one set of autosomes.
Essay # 3. Animals with Heterogametic Females:
In many birds, moths and some fish, the sex determination mechanism is identical to the XX-XY mechanism but the females are heterogametic (ZW) and males are homogametic (ZZ). This mechanism of sex determination is called ZZ-ZW.
In this mechanism the relationship between sex chromosomes and sex phenotypes is reversed. In birds the chromosome composition of the egg determines the sex of the offspring, whereas in humans and fruit flies, the chromosome composition of the sperm determines the sex of the offspring.
Essay # 4. Process of Sex Determination in Human Beings:
In human beings, sex is determined by the number of X chromosomes or by the presence or absence of the Y chromosome. In human beings and other placental mammals, maleness is due to a dominant effect of the Y chromosome. The dominant effect of the Y chromosome is manifested early in development when it directs the primordial gonads to differentiate into testes.
Once the testes are formed, they secrete testosterone that stimulates the development of male secondary sexual characteristics. Testis determining factor (TDF) is the product of a gene called SRY (Sex determining Region of Y), which is located in the short arm of the Y chromosome of the mouse. SRY was discovered in unusual individuals whose sex was not consistent with their chromosome constitution – males with XX chromosomes and females with XY chromosomes.
Some of the XX males carried a small piece of the Y chromosome inserted into one of the X chromosomes. It is evident that this small piece carried genes for maleness. Some of the XY females carried an incomplete Y chromosome. The part of the Y chromosome that was missing corresponded to the piece that was present in the XX males.
Its absence in the XY females prevented them from developing testes. These observations show that a particular segment of the Y chromosome was required for the development of the male. Further studies showed that the SRY gene is located in this male determining segment. Like that of the human SRY gene is present in the Y chromosome of the mouse and it specifies male development (Fig. 5).
After the formation of the testes, testosterone secretion initiates the development of male sexual characteristics. The hormone testosterone binds to receptors of several types of cells. This binding leads to the formation of a hormone – receptor complex that transmits signals to the cell instructing how to differentiate.
The combined differentiation of many types of cells leads to the development of male characteristic like beard, heavy musculature and deep voice. Failure of the testosterone signaling system leads to nonappearance of the male characters and the individual develops into a female. One of the reasons for failure is an inability to make the testosterone receptor (Fig. 6).
Individuals with XY chromosomal composition having this biochemical deficiency first develop into males. In such males, although testis is formed and testosterone secreted, it has no effect because it cannot reach the target cell to transmit the developmental signal. Individuals lacking the testosterone receptor therefore can change sexes during embryological development and acquire female sexual characteristics.
However, such individuals do not develop ovaries and remain sterile. This syndrome known as testicular feminization is due to a mutation in an X-linked gene, tfm that codes for the testosterone receptor. The tfm mutation is transmitted from mothers to sons who are actually phenotypically female in a typical X- linked manner.
Master Regulatory Gene:
In human beings irregular sex chromosome constitutions occur occasionally. Any number of X chromosomes (XXX or XXXX), in the absence of a Y chromosome give rise to a female. For maleness, the presence of a Y chromosome is essential and even if several X chromosomes are present (XXXXY), the presence of a single Y chromosome leads to maleness.
The Y chromosome induces the development of the undifferentiated gonad medulla into testis, whereas an XX chromosomal set induces the undifferentiated gonadal cortex to develop into ovaries. The gene on the Y chromosome that induces the development of testes is called as Testis Determining Factor (TDF). It has been isolated, characterized and found to encode a protein that regulates the expression of other genes.
Thus, the TDF gene is the master regulator gene that triggers the expression of large number of genes that produce male sex phenotype. In the absence of TDF gene, the genes that produce femaleness predominate and express to produce a female phenotype. The TDF exerts a very dominant effect on development of the sex phenotype.
Essay # 5. Genic Balance Theory of Sex Determination in Drosophila:
In Drosophila investigations by C.B. Bridges have shown that X chromosomes contain female determining genes and male determining genes are located on the autosomes and many chromosome segments are involved. The genie balance theory of sex determination in Drosophila explains the mechanism involved in sex determination in this fly.
The Y chromosome in Drosophila does not play any role in sex determination. Sex in this animal is determined by the ratio of X chromosomes to autosomes. Normal diploid insects have a pair of sex chromosomes, either XX or XY, and three pairs of autosomes. These are denoted by AA, each A representing one set of haploid autosomes. Flies with abnormal number of autosomes can be produced by genetic manipulation as shown in Table 1.
Whenever the ratio of X chromosomes to autosomes is 1.0 or above, the sex of the fly is female, and whenever it is 0.5 or less, the fly is male. If the ratio is between 0.5 and 1.0, it is an intersex with both male and female characters. In all these phenotypes, Y chromosome has no role to play but it is required for the fertility of the male. In Drosophila sex determination mechanism, an X-linked gene called Sex lethal (Sxl) plays an important role (Fig. 7).
A number of X linked genes sets the level of Sxl activity in a zygote. If the ratio between X chromosomes and autosomes is 1.0 or above, the Sxl gene becomes activated and the zygote develops into a female. If the ratio is 0.5 or less, the Sxl gene is inactivated and the zygote develops into a male. A ratio between 0.5 and 1.0 leads to mixing of signals and the zygote develops into an intersex with a mixing of male and female characters.
The sex ratio of X chromosomes to autosomes and the phenotype of Drosophila determination pathway in Drosophila has three components:
(i) A system to ascertain the X : A ratio in the early embryo,
(ii) A system to convert this ratio into a developmental signal, and
(iii) A system to respond to this signal by producing either male or female structures.
The system to ascertain the X : A ratio involves interactions between maternally synthesized proteins that have been deposited in the eggs cytoplasm and embryologically synthesized proteins that are coded by several X-linked genes. These latter proteins are twice as abundant in XX embryos as in XY embryos and therefore provide a means for counting the number of X chromosomes present.
Because the genes that encode these proteins effect the numerator of the X : A ratio, they are called numerator elements. Other genes located on the autosomes affect the denominator of X : A ratio and are therefore called as denominator elements. These encode proteins that antagonize the products of numerator elements (Fig. 8).
The system for ascertainment of the X : A ratio in Drosophila is therefore based on antagonism between X-linked (numerator) and autosomal (denominator) gene products. Once the X : A ratio is ascertained, it is converted into a molecular signal that controls expression of the X-linked sex lethal gene (Sxl), the master regulator of the sex determination pathway.
Early in development, this signal activates transcription of the Sxl gene from PE’ the gene’s ‘early’ promoter, but only in XX embryos. The early transcripts from this promoter are processed and translated to produce functional sex-lethal proteins, denoted Sxl. After only a few cell divisions, transcription from the PE promoter is replaced by transcription from another promoter, PM.
The so called maintenance promoter of the Sxl gene. Interestingly, transcription from the PM promoter is also initiated in XY embryo. However, the transcripts from PM are correctly processed only if Sxl protein is present. Consequently, in XY embryos, where this protein is not synthesized, the Sxl transcripts are alternately spliced to include an exon with a stop codon, and when these alternately spliced transcripts are translated, they generate a short polypeptide without regulatory function.
Thus, alternate splicing of the Sxl transcripts in XY embryos does not lead to the production of functional Sxl protein and in the absence of this protein, these embryos develop as males. In XX embryos, where Sxl protein was initially made in response to X : A signal, Sxl transcripts from the PM promoter are spliced to produce more Sxl proteins.
In XX embryos, this protein is therefore, a positive regulator of its own synthesis forming a feedback mechanism that maintains the expression of the Sxl proteins in XX embryos and prevents its expression in XY embryos. The Sxl protein also regulates the splicing of transcription from another gene in the sex determination pathways, transformers (tra). These transcripts can be processed in two different ways.
In chromosomal males, where the Sxl protein is absent, the splicing apparatus always leaves a stop codon in the second exon of the tra RNA. Thus, when spliced tra RNA is translated, it generates a truncated polypeptide. In females, where the Sxl protein is present, this premature stop codon is removed by alternate splicing in at least some of the transcripts. Thus, when they are translated, some functional transformer protein tra is produced. The Sxl protein therefore allows the synthesis of functional tra protein in XX embryos but not in XY embryos (Fig. 9).
The tra protein also turns out to be a regulator of RNA processing. Along with tra 2, a protein encoded by the transformer 2 (tra 2) gene, it encodes the expression of double sex (dsx) an autosomal gene that can produce two different proteins -through alternate splicing of its RNA. In XX embryos, where the tra protein is present, dsx transcripts are processed to encode a DSX protein that represses the genes required for male development.
Therefore, such embryos develop into females. In XY embryos, where the TRA protein is absent, dsx transcripts are processes to encode a DSX protein that represses the gene required for female development. Consequently, such embryos develop into males. The dsx gene is therefore, the switch point at which a male or female developmental pathway is chosen. From this point, different sets of genes are specifically expressed in males and females to bring about sexual differentiation.
Essay # 6. Haplodiploidy and Sex Determination in Hymenoptera:
In the order hymenoptera including bees, wasps, ants and sand flies, males develop parthenogenetically from unfertilized eggs and have a haploid chromosomal number (in honey bee drone, there are 16 chromosomes). The queen honeybee and workers develop from fertilized eggs and carry the diploid number of 32 chromosomes. Because the normal males are haploid and normal females are diploid, this mechanism is known as haplodiploidy.
The hemizygous, hortiozygous and heterozygous status of certain chromosome segments controls sex determination. Female determination depends on heterozygosity for part of a chromosome. If different forms of this segment of chromosome involved are designated Xa, Xb and Xc, then individuals of chromosome make up XaXb, XaXc and XbXc are all females.
Hemizygous individuals Xa, Xb, or Xc cannot be heterozygous and are therefore male. Genetic manipulations to produce homozygous diploid males showed that sex determination depends on the genetic composition of this region and not on diploidy versus haploidy (XaXa, XbXb, or XcXc).
Mosaics and Gynandromorphs:
Abnormal chromosomal behaviour in insects produces sexual mosaics or gynandromorphs. In these forms some parts of the animal are male and others are female. When such abnormal chromosomal transmission involves autosomes lodging genes that control easily recognized phenotypes, individuals may also be produced that are mosaic for phenotypes unrelated to sex phenotype. Some gynandromorphs in Drosophyla are bilateral intersexes (Fig. 10) with male color pattern; body shape and sex comb on one half of the body and female characteristics on the other half of the body. Both male and female gonads and genitalia are present.
The reason for bilateral gynandromorphism is irregularity in mitosis at the first cleavage of the zygote (Fig. 11). A chromosome lags behind in division and does not arrive at the pole in tine to be included in the newly formed daughter nucleus. When one of the X chromosomes of an XX female zygote lags behind in the spindle, one daughter nucleus receives only one X chromosome, while the other receives two X chromosomes resulting in a mosaic body pattern.
One nucleus in the two nuclei stage would be XO male. If the cleavage plane is so oriented that one daughter nucleus goes towards the right, that part will give rise to all cells that make up the right half of the adult body and the other half gives rise to the left half. If the loss of chromosome occurs at a later stage in cell division, smaller parts of the adult body would be male.
Position and size of the mosaic sector are determined by the place and time of the division abnormality.
Essay # 7. Process of Sex Determination in Coenorhabditis Elegans:
Coenorhabditis elegans is a nematode hermaphrodite species having two X chromosomes and five pairs of autosomes. Occasionally animals with a single X chromosome and five pairs of autosomes are produced by meiotic non disjunction. These animals are males capable of producing sperms but not eggs. Hermaphrodites are females in their vegetative parts (soma) but mixed in their genetic composition.
The somatic sex determination pathway in C.elgans involves atleast 10 different genes. The tra-1 and tra-2 gene products are required for normal hermaphrodite development and that the her-1 gene product is needed for normal male development. The fem gene products fem-1, fem-2, fem-3 are also needed for normal male development. The gene her-1, encodes a secreted protein that is likely to be a signaling molecule.
The next gene, tra-2i encodes a membrane bound protein, which may function as a receptor for the her-1 signalling protein. The products of the fem genes are cytoplasmic proteins that may transduce the her-1 signal and the last gene in the pathway, tra-1 encodes a zinc finger type transcription factor, which may regulate the gene involved in sexual differentiation (Fig. 12).
In Coenorhabditis elegans the sex determination pathway involves a series of negative regulators of gene expression. In XO animals the secreted her-1 gene product apparently interacts with the tra-2 gene product, causing it to become inactive. This interaction allows the three fem gene products to be activated and they collectively inactivate the tra-1 gene product that is a positive regulator of female differentiation. Because the animal cannot develop as a hermaphrodite without active tra-1 protein, it develops into a male.
In XX animals, the her-1 protein is not formed, therefore its putative receptor, the tra-2 protein remains active. Active tra-2 protein causes the fem gene products to be inactivated, which in turn allows the tra-1 protein to stimulate differentiation of the female. The animal therefore develops into a hermaphrodite.
Sexual development in Caenorhabditis fundamentally depends on the X : A ratio, just as it does in Drosophila. The X : A ratio is somehow converted into a. molecular signal that controls sexual differentiation. The signal from the X : A ratio is directed into the sex determination and dosage compensation pathways through a short pathway involving at least four genes. One of these genes, xol -I is required in males but not in hermaphrodites. Three other genes, Sdc-1, Sdc-2 and Sdc-3 are negatively regulated by Xol-i. These Sdc genes are needed in hermaphrodites but not in males.
Development of animals is sensitive to an imbalance in the number of genes. Normally each gene is present in two copies. Departures from this condition, either up or down can produce abnormal phenotypes and sometimes even death. It is therefore, puzzling that many species have a sex determination system based on females with two X chromosomes and males with only one X chromosome.
Normal females have IX chromosomes when male has IX chromosome. This is an unique situation as the number of chromosomes is same in males and females. Such disparities or differences create a “genetic dosage” problem between males and females for all the X-linked genes.
Some females have two copies of X-chromosome and males only one. Therefore, there is potential for females to produce twice as much of each gene product for all the X-linked genes. For compensating this dosage problem, it is proposed that one of the X-chromosome becomes heterochromatin in the case of the female, so that dosage of genetic information expressed in both females and males is equal.
Dosage Compensation in Drosophila:
In Drosophila dosage compensation of X-linked genes is achieved by an increase in the activity of these genes in males. This phenomenon, called “hyperactivation” involves complex of different proteins that binds to many sites on the X-chromosome in males and triggers a doubling of gene activity. When this protein complex does not bind, as in the case of females, hyperactivation of X-linked genes does not occur. In this way total X-linked gene activity in males and females is approximately equal (Fig. 13).
Dosage Compensation in Humans:
In human beings dosage compensation of X-linked genes is achieved by the “inactivation” of one of the females X-chromosomes. This mechanism was first proposed by Mary Lyon in 1961. The chromosome to be inactivated is chosen at random. Once chosen it remains inactivated in all the descendants of that cell. In human embryos sex chromatin bodies have been observed by the 16th day of gestation. Some human traits are influenced by both X chromosomes during the first 16 days. Later only one X chromosome is functional.
Thus, the female is a mosaic with some parts having the alternate allele expressed. X chromosome inactivation occurs only when at least two X chromosomes are present. When a number of X chromosomes are present in the same nucleus, all but one are inactivated. The number of sex chromatin bodies present after inactivation is one less than the number of X chromosomes present in the original cell.
Dosage Compensation in Caenorhabditis Elegans:
In C.elegans dosage compensation involves the partial repression of X-linked genes in the somatic cells of hermaphrotites. In C.elegans dosage compensation is achieved by “hypoactivating” the two X chromosomes in XX hermaphrodites.
Essay # 8. Environmental Factors and Sex Determination:
The environmental factors determine whether an individual develops into a male and female. They live as parasites in the reproductive tract of the well developed and bigger female. In male all organs except the reproductive system are degenerate. During reproduction, the female releases eggs into the seawater. The eggs hatch out to release young worms. Some of the young worms reach the proboscis of the female and become males.
They reach the female reproductive tract and lie as permanent parasites on the female. The young worms, which fail to reach a female, develop to become females. Genetic determiners for both the sexes are present in all young worms. It has been observed that the young worms become attracted towards the extracts of the female proboscis and become males.
In some reptiles, temperature plays an important role in determining the sex. In the turtle Chrysema picta incubation of eggs prior to hatching at high temperature leads to the development of females. However, in the lizard
Agama high incubation temperature leads to male progeny.
Although the segregation of specific sex determining genes and chromosomes is responsible for sex in most animals, the genetic potential for both maleness and femaleness exists in every zygote and some specific factor in the environment triggers the expression of maleness or femaleness producing genes resulting in the production of male phenotype or female phenotype.