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Home , Calvin Bridges, Gamete, Parthenogenesis, Sexual differentiation, Sex reversal

DETERMINATION OF

The word “sex” is derived from the Latin word “sexus” means sections. The sexually reproducing are classified into two types viz, monoecious () and dioecious. In monoecious organisms, both male and (sex cells) are produced by a single individual. The organisms in which both male and female gametes are produced by different individuals are called dioecious. The sex cells and reproductive organs form the primary sexual characters of both . Besides this, male and female sexes differ from each other in many characters known as secondary sexual characters. The phenomenon of molecular, morphological physiological or behavioral differentiation between male and female sexes is called .

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 and secondary sexual characters.

Significant progress has been made in understanding the mechanism of sex in beings and other mammals and new have been identified.

The primary function of such a of a into male and female sexes is to prevent combination of gametes derived from the same and the maintenance of a high degree of heterozygosity and genetic exchange.

Theories of Sex Determination

The problem of sex determination has been one of the most important biological puzzle up to the year 1900. A number of theories were postulated from time to time by the biologists to explain this critical phenomenon.

Chromosome Theory of Sex Determination:

Sex determination in higher is controlled by the action of one or more genes. The testis determining factor (TDF) is the dominant sex determining factor in human beings.

Hemking a German biologist identified a particular nuclear structure throughout the in of squash bug, Pyrrhocoris. He named it as “X-body” and showed that differed by its presence or absence. After three years, Miss Steven and Wilson succeeded in understanding and spermatogenesis in Protenor bugs. The X body was later found to be a that determined sex. It was identified in several and is known as the sex or .

In males, all the are paired, but the chromosome analogues to X chromosome is smaller and is called as .

Thus, the chromosome theory of sex determination states that female and male individuals differ in their chromosomes. In majority of sexually reproducing animals two types of chromosomes are found:

(i) Autosomes:

The chromosomes which have no relation with sex and contain the genes which determine the somatic characters of the individuals are called as autosomes (A). They are found in all cells. The two members of this pair are similar in shape, ie, homologous pair (homomorphic).

(ii) Sex Chromosomes or Allosomes:

The chromosomes which carry genes for sex determination are called allosomes. A pair of them determines the sex. They are variously named as X and Y chromosomes (Man and Drosophila),

Z and W chromosomes ( and Moth), odd chromosomes, idiosoines, heterosomes or allosomes. The two members of this pair are often dissimilar in male and are represented as X and Y chromosomes or as Z and W chromosomes.

Types of chromosomal mechanisms of sex determination

In dioecious diploidic organisms following two systems of sex chromosomal mechanisms of sex determination have been recognized

1. Heterogametic male

In this type of sex determination female has two X chromosomes, hence produces similar type of gametes and called as homogametic sex, But male has only one X chromosomes, hence during it produces two types of gametes, 50% carry X chromosomes while rest either lacks X chromosomes (XO) or has a Y chromosome (XY) a. XX Female - XO Male Type:

Mc Clung and Wilson described this type of sex mechanism in insects especially grasshopper. In male there is no mate for X chromosome, hence the name Xo is given, there is no Y chromosome They produce sperm of two types, 50% with X chromosome and 50% without X. In

there are two similar or homomorphic sex chromosomes XX (homogametic) (Fig 18.3)

b. XX Female - XY Male Types: This type of sex mechanism is found in Drosophila (fruitfly) and majority of mammals including man. In this type the female is homogametic (XX) and male is heterogametic (XY) consisting of two dissimilar chromosomes X and Y. The females produce ova all of one type having X chromosome (homogametic. Males produce two types of : -50% with X-chromosome and remaining 50% with Y-chromosome. Thus, the sex chromosomes in female are homomorphic and those of male are heteromorphic or heterogametic (Fig. 5.13).

Fig: 5.13 XX-XY type of sex determination

2. Heterogametic females

In this type of sex chromosomal sex determination, the male possess two homomorphic X chromosomes, thus is homogametic and produces gametes with a single X chromosome. The female either, has only one X chromosomes, or has one X and one Y, hence during gametogenesis it produces two types of gametes, 50% gamete carry X chromosomes while rest either lacks X chromosomes or has a Y chromosome. To avoid confusion with that of XX-XO and XX-XY methods of sex determination, instead of X and Y Z and W alphabets are used respectively a. ZO Female -ZZ Male Type

This system of sex determination is found in moths and . Feale hence ZO and is heterogametic producing 50 % gametes with Z and 50 % without Z. Males possess two Z chromosomes hence homogametic and produces single types of gametes(with Z chromosome) and are called as homogametic. The sex of offspring depends on as shown below

b. ZW Female - ZZ Male Type

In certain insects, , and birds, the female is heterogametic; having dissimilar Z and W chromosomes, whereas the male is homogametic having similar ZZ chromosomes (It is a convention to designate female as ZW instead of

XY and male as ZZ instead of XX). The situation here is just reverse to first type. SEX DETERMINATION IN The sex of a human being in decided at the moment of conception. Sex is determined by the sort of , X-bearing or Y-bearing, which happens to fertilize the ovum. In human beings, sex is determined by genetic inheritance. Genes inherited from the determine whether an offspring will be a boy or a girl.

We have a total of 46 chromosomes. Half of them come from the mother and the rest, from the father. Out of these 46 chromosomes, 44 are autonomies and 2 are sex chromosomes. The sex chromosomes are not always a perfect pair. In females there are 44 autonomies and two X chromosomes (44 + XX), in males there are 44 autonomies, one X chromosome and one Y chromosome (44 + XY).

The human female produces only one sort of ovum —each containing 22 autosomes and an X chromosome (22A + X). The human male, on the other hand, produces two sorts of spermatozoon, those containing 22 autosomes and an X chromosome (22A + X); and other those containing 22 autosomes and a Y-chromosome(22A + Y).

There are two possibilities when fertilization takes place: 1. An ovum (bearing an X- chromosome) may be fertilized by a spermatozoon bearing an X chromosome; the result is an individual bearing 44 autosomes and two X-chromosome—that is a female.

2. An ovum bearing an X-chromosome may be fertilized by a spermatozoon bearing a Y- chromosome; the result is an individual bearing 44 autosomes and one X- chromosome and one Y-chromosome that is a male.

Let us see the inheritance pattern of X and Y chromosomes.

During gamete formation, the normal diploid chromosome number is halved. This is called the haploid condition. All the of a female have 22 + X chromosomes. A male produces two types of sperms—one type bears the 22 + X composition and the other, 22 + Y. Therefore, in every 100 sperms, 50 have Y chromosomes and 50 have X chromosomes (Fig 7.5). Any one of the two types of sperms can fertilize the egg. If a Y-bearing sperm fertilizes the egg, the has the 44 + XY composition, and the resulting grows to be a boy. When an X-bearing sperm fertilizes the egg, the resulting zygote has the 44 + XX composition. This embryo develops into a girl. All the children inherit one X chromosome from the mother.

Therefore, sex is always determined by the other that they inherit from the father. One who inherits the X chromosome of the father is a girl, while one who inherits the Y chromosome of the father is a boy (Fig 7.5).

SRY GENE AND ITS ROLE

A small region of the Y chromosome, perhaps a single gene, is responsible for initiating the sequence of events that lead to testis formation and, hence, to male development. This gene is known as TDF (Testis determining factor).Testis-determining factor (TDF), also known as Sex-determining Region Y (SRY) protein, is a DNA-binding protein (also known as gene- regulatory protein/transcription factor) encoded by the SRY gene that is responsible for the initiation of male sex determination in humans. SRY is an intronless sex- determining gene located on the short arm of Y chromosome in therians (placental mammals and marsupials). In the absence of TDF gene female development ensures. The absence of TDF 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 (Fig. 5).

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 to differentiate into testes. Once the testes are formed, they secrete that stimulates the development of male secondary .

The 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 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 is due to a 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 medulla into testis, whereas an XX chromosomal set induces the undifferentiated gonadal cortex to develop into ovaries. Thus, TDF gene is the master regulator gene that triggers the expression of large number of genes that produce male sex . 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.

HORMONAL THEORY OF SEX DETERMINATION are the secretion of the endocrine glands which in many instances modify the sex rather determining the sex. They are mainly responsible for the expression of secondary sexual characters. This theory is based upon the observation of Crew in chicks.

In course of his investigation he found a hen which laid fertile eggs, accidentally lost its , stopped laying eggs, and developed male characters such as comb and male plumage and became a cock. The above case of is explained by assuming that destruction or removal of the ovary led to stoppage of production of the ovarian hormones.

But the rudiment of testis (present in all female birds) became functional following the loss of ovary and produced male hormone which is responsible for the appearance of male secondary sexual characters. Such a male produced sperms and became father of two .

Another classical example of sex reversal by the action of hormone is observed in free martin. In , when twin calves of opposite sex are born, the female is usually somewhat abnormal and sterile. Such a calf is called freemartin. Since the male hormone appears earlier in the development, it passes into the body of the under developed female through the circulation and causes partial sex reversal of the female

FREEMARTIN

 A freemartin is an infertile female mammal with masculinized behavior and non-functioning ovaries. The 18th-century physician John Hunter discovered the freemartin.  In most cattle twins, the blood vessels in the chorions become interconnected, creating a shared circulation for both twins.  If both foetuses are the same sex this is of no significance, but if they are different, male hormones pass from the male twin to the female twin.  The male hormones (testosterone and anti-Müllerian hormone) then masculinize the female twin, and the result is a freemartin.  The degree of masculinization is greater if the fusion occurs earlier in the – in about 10% of cases no fusion takes place and the female remains fertile.  The male twin is largely unaffected by the fusion, although the size of the may be slightly reduced.  size is associated with , so there may be some reduction in bull fertility.  Freemartins behave and grow in a similar way to castrated male cattle.  Genetically the is chimeric*: Karyotyping of a sample of cells shows XX/XY chromosomes. (*An animal chimera is a single that is composed of two or more different populations of genetically distinct cells that originated from different involved in sexual .)  The animal originates as a female (XX), but acquires the male (XY) component in utero by exchange of some cellular material from a male twin, via vascular connections between .  Freemartinism is the normal outcome of mixed-sex twins in all cattle species that have been studied, and it also occurs occasionally in other mammals including sheep, and pigs.  Freemartins are occasionally used in and immunology research.

Sexual differentiation

 Sexual differentiation is the process of development of the differences between males and females from an undifferentiated zygote.  In the first weeks of life, a foetus has no anatomic or hormonal sex, and only a distinguishes male from female.  Specific genes induce gonadal differences, which produce hormonal differences, which cause anatomic differences, leading to psychological and behavioural differences, some of which are innate and some induced by the social environment.  During there is a sexually indifferent stage in which the embryo has the potential to develop either male or female structures.  Internally, adjacent to each developing gonad, are two primitive ducts that can give rise to either the male (Wolffian/ Mesonephric ducts) or the female (Müllerian/ ) reproductive tracts.

 Sexual differentiation begins with sexual determination, which depends upon the sex chromosomes, X and Y.  Sexual determination involves the specification of the gonads as either testes or ovaries.  If the embryo is XY, the presence of the SRY gene (for Sex-determining Region of the Y chromosome) will direct the gonads to develop as testes.  In the absence of a Y chromosome and SRY gene, the gonads develop as ovaries.  Once the gonad begins to develop as a testis, the two support cells in the testis differentiate and begin to generate important regulatory molecules that direct sexual differentiation.  The Leydig cells produce testosterone, which promotes development of the Wolffian ducts.  The Wolffian ducts then differentiate to form the epididymis, vas deferens, seminal vesicles, and ejaculatory ducts.  The Sertoli cells produce Müllerian inhibiting substance (MIS; also known as Anti-Müllerian hormone, AMH), a peptide hormone that causes the Müllerian ducts to regress.  Female development proceeds when there is an absence of the SRY gene.  No testosterone or anti Mullerian hormone is made.  The Wolffian ducts regress, and the Müllerian ducts persist, developing into the fallopian tubes, the uterus and the upper part of the vagina.

GENIC BALANCE THEORY OF SEX DETERMINATION

There seems to exist a delicate balance of masculine and feminine tendencies in the hereditary compliment of an individual. Such genic balance mechanism of sex determination was first studied in Drosophila by C. B. Bridges in 1921. The genie balance theory of sex determination in Drosophila explains the mechanism involved in sex determination in this . The theory of genic balance given by Calvin Bridges (1926) states that instead of XY chromosomes, sex is determined by the genic balance or ratio between X-chromosomes and autosome . 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. Y chromosome does not take part in sex determination.

From the Fig. 18.6 it is clear that the genic balance is governed by the ratio of the number of X- chromosomes to the number of sets of autosomes in the zygote at fertilization.

In a normal male (AA+XY), the male and female determinants are in the ratio of 2:1 (2/1=0.5) and therefore the genic balance is in the favour of maleness. A normal female (AAXX) has a male and female ratio of 2:2 (2/2=1) and therefore the balance is in favour of femaleness (fig 18.6 ). Bridges could drew such conclusions by crossing certain triploid female Drosophila (3A:3X) with diploid males (2A: XY). The results obtained from such cross are shown below .

From this cross he obtained normal diploid male, triploid males, triploid females, , super males and super females. The occurrence of triploid intersexes from such a cross clearly indicates the autosomes also carry genes for sex determination. The occurrence of such intersexes, super males and super females were explained by him by genic balance mechanism. Different combinations of X chromosomes and autosomes and corresponding sex expressions in Dosophila can be summarized in Table 1. He found that the genic ratio X /А of 1.0 produces fertile females whether the have XX + 2A or XXX + ЗА chromosome complement. A genic ratio (X /А) of 0.5 forms a male fruitfully. This occurs in XY + 2A as well as X0 + 2A. It means that expression of maleness is not controlled by Y- chromosome but is instead localised on autosomes. A genic ratio (X /А) of less than 0.5 produce metamales (sterile and week) above 1 produces metafemales (with serious developmental problems.), between more than 0.5 and less than 1 produces (sterile individuals and had phenotype in between male and female sexes).

SEX DETERMINATION IN MELANDRIUM

Y CHROMOSOMAL SEGMENTS AND ITS FUNCTIONS @.segment – I: Suppress femaleness; it inhibits the development of female structures in the .

In Melandrium four whorls of floral organs are observed in both male and female floral meristems. They are sepals, petals, and carpels. At the early stages of flower development the floral meristem is completely undifferentiated and similar in male and female . When the floral development proceeds, there will be considerable difference in the relative proportion of different floral parts in both male and female. In male (AAXY), a sterile filament is developed in place of the carpels. Similarly in female flowers (AAXX) The development od anthers are first initiated but rapidly degenerates, whereas five carpels are developed in the centre.

SEX DETERMINATION IN SPHAEROCARPUS

Sphaerocarpus is a genus of plants known as bottle liverworts. There are eight or nine species in this genus. It is dioecious and there are separate male and female . In sphaerocarpos the sex is determined by chromosomes. The XY mechanism is followed

The female has 7 autosomes and one X chromosome. The male gametophyte has 7 autosomes and one Y chromosome. The Y chromosome is dot like and X chromosome is elongated. Allen (1919) found that the of Sphaerocarpus produces two kinds of meiospores-some with X and other with Y chromosomes. They germinate into sexually reproducing female gametophyte and meiospores with Y chromosomes germinate into sexually reproducing male gametophyte. The gametophytes are haploid, fuse to form the which is diploid. The sporophyte produces through .

PARTHENOGENESIS Usually an un-fertilised ovum develops into a new individual only after the union with the sperm or but in certain cases the development of the egg takes place without the fertilisation. This peculiar mode of in which egg development occurs without the fertilisation is known as the (Gr., parthenos = virgin; genesis = origin). The phenomenon of parthenogenesis occurs in different groups of the animals as in certain insects (, Homoptera, Coleoptera), and .In certain animals the parthenogenesis occurs regularly, constantly and naturally in their life cycles and is known as the natural parthenogenesis.

The natural parthenogenesis may be of two types, viz., complete or incomplete: (i) Complete Parthenogenesis: Certain insects have no sexual phase and no males. They depend exclusively on the parthenogenesis for the self-reproduction. This type of parthenogenesis is known as the complete parthenogenesis or obligatory parthenogenesis. It is found in some species of , badelloid rotifers, grasshoppers, roaches, phasmids, moths, gall flies, fishes, and . (ii) Incomplete Parthenogenesis: The life cycle of certain insects includes two generations, the sexual generation and parthenogenetic generation, both of which alternate to each other. In such cases, the diploid eggs produce females and the un- fertilised eggs produce males. This type of parthenogenesis is known as the partial or incomplete or cyclic parthenogenesis. The complete or incomplete type of natural parthenogenesis may be of following two types: 1. HAPLOID OR ARRHENOTOKOUS PARTHENOGENESIS: In the arrhenotokous parthenogenesis, the haploid eggs are not fertilised by the sperms and develop into the haploid individuals. In these cases, the haploid individuals are always males and the diploid individuals are the females (hence also known as ), e.g. a. Insects: Hymenoptera ( and wasps), Homoptera, Coleoptera, Thysanoptera b. Arachnids: Arachnids, e.g., ticks, mites and certain spiders c. Rotifers: Rotifers, e.g., Asplanchne amphora.

2. DIPLOID OR THELYTOKOUS PARTHENOGENESIS: In the diploid parthenogenesis, the young individuals develop from the un-fertilised diploid eggs.

Types of the : (i) Ameiotic or apomictic Parthenogenesis: Sometimes during the oogenesis, first meiotic or reduction division does not occur but second meiotic division occurs as usual. Such eggs contain diploid number of chromosomes and develop into new individuals without the fertilisation. This type of parthenogenesis is known as apomictic or ameiotic parthenogenesis and occurs in Trichoniscus (Isopoda), pulex (Crustacea), Campelona rufum (Mollusca), weevils and long-horned grasshoppers.

(ii) Meiotic Parthenogenesis: Certain eggs develop by the usual process of oogenesis but at certain stages diplosis or doubling of chromosome number and production of diploid eggs occur. Such eggs develop into the diploid individuals and this phenomenon is known as the meiotic parthenogenesis.

Meiotic Parthenogenesis may occur by the following methods: (a) By Autofertiiisation: In certain cases, the divides meiotically up to the formation of ootid or ovum and secondary polocyte. But the ootid and the secondary polocyte unite together to form a diploid egg which develops into a new individual, e.g., (Crustacea) and various other organisms. (b) By Restitution: Sometimes in primary oocyte, karyokinesis forms a nucleus of the secondary oocyte and nucleus of the first polocyte. But the karyokinesis is not followed by the cytokinesis. The chromosomes of both daughter nuclei are arranged on the equator and undergo second meiotic division to form a diploid ootid and a diploid polocyte.

The diploid ootid or ovum develops into a parthenogenetic diploid individual. This type of diplosis is known as the restitution, e.g., insects of order Hymenoptera (Nemertis conesceus) and . Significance of Parthenogenesis: 1. The parthenogenesis serves as the means for the determination of sex in the honey bees, wasps, etc. 2. The parthenogenesis supports the chromosome theory of inheritance. 3. The parthenogenesis is the most simple, stable and easy process of reproduction. 4. The parthenogenesis eliminates the variation from the populations. 5. The parthenogenesis is the best way of high rate of multiplication in certain insects, e.g., . 6. The parthenogenesis causes the in the organisms. 7. The parthenogenesis encourages development of the advantageous mutant characters. 8. The parthenogenesis checks the non-adaptive combination of genes which may be caused due to the mutation. 9. organisms need not waste their energy in the process of but it allows them to utilise that amount of energy in the feeding and reproduction. 10. The parthenogenesis avoids the sterility in the races. SEX-LIMITED GENES Sex-limited genes are autosomal genes that are present in both sexes of sexually reproducing species but are expressed in only one sex and remain 'turned off' in the other. Sex-limited genes cause the two sexes to show different traits or phenotypes, despite having the same genotype. Many of the secondary sexual characters of man depend on sex limited genes. ie, Development of beard in man and development of mammary glands () in women. Though both man and women have genes for beard and mammary gland development, due to sex hormones the female lack beard and male lack breasts. Another example for sex-limited genes if the plumage pattern or feathering in birds.

COCK FEATHERING Many birds display a marked sexual dimorphism in plumage pattern, for example in domestic fowl of leghorn breed, males have long, pointed, curved, fringed feathers on tail Sex-limited genes are responsible for sexual dimorphism, which is a phenotypic difference between males and females of the same species. These differences can be reflected in size, color, behaviour, and morphology.

SEX-INFLUENCED TRAITS

There are a number of autosomal and sex linked genes whose expression will be either as if the individual is male or as recessive if the individual is female. Such genes are called sex- influenced genes. These kinds of traits appear more often in one sex than the other. Although these traits may appear more often in males than in females, they are not sex-linked, because they do not appear on the sex chromosomes. The genes for sex-influenced traits are on the autosomes not the sex chromosomes. Sex-influenced traits are influenced by both genes and hormones, not just genes. Example: Inheritance of BALDNESS IN MAN The of a young man in his twenties or early thirties gradually become thin on head top, leaving ultimately a fringe of low on head and commonly known as pattern baldness. The gene for baldness is found to be dominant in man (B) and recessive in (b). In heterozygous condition it express only in the presence of male androgenic hormone testosterone (in males). The inheritance of gene B for Baldness can be tabulated as follows. Phenotype Genotype Man Woman BB Bald Bald Bb Bald Non Bald bb Non Bald Non Bald

• In males, the testosterone causes the for baldness to be dominant over the normal allele whereas in females, who has minimal amount of testosterone has an opposite effect. • All humans have testosterone, but males have much higher levels of this hormone than females. The presence of high levels of testosterone, the baldness allele has a very powerful influence (in case of males). In the presence of low levels of testosterone, this allele is quite ineffectual (in case of females). • The result is that in males, the baldness allele behaves like a dominant allele, while in females it behaves like a recessive allele. This means that heterozygous males will experience hair loss and heterozygous females will not. Even homozygous females may experience no more than a thinning of their hair, but many develop bald spots or have receding hairlines.