Contents
Environmental biology is one of the critical Biology Topics that involves understanding how humans impact the environment and how to address environmental issues.
Process of Sex Determination – Types of Chromosomal in Humans, Birds, Honey Bee
Sexual dimorphism is an interesting aspect of diversity in the living world. Hence, an attempt to reveal the mechanism of sex determination in the living world has been taken as an important investigation in biological systems. However, some planned investigations on this matter could only be started after 1870 when Von Beneden and Hartwig revealed the process of fertilization during sexual reproduction. During the early part of the 20th century, the discovery of sex chromosomes first threw some light on the basis of sexual dimorphism in animals. However, during the subsequent period, it was realized that sexual dimorphism may hardly be explained with the help of sex chromosomes. There are some organisms or animals that do not contain any sex chromosome.
In some animals males and females are developed depending on their environmental condition. Because of these sex determination and sex differentiation have been considered as two different aspects of sexual dimorphism in animals. Considering this fact it may be said that in sexual differentiation dimorphic characters in males and females are developed and sex determination directs this differentiation path in the zygotes. However, both processes are affected by the involvement of many genes. After the completion of the effects of sex-determining genes the differentiating genes start their work to develop the male and female-specific characters in the growing embryo. A concept at the present time has been developed that states that some primary signals trigger the sex-determining genes to be active to promote the development of the early embryo establishing its sex.
Modern Concept on Sexual Dimorphism
According to modern concepts, sexual dimorphic is established in animals following three sequential steps. In this regard, McLaren (1988) advocated a generalized model to explain the differentiation of the sexes.
The model states that the environment, chromosomes, and genes may act at the initial level as primary signals for promoting the activation of the sex-determining genes. The sex-determining genes then being activated influence the sex-differentiating genes for the development of sex-specific characters. In this context, it may be pointed out that environment acts as the primary signal in the sex determination of Bonellia, Agama, and Crocodiles whereas, in Drosophila melanogaster chromosome combination in the zygote acts as the primary signal. The genic signal acts in the sex determination of man and other mammalian individuals. Primary signals in sex determination appear to be of three different types namely environment, chromosome, and gene. Different factors in the process of development in several organisms promoting their sex determination may be described as under:
Environment as Sex-Determining Signal
Depending on the environmental condition sexes in many organisms may be determined. Various environmental factors are found to act on this process. Several environmental factors involved in the process of sex determination in some organisms may be indicated in the following way.
1. Temperature:
In some species of reptiles sexual dimorphism is found to be greatly influenced by environmental temperature. When the eggs of crocodiles are incubated at 33°C only male crocodiles are developed from the eggs. On the other hand, their eggs incubated at 31°C develop the female crocodiles. A similar observation could be noted in a lizard named Agama Agama. At 26 – 27°C the eggs of this lizard develop only female offspring. Conversely, their eggs incubated at 29°C develop only male progeny. In the case of a turtle, Graptemys, a temperature lower than 30°C promotes male development, while a female may be developed at a temperature above 30°C.
2. Association:
Sometimes association has been found to trigger sexual differentiation in the organisms. Male and female development in marine echiura Bonellia sp. is dependent on association. When the larvae of Bonellia emerged from their eggs and remain in association with female Bonellia in the surroundings the larvae develop into male individuals. On the other hand, if the larvae develop in an isolated fashion in the absence of any females in the surroundings, they develop into females. It is to be mentioned here that the males of Bonellia live as parasites in the proboscis of the female. It has also been revealed by some investigators that the females of Bonellia secrete certain hormone-like substances in the surrounding medium and under the influence of such hormones larval differentiation in a female manner is prevented. It has been found in some coral fishes that if the surrounding environment remains overpopulated by small fishes, a female coralfish may be transformed into a male.
3. Nutrition:
The plant Equisetum grown in high-nutrient soil develops more female features. Conversely, when they are grown in less nutrient soil they develop more male features. From the above discussion, it becomes apparent that environmental conditions may have some influence on the functional activity of the sex-determining genes. Though a detailed account of the mode of imposing the impact of environmental factors on gene activity is yet to be revealed. But it may emphatically be said that under certain environmental conditions, the male-promoting genes are activated and in other conditions, the female sex-triggering genes are activated. On the basis of this, either male or female sexual differentiation may be established.
Chromosomal Signal
Before a discussion on the chromosomes acting as signals in sex determination, we should be aware of the history of the discovery of sex chromosomes and their distinction. In 1891 Henking while studying Pyrrhocoris on their spermatogenesis observed a darkly stained body in half of the spermatozoa of the testis. He called these X-bodies, but he could not be able to reveal the function of these X-bodies. In 1902 McClung observed variable chromosome complements in the somatic cells of males and females of different grasshoppers. He also observed similar objects {i.e., X bodies) in the cells produced through spermatogenesis as observed by Henking 1891.
However, he failed to trace similar bodies in the cells during oogenesis. McClung then correlated the X-body in male sex determination in grasshoppers. Subsequently by 1905 Wilson and his colleagues could be able to reveal the significance of the X-body. Wilson observed in a species of grasshopper, Pronetor sp. that males and females of this species differ by their chromosome number.
He observed that the sperm mother cell contained 13 chromosomes and the ovum mother cell contained 14 chromosomes. He also noted that the male Pronetor could produce two types of spermatozoa. One type with 7 chromosomes and the other type with 6 chromosomes. On the contrary, the females of the species could produce eggs with always 7 chromosomes. Wilson also realized that both male and female Pronetor carried X-body, but the difference was that when the female of the species carried two X-bodies in a cell, the male of the species carried a single X-body in a cell. On the basis of this, he advocated that X-body was a chromosome that was responsible for sex determination in Pronetor. When a zygote carried one X-body it developed into a male and when carried two X-bodies, developed into a female. Hence, Wilson designated the X-body of Henking as X-chromosome.
Wilson also studied the chromosome in combination in male and female insects of Lygaeus Turcios. The male and female members of this species contained the same number of chromosomes, but the male specimen showed the presence of an X-chromosome and a comparatively small chromosome in its somatic cell and this small chromosome was found only in male insects. He named this small chromosome as Y chromosome. However, the females of Lygaeus contained 2X chromosomes in their somatic cells. Based on such observation Wilson came to the conclusion that in Lygaeus a zygote with an XY chromosome combination develops a male, while a zygote with 2X chromosomes develops a female.
Subsequently, after the discovery of X and Y chromosomes in several insects by Wilson and his colleagues similar chromosome combination was also found in many other animal and plant species, including Fruit fly Drosophila, most mammals including man and plant Melandrium showed the presence of X and Y chromosome in male somatic cells, but 2X chromosomes in the female somatic cells. Because of two X-chromosomes in the ovum mother cell, the females of the organisms may produce one type of gametes, e.g., the X- chromosome bearing gametes. These females are called homogametic. On the other hand, the males of these organisms have one X and one Y chromosome in their somatic cells, they may produce two types of gametes, i.e., X-chromosome-bearing gamete and Y chromosome-bearing gamete. Hence the males of these organisms are called heterogametic.
In some organisms, the situation is just reversed. The females contain X and Y chromosomes in their somatic cells and the males contain two X-chromosomes in their somatic cells. Therefore, the females in these cases are heterogametic and the males are homogametic. In silk moths the females in their body cells contain X and Y chromosomes, whereas the males in their body cells contain two X-chromosomes. A similar condition is found in birds, but the equivalent of an X-chromosome in bird is called a Z chromosome, and that for a Y chromosome is called a W chromosome. Hence, the female sex of birds is designated as ZW and the male sex is designated as ZZ.
Though in many animals, apparently atleast at the primary stage sex chromosomes, i.e., X and Y become the determinants for sexual dimorphism, yet in certain cases autosomes also play a role along with the sex chromosomes in determining the sexes in animals. In Drosophila melanogaster role of autosome along with the sex chromosomes could be revealed. Scientists feel that the relative number of X chromosomes and autosomal set exerts some influence in the determination of sex in this fly species. It has also been revealed that in this fly Y chromosome has no role in the determination of sex. The presence of Y chromosomes in males is necessary for the maintenance of their fertility. However, chromosomal signals in sex determination may broadly be divided into several types as mentioned in the following table:
Major Sex-Determining Categories in Animals:
Type | Designation | Example (Where Encountered) |
1. XX/XY | Homogametic female and heterogametic male. | Man and other mammals |
2. ZW/ZZ | Heterozygotic female and homogametic male. | Birds |
3. XX/XO | Homogametic female with 2X chromosomes and heterogametic male with IX chromosome but no Y chromosome. | Grasshoppers |
4. X chromosome and autosome ratio (X : A) | The number of X chromosomes and the number of sets of autosomes determine the signals for the sex. | Drosophila melanogaster Soil nematode |
5. Haploidy/Diploidy | The male contains a haploid number of chromosomes and the female contains a diploid number of chromosomes. | Honey bees and wasps |
Sex Determination in Human
The genic signal in man and other mammals influences the sex-determining genes that establish sexual differentiation in the early embryonic stage. As already mentioned the genic signal in man is emanated through the expression of the genes SRY present on the Y chromosome. The SRY gene product may activate the male pathway genes for the development of the male sex. On the other hand, a zygote lacking the SRY gene may only activate the female pathway genes (FPG) for establishing the female sex.
Before a discussion on the genic mechanism of sex determination in men, the developmental history related to sexual differentiation in the embryo is needed to be mentioned. The human embryo at the early developmental period is bipotential and may either be developed into a male or a female. Being associated with the kidney the primordial gonads appear as a pair of genital ridges. The primordial germ cells then migrate to the gonadal ridges and they become localized at the outer cortex and inner medulla. The cortex of the early gonad may develop an ovary and the differentiation of the medulla may develop a testis. Depending upon the presence or absence of the Y chromosome in the developing embryo, gonadal differentiation is established. In humans, this differentiation is established only in the embryo between 4-6 weeks of age. Besides the gonads, the embryo also develops two sets of duct systems known as Wolffian and Mullerian ducts.
From the Mullerian ducts female genital ducts are differentiated, whereas in males the Mullerian ducts degenerate, and Wolffian ducts are flourished with the development of epididymis, semi¬nal vesicle, and vas deferens. This differentiation may be started after the development of testes with the release of two hormones namely testosterone and Mullerian inhibiting hormone (MIH). The gene SRY is behind the development of testis with the promotion of sex determination. It is presumed that the SRY gene product is a transcription factor that binds with the DNA to control the functions of other genes. The sex-determining genes and their pathway of action are not completely known. However, a generalized model for genic action with differentiation of two sexes has been proposed which may be shown in the following diagram.
According to this model, DSS is the first sex-determining gene that is inhibited by the protein synthesized from SRY. The full form of DSS is Dosage Sensitive Sex Reversal and it is present over X-chromosome at the Xp21 locus. The inactivity of the DSS promotes the activation of several MPGs (Male pathway genes). The MPGs induce the bipotential gonad to be differentiated into the testis. On the other hand when the zygote contains two X chromosomes, the SRY gene remains absent, then DSS becomes active which in turn inactivates the MPGs. In this situation, the FPGs (Female Pathway Genes) become active inducing differentiation of the bipotential gonad into the ovary. After its formation, the ovary secretes the female hormones that promote the development of secondary sexual features in females.
Though the previous model explains the genetic mechanism of sex determination in males and females, the genes, in general, are called as MPGs (Male pathway genes) for male sex determination and FPGs (Female pathway genes) for female sex determination. However, by this time several genes have been identified in male and female mammals these have been suggested to be involved in the differentiation of embryos either in a male or female manner. Accordingly, a model for the differentiation of males and females in mammals by the activity of these genes has been put forward by scientists. The genes involved in the pathway of sex determination are SFI (Steroidogenic Factor-I), WTI (Wilm’s Tumour gene), SRY (Sex determining region of Y chromosome), DMRT I (Double sex and Mab 3 related transcription Factor I), DAX I (Dosage sensitive, sex reversal, adrenal hypoplasia critical region on chromosome X, Gene I) and AMH (Anti Mullerian hormone).
Among these genes, SFI and WTI are expressed in the early embryos prior to the differentiation of sexes and it has been predicted that these genes are essential for SRY activation. Once SRY is activated, this promotes the expression of SOX 9 to give a protein, a transcription factor by nature and its expression is essential for the development of males. It has been found that one XY zygote with loss of function of SOX 9 develops a female in mice. Again SOX 9 expression along with WTI and SFI results in activation of another gene known as AMH which produces an antimullerian hormone in the male embryo.” Under the influence of the antimullerian hormone Mullerian duct is regressed in the male embryo. The scientists have observed that males that lacked the AMH gene appeared infertile due to the retention of Mullerian duct-derived structures.
In this context, it may be inferred that the AMH gene is necessary for the proper development of males in mammals but it is not required for testis development. Sex determination is also assisted by two other genes namely DMRT I and DAX I. DAX I is X linked and its activity hinders male sex differentiation. A male may contain DAX I product but its amount is much less compared to that in females as a female contains two X-chromosomes. It was found that one XY zygote having two copies of the DAX I gene produced a female. The DAX I expression is suggested to repress the activity of the SRY gene. DAX I is known to synthesize a hormone receptor protein. The other gene DMRT I is expressed in males only and it is needed for proper differentiation of the testis in males. It has been observed that when DMRT I is absent in XY mice it was developed into an infertile male containing impaired testis.
Dosage Compensation in Mammals
Mammals in general exhibit a difference in the combination of sex chromosomes in males and females. Individuals identified as male and female are equivalent with regard to the possession of autosomes. It has also been elucidated that the Y chromosome is related to sex determination in men as well as in other animals. But the difference in the number of X chromosomes in male and female mammals could not be related to sex determination in this case. Sex-determining genes are present both in autosomes and sex chromosomes.
However, the presence of one X chromosome in male and two X chromosomes in female mammal create a dosage problem for the X-linked genes in male and female. The X-linked genes are present in double dosage in females, while the same genes remain in a single dose in males. In spite of this fact, most of the X-linked genes show equal activity in both males and females. This is possible due to a natural phenomenon known as dosage compensation. The dosage compensation in mammals is affected by the inactivation of one of the two X-chromosomes in mammalian females. Because of such inactivation of one of the two X-chromosomes in the mammalian female, the X-linked genes may show equal activity in both male and female mammals.
It has also been found that whenever the number of X-chromosomes becomes two or more in a mammalian individual, all the additional X chromosomes except one get inactivated. This inactivation of the X-chromosome is effected through heterochromatinization of the X-chromosome in the female somatic cell. In the interphase nucleus, the heterochromatinized X-chromosome appears as a darkly stained body at the periphery of the nucleus. This heterochromatic body in the interphase nucleus of the nerve cells of female cats was first noticed by Murray L. Barr and Ewart G. Bertram (1948). In human females, this heterochromatic body may easily be demonstrated by staining the cells of the buccal epithelium. This heterochromatinized darkly stained body of the mammalian somatic cell nucleus is called sex chromatin or Barr body.
That the sex chromatin or Barr body appears from one of the two X chromosomes of mammalian females was first suggested by Ohno. Indirect but convincing evidence on the heterochromatinization of additional X-chromosomes but one in the mammalian female comes from the study of syndromes in man where the human subjects contain two, three, or more X chromosomes. Individuals; exhibiting the symptoms of Klinefelter syndrome containing 47, XXY chromosome combination, females with XXX chromosomes, and females with 4X chromosomes show one, two, and three Barr bodies in the somatic cell nucleus respectively.
The inactivation of additional X chromosomes has raised some questions. Several such questions are
- Why is Turner female having only one X chromosome in not normal like a female who faces inactivation of one of her two X-chromosomes?
- Why a Klinefelter individual with 47, XXY chromosome complement is not normal like a male with 46, XY chromosome combination
- Why female with XXX or XXXX chromosomes is not normal as a female who is with XX chromosomes.
Answers to all of these questions has come from Mary Lyon in 1961 who proposed the Lyon hypothesis on the X- inactivation in mammalian females. Lyon’s hypothesis stated that out of the two X chromosomes of the mammalian female body cells, one gets inactivated randomly in a fixed manner at an early embryonic life leading to the development of mosaic cell lines for X chromosomes in the female body of mammals. Lyon hypothesis was renamed Lyon Law on July 22, 2011, at the EMBO 50 Years of X-inactivation conference in Oxford, U.K. X-chromosome inactivation is also called Lyonization.
Lyonization is based on four principal points:
- In the somatic cell of a mammalian female one of the two X chromosomes becomes inactivated and the inactive X-chromosome appears as Barr body in the somatic cell interphase nucleus.
- Inactivation of the X-chromosome in females occurs at random, i.e., in some cells, the paternally contributed X-chromosome becomes inactive and in others, the maternally contributed X-chromosome becomes inactive.
- Inactivation of the X-chromosome occurs at an early embryonic stage of the female (usually in the early blastocyst stage).
- X-chromosome undergoing inactivation of a particular X-chromosome maintains inactivation of the same X-chromosome in all the succeeding cell generations.
Supportive Evidence in Favor of X-chromosome Inactivation and Dosage Compensation
- Glucose 6-phosphate dehydrogenase is produced in man by the activity of a gene product present on the X-chromosome. It has been found that both normal men and women (homozygous for the G-6 PD gene) produce equal amounts of enzyme.
- Women heterozygous for G-6-PD deficiency usually produce two clones of RBC. One type of RBCs produces the enzyme while the other type fails to produce the enzyme. Not only that the populations of G-6-PD deficient cells and G-6-PD containing cells remain exactly in 50 : 50 ratio.
- Anhidrotic ectodermal dysplasia is an X-linked recessive disorder in man and the affected individual shows the absence of teeth, sparse hair growth, and absence of sweat glands. It has been found that females heterozygous for the defect contain a random distribution of skin tissue with regard to the presence and absence of sweat glands.
Mechanism of X-chromosome Inactivation
In mammalian females normally there are two X chromosomes one contributed by the male parent and the other contributed by the female parent. Out of these two X chromosomes, one undergoes inactivation in a random fashion. However, when an individual contains more than two X-chromosomes, all but one X-chromosome becomes inactivated. Therefore, in the process of inactivation, there is an invariable selection for one X-chromosome that may remain protected from inactivation. The X-chromosome that may escape inactivation is designated as Xa, while the X-chromosome that undergoes inactivation is called Xi. Selection of one X-chromosome so as to protect it from inactivation may occur after a process of counting of X chromosomes in a cell. If in this counting number of X-chromosomes comes to one then none undergoes inactivation. For this reason, no Barr body is available in normal human males and in Turner syndrome.
X-chromosomes and Number of Barr Bodies in Several Human Individuals:
Phenotype | X chromosome Position | Barr bodies and their Number |
1. Normal Male | XY | 0 |
2. Turner Syndrome | X | 0 |
3. Normal Female | XX | 1 |
4. Klinefelter Syndrome | XXY | 1 |
5. Triple X Syndrome | XXX | 2 |
6. Female with 4 X-chromosomes | XXXX | 3 |
Since the cells of mammals have the ability to count the number of X chromosomes in the cell, the cell desires a need for the inactivation of X chromosomes depending upon the situation. In case the number of X chromosomes exceeds one, all additional X chromosomes are converted into Barr body following inactivation. Following this counting the mechanism to protect one X chromosome from inactivation comes into operation.
It is hypothesized that the selection of Xa is mediated through some autosomal gene. The autosomally encoded blocking factor binds with one X chromosome and protects it from inactivation. On the other hand, blocking factors that normally cannot bind with the additional X-chromosome, cannot protect it from inactivation. It has been found experimentally that a cell line containing twice the normal number of autosomes may produce two Xa which supports the role of autosomes in the regulation activation and inactivation of X-chromosomes in mammalian female body cells.
Further, it has been elucidated that silencing of the X chromosome or its inactivation is mediated by XIC (X chromosome inactivation center) present on the X chromosome. It has been found that a translocation placing the XIC from one X-chromosome to an autosome failed to show the inactivation of the X-chromosome. Conversely, the autosome gaining the XIC gets inactivated. The hypothetical blocking factor for the selection of Xa probably binds with the XIC on X-chromosome.
The XIC (or Xic) is 1Mb long and is located on the p arm of the human X chromosome at its proximal end. The XIC contains four genes, out of which a gene known as XIST (or Xist) plays an important role in X-chromosome inactivation. A transcript is produced from the XIST gene that spreads over the X chromosome creating some sort of molecular trap causing inactivation of the chromosome. The X chromosome which is to be inactivated produces the XIST RNA from its Xist gene. Hence, the gene is said to be cis-acting. That the Xist gene is related to X-inactivation is evidenced by the finding of Graeme Penney and his colleagues in 1996. Experimentally when they introduced a small deletion (-7kb) in this gene, the X- chromosome failed to show inactivation.
The majority of the genes present in X-chromosomes are silenced because of their inactivation. The repressive heterochromatin compacts the Xi DNA resulting prevention of the expression of most of the genes. Besides, the DNA of Xi shows a high level of DNA methylation, a low level of histone acetylation, a low level of H3-lysine-4 methylation, and an increased level of histone 3 lysine-9 methylation. Along the nucleosomes of Xi, H2AFY (macro H2A) variant histone is exclusively found to be present and therefore, may be concerned with gene silencing in the Xi.
Genes are expressed by Xi
Though Xi denotes the inactive X chromosome of a female there are many genes over it those escape inactivation. A prominent example with regard to this may be cited from the human Xg blood group. This blood group gene (Xga) is linked with the X chromosome and in this blood group Xga antigen appears in the RBC. There is an allele of the Xga gene which may be designated as Xg. This allele is recessive in nature and due to this allele no Xga antigen may be produced in the RBC.
A woman heterozygous for these alleles (Xga/Xg) should possess two clones of cells, one showing the presence of Xga antigen and the other lacking this antigen in the RBC. But studies made by Gorman et. al, 1963 and Russel (1963) on the Xg blood group of females heterozygous for these alleles showed the presence of only a single category of red cells all containing the Xga antigens. However, the genes that are present within the pseudoautosomal region escape X chromosome inactivation. It has been estimated that about 15% of the human X-linked genes and 3% of the mouse X-linked genes escape X-inactivation. The Xist gene of the inactive X-chromosome also escapes inactivation and this gene shows a high level of expression in Xi on the contrary the same gene on Xa remains repressed. Pseudoautosomal genes comprise a set of genes that are present in the homologous portions of both the X and Y chromosomes. Since these genes are equally available in male and female individuals as autosomal genes, they do not require inactivation for dosage compensation.
Sex Determination in Bird
Sex determination in birds follows a system in which the male is homogametic and the female is heterogametic, a system that is just opposite to that found in humans and other mammals. The method is known as the ZZ/ZW method of sex determination. In this case, the chromosomes having some relation with sex determination are Z and W, when Z is equivalent to X as found in mammals and W is equivalent to Y as found in mammalian males. The male birds contain two Z chromosomes so they are homogametic, while the female birds contain one Z chromosome and one W chromosome. Therefore, the females in this case are heterogametic.
However, in the present period, scientists have come across certain sex-determining genes related to sex determination in birds. One such gene is DMRT I present on the Z chromosome of a bird. Hence, a male bird contains two DMRT I genes those help in the differentiation of the male sex in birds. It is noteworthy to point out that DMRT I is also present in the mammalian chromosome and on the autosomes. The DMRT I of the autosomes in combination with the SRY gene guides the differentiation of testis in males. On the other hand two other genes namely FET I and ASN present on the W chromosome in female bird guide differentiation of female sexual differentiation (Smith and Sindair, 2004). During the early stage of development in birds also the gonads in both males and females appear identical. At about 4th day of incubation, the gonad in females is differentiated into ovary in which estrogen appears to be necessary. If during early development the egg (genetically potential to develop a male) is injected with estrogen, and the egg develops into a female.
In many birds, sexual dimorphism is hardly detectable from their appearance. However, in certain cases, sexual dimorphism is established only during their breeding period, In several cases the male and female birds can be distinguished by their external features. The dimorphic features as observed in the figures may be called secondary sexual characters probably affected by the influence of sex-limited gene activity. These characters are established after the determination of the sexes and their differentiation.
Most of our knowledge on sex determination in bird has come from studies on avian model species Gallus domesticus (the chickens) [Hillier et. al, 2004], The sex in almost all birds are determined by the inheritance of sex chromosomes Z and W, with two Z chromosomes in male and Z + W chromosome in female.
The gene, DMRT I (Double Sex and Mab3 Related transcription Factor I) present on the Z chromosome triggers the development of the male sex by converting a bipotential gonad into the testis. To differentiate a bipotential gonad into a testis double dose of the gene product (DMRT I) is required and that is available only in the male birds. On the other hand only in female bird does a single dose of the DMRT I gene product becomes insufficient to convert bipotential gonad into a testis. DMRT I in birds along with gonadal sex differentiation carries out somatic sex differentiation of sexes at the early period of birds development. Sex determination and differentiation of sexes have two different aspects. While the determination of sex is established from the stage of fertilization, sex differentiation along with gonadal differentiation occurs at the later period of development probably at about 3.5 days of development. Studies on gynandromorphic chickens suggest the determination of sex occurs autonomously under the action of the sex-determining genes in the body cells.
In the case of gynandromorph is chicken half of the body completely develops as female and the other half develops as male. Hormonal influence has no role in the development of secondary sexual features. This indicates that genetic sex in a cell after its determination shows a directional development based on its genetic sex. This determination occurs only during the early embryonic period before its gonadal sex determination.
The candidate gene for testis differentiation and male sex determination is DMRT I on Z and in males high levels of DMRT I expression may act to inhibit the female pathway genes. DMRT I activates SOX I (at the 6th day of development) in the pathway of male sex differentiation. On the other hand in the case of ZW combination low levels of DMRT I expression and ovarian development under the influence of S the genes, namely FOXL2 and RSPOI. These genes are activators of the WNT/β-catenin pathway in female birds for ovary development. FOXL2 activates aromatase promoting estrogen synthesis and ovarian development while the WNT4 gene activates β-catenin signaling required for ovarian development (Smith et. al, 2007).
No W-linked good candidate gene has been identified yet to regulate female sex determination as sry in mammalian males, but a W-linked sequence known as HINT W that encodes a histidine-tied nucleotide-binding protein which may operate as a dominant negative regulator of Z-linked gene to di¬rect female development (Chue and Smith, 2010).
Sex Determination in Honey Bee
In a bee hive, thousands of bees may be encountered and these may be grouped into 3 classes namely male, female, and workers. The number of male and female in the colony of bees are rather limited. The number of reproductively potential females in a colony is one. The workers are many in number and they are actually sterile females. On the other hand, few males may be encountered in a bee colony. Therefore, among the honey bees, there are two sexually differentiated bees, i.e., male and female. The development of a fertile female from the larva depends on how it has been nourished by the workers. Only one larva amongst many in the bee hive is fed with royal jelly by the workers and therefore, it may be developed into a fertile female. On the other hand, the larvae genetically equivalent to a female, being deprived of royal jelly develop into sterile females and they after their emergence serve the colony as workers.
The first report on the process of sex determination in honey bees came from Dzierzon in 1845 more than 50 years before the discovery of the sex chromosomes in 1902. He reported that males were developed from the unfertilized eggs of the female and females as well as workers could be developed from the fertilized egg. This finding of Dzierzon was supported later on the basis of cytological studies. The unfertilized egg contains a haploid set of 16 chromosomes and fertilized egg contains diploid sets of 32 chromosomes. On the basis of such findings, the workers of the initial period of studies on sex determination in honey bees came to the conclusion that the primary signal for sex determination in this insect emanates from haplo-diploidy.
Subsequently, scientists revealed that the primary sex-determining signal in honey bees comes through a complementary sex-determining system, and in this system, a signal from a sex-determining locus called SDL determines the primary sex of the embryo directing male or female sex development. The female honey bee appears to be heterozygous for SDL. But the homozygous SDL may also appear, as a male. Such a homozygous male is consumed by the worker bees immediately after emergence from the egg. Following this discovery, geneticists isolated a candidate gene from the SDL locus and this gene is called CSD (complementary sex determiner) which produces a potential splicing factor needed for the determination of the female sex (Beye et al., 2003).
When the CSD gene remains inactive in the embryo then a male may be developed from the embryo. Besides this, scientists could be able to trace another gene called fem (Feminizer) which produces an effective protein in the fertilized egg, and under the influence of this protein differentiation of a female may occur. On the other hand in the unfertilized egg, a defective splicing of the fem gene product results in a male honey bee being developed.