Unit 5
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UNIT- 5
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SEX DETERMINATION
Chromosomal
mechanisms and methods
(02 Lectures)
Chromosomal mechanisms and
methods
Members of almost all species are often divided into
two sections according to the kinds of gametes of sex cells produced by them
i.e. male sex and female sex. The word sex has been derived from Latin word sexus
meaning section or separation. However, some of the lowest forms of plant and
animals like are found to have several sexes. For example, in one variety of
ciliated protozoan, Paramecium bursaria there are eight sexes or ‘mating
types’all morphologically identical. Each mating type is physiologically
capable of conjugating with its own type, but may exchange genetic material
with any of the seven other types within the same variety. Further, in
organisms in which the number of sex reduced to just two, sexes may reside in
different individuals or within the same individual. An animal possessing both
male and female reproductive organs is usually referred to as a hermaphrodite.
The sex cells and reproductive organs form the primary sexual characters of
male and female sexes. Besides these primary sexual characters, the male and
female sexes differ from each other in many somatic characters known as
secondary sexual characters. The phenomenon of molecular, morphological,
physiological or behavioral differentiation between male and females sexes is
called sexual dimorphism.
The phenomenon of sexual dimorphism has been a
biological riddle for the thinkers and biologists of all time. People always
tried to know those factors which determine the male and female sexes of a
species. Literally hundreds of mistaken hypotheses and wild guesses were
proposed before 1900 in vain attempts to find out a solution to the problem of
determination of sex. Modern genetics have reported many different mechanisms
of determination of sex in living organisms.
The X chromosomes have large amount of euchromatin
and small amount of heterochromatin as compared to Y chromosome. The
euchromatin has large amount of DNA material, hence, X chromosomes is
genetically active. The Y chromosome
contains small amount of euchromatin and large amount of heterochromatin. The
chromosome has little genetic information; therefore, sometimes it is referred
to as genetically inert or inactive chromosome.
Sex Determination in Animals:
Biologically,
sex is an aggregate of those morphological, Physiological and behavioral
qualities that differentiates the organisms producing eggs from those producing
sperms. This separation of species or section of animals into two sexes is
called as Gonochorism. The sex
chromosome behaves as a Mendelian inheritance follows the law of segregation.
Many mechanisms are given to explain the determination and differentiation of
sexes.
These
mechanisms are –
A) Genetically
controlled sex determining mechanism
i)
Chromosomal theory
of sex determination or sex chromosome mechanism
ii)
Genic balance
mechanism
iii)
Haplodiploidy
mechanism
iv)
Single gene effects or single gene
control of sex
Among these
sex chromosomes, theory of sex determination is best explanatory in explaining
the problem of sex determination. In plants male and female do not differ
morphologically, except in the floral characters and here gonochorism is a rare
phenomenon.
i) Chromosomal Theory of Sex determination or Sex Chromosomal Mechanisms or
Heterogamesis:
a) According to the chromosome
theory of sex determination the male (♂) and female (♀) individuals normally
differ in their chromosomal constituents. There may be two types of chromosomes
present in these individuals, theseare :
Sex
chromosomes or Allosomes,i.e.,
chromosomes, which are responsible for determination of sex, e.g., X and
Y-chromosomes.
Autosomes,i.e., chromosomes, which have no relation with the sex and
contain genes, which determine the somatic characters.
b) The X-chromosome was first
observed by German biologist Henking
in 1891, during his studies on spermatogenesis in male bug. He called it
X-body.
c) The chromosomal theory of sex determination
was worked out by E. B. Wilson and
Stevens (1902-1905). They named X and Y chromosomes as allosomes or sex
chromosomes and other chromosomes as autosomes.
d) The parents can be of two types -
• Homogametic, i.e.,
with similar gametes.
• Heterogametic, i.e.,
with different gametes.
e) In some cases, males are heterogametic, e.g., Man,
Drosophila, bug, etc., while in some other cases, females are heterogametic,
e.g., birds. Thus, we have two following types of systems among diploid
organisms, on the basis of chromosomal theory of sex determination.
i.
System that is
having heterogametic males.
ii.
System that is
having heterogametic females.
f) In the first system, i.e., males heterogametic, the females have two X-chromosomes,
while males have one X- chromosome and Y- chromosome because of which male
during gametogenesis produces two types of gametes (sperms), i.e.,
• 50% that carry X- chromosome.
• 50% that carry Y- chromosome.
g) This chromosomal pattern varies in different
organisms in the way that male either contain Y-chromosome (i.e., XX - XY
system or lygaeus type) or contain no chromosome along with X- chromosome,
i.e., XX - XO type or protenor type.
Structure
of sex chromosomes
Fig. Male and Female Drosophila melanogaster and their chromosome
Discovery
of sex chromosomes
In sexually
dimorphic dioecious organisms, besides morphological and behavioral differences
between both sexes, the sexual diversity also occurs at the level of
chromosomes. The chromosomal differences between the sexes of several dioecious
species were found earlier in the course of cytological investigations. A
German biologist, Herman Henking in 1891 while studying spermatogenesis of the
squash bug, Pyrrhocoris, noted that meiotic nuclei contained 11 pairs of
chromosomes and an unpaired element is moved to one of the poles during the
first meiotic division. Henking called this unpaired element a ‘X-body’ and
interpreted it as a nucleolus. The significance of X-body was not immediately
understood, but in 1902 an American geneticist, McClung, who had made extensive
observations of gametogenesis in grasshoppers, suggested that the X-body was
involved in some way with the determination of sex. He reported that somatic
cells of the female grasshopper (Xyphidium faciatum) contained 24
chromosomes, whereas those of the male had only 23. He demonstrated that
karyotype of a cell is composed not only of common chromosomes (autosomes) but
also of one or more special chromosomes that are distinguished from the
autosomes by their morphological characteristics and behavior. These were
called accessory chromosomes, allosomes, heterochromosomes or sex chromosomes.
In 1905, Edmond Wilson noted that females of Protenor, a hemipteran bug
have 7 pairs of chromosomes, while the males have 6 pairs and an unpaired
chromosome, which he called the X chromosome. The X and Y chromosomes were
first discovered in beetles by Nettie Stevens in 1906. Stevens found a similar
situation in Drosophila melanogaster,
which has four pairs of chromosomes, with one of the pairs being heteromorphic
in nature.
Fig. Sex determination pathways in
diverse model organisms.
Types
of Sex Chromosomal Mechanism of Sex Determination:
In dioecious
diploidic organisms following two systems of sex chromosomal determination of
sex have been recognized;
(a) Heterogametic
males and (b) Heterogametic females
a)
Heterogametic Males:
In this type
of sex chromosomal determination of sex, the female sex has two X-chromosomes,
while the male sex has only one X chromosome. Because, male lacks a X
chromosome, therefore, during gametogenesis it produces two types of gametes,
50 per cent gametes carry the X-chromosomes, while the rest lack in X
chromosomes.
Such as sex
which produces two different types of gametes in terms of sex chromosomes is
called heterogametic sex. The female sex is therefore called homogametic sex.
The heterogametic males may be of the following two types.
i) XX - XY method:
The XX - XY method is the most common method of sex determination. It is found
in mammals and certain insects like Drosophila, etc. In these animals the
female possesses two homomorphic X- chromosomes (i.e., XX) and being
homogametic produces eggs of one kind only. These all eggs have one X-
chromosome in them.
The male, here
possesses one X and one Y- chromosome (hence, called XY) and produces two kinds
of sperms, i.e., 50% with X- chromosome and 50% with Y- chromosome.
The sex of embryo depends upon the kind of sperm.
A female individual is produced, if egg fertilizes with a sperm having
X-chromosome and a male is produced if same is fertilized with Y-chromosome
bearing sperm.
In human being
there are 46 chromosomes (23pairs) in all the body cells except gametes. Of
these 46 chromosomes, 44 are autosomes which are common in both the sexes and
the remaining two are sex chromosomes either with XX combination or with XY
combination. The female has 44 autosomes and two sex chromosomes- XX; the male
has 44 autosomes and two sex chromosomes- XY. Thus combination of sex
chromosomes determines the sex, two XX chromosomes gives rise to a female
whereas X and Y combination gives rise to a male.
Fig. XX - XY pattern
ii) XX – XO pattern - The other method, i.e., XX – XO type is found in certain insects
especially those of class – Hemiptera (true bugs) and order – Orthoptera, In
this method, the female has 2X–chromosomes (called while male has only one X–
chromosome (called XO).
The
Y–chromosome is completely lacking here, thus, the presence of unpaired X –
chromosome determines masculine sex. The female just like lygaeus method
produces only one type of eggs and male produces types of sperms, i.e., 50%
with one X – chromosome and 50% without any sex chromosome. The sex of
offspring depends upon the type of sperm, which fertilizes the egg, e.g., in
grasshopper.
Example
Grasshopper In
1902 C. E. McClung while studying meiosis in the testes of the Grasshopper, he
noted that there were 11 pairs (22) of chromosomes and an odd chromosome with
no mate. He associated the odd chromosome with the sex determination. Thus the
female grasshopper possesses 22 autosomes and a pair of sex chromosomes i.e. XX
and the male possesses 22 autosomes and only one sex chromosome, the X. The female is homogametic and produces only
one kind of egg with 11 autosomes and one sex chromosome, the X. The male is
heterogametic and produces two types of sperms one with 11 autosomes and one sex
chromosome, X and other with 11 autosomes only (that is without a sex
chromosome). The sex determination in grasshopper follows a different pattern
as illustrated below;
In this type of sex chromosomal determination of
sex, the male sex possesses two homomorphic X chromosomes, therefore, is
homogametic and produces single type of gametes, each carries a single X
chromosome. The female sex either consists of single X chromosome or one X
chromosome and one Y chromosome. The female sex is, thus, heterogametic and
produces two types of eggs, half with a X chromosome and half without a X
chromosome (with or without a Y chromosome). To avoid confusion with that of
XX-XO and XX-XY types of sex determining mechanisms, instead of the X and Y
alphabets, Z and W alphabets are generally used respectively. The heterogametic
females may be of following two types:
Fig. Haploid male, Diploid female and XX-XO type
sex determination mechanism of insects.
i)
ZO-ZZ
method (Example butterflies and moth)
This system of sex determination is found in certain
moths and butterflies. In this case, the female possesses single Z chromosome
in its body cells (hence, is referred to as ZO) and is heterogametic, producing
two kinds of eggs, half with a Z chromosome and half without any Z chromosome.
The male possesses two Z chromosomes (hence, referred to as ZZ) and is
homogametic, producing single type of sperms, each of which carries a single Z
chromosome. The sex of the offspring depends on the kind of egg as shown below;
Fig. ZO-ZZ determination of sex in butterfly
ii) ZZ-ZW method
(Example Birds)
Fig The ZW-ZZ type of determination of sex in chicken
ii) Genic Balance Mechanism:
- 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 genomes.
- The sex determination in Drosophila is quite
different from humans. Drosophila has eight chromosomes (n = 4),
three pairs of autosomes and one pair of sex chromosomes. Even though Drosophila
possesses XX and XY sex chromosomal organization, unlike human beings, the
Y chromosome does not have any role in determining the sex of individuals.
The sex in Drosophila is determined by the ratio of the number of X
chromosomes to that of the number of sets of autosomes. In simpler terms,
the sex determination is achieved by a balance of female determinants on
the X chromosome (X) and male determinants on the autosomes (A). This type
of sex determination is called genic balance system.
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Chromosome Complement
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X / A Ratio
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Sexual Morphology
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X X X + 2A
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3/2 or 1.5
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Metafemale
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X X X + ЗА
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3/3 or 1.0
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Female
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XX + 2A
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2/2 or 1.0
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Female
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X X + ЗА
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2/3 or 0.67
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Inter sex
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X X X + 4A
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3/4 or 0.75
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Inter sex
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XO + 2A
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1/2 or 0.5
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Male
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XY + 2A
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1/2 or 0.5
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Male
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XY + ЗА
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1/3 or 0.33
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Metamale
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a)
Triploid or polyploid intersexes in Drosophila
In many cases
of different organisms, it has been observed that these generally have inherent
potentialities both the sexes and each individual is found to be more or less
intermediate between male and female sex. In Drosophila, in fact, it is the ratio between the X–chromosome and
autosome, which governs or determines sex. This was worked by CB Bridges
(1922), on triploid Drosophila.
He performed
experiments on these triploids, on the basis of which he developed his ratio
theory of sex determination or ultimately the genic balance theory. According
to Bridges in Drosophila, it is the
a) X–chromosome – Which is responsible for female
characteristics.
b) Y–chromosome – Which is responsible for male
characteristics.
c) Autosome – It is necessary for the fertility
of sex, but not responsible for any sex determination.
Thus,
according to him, it is the delicate balance between the X–chromosomes and
autosomes, which is responsible for sex determination. This theory is very
helpful in predicting the sex of individuals that arise from non-disjunction of
X-chromosome during meiosis in female. According to this theory, a
singleX–chromosome in a diploid organism produces male sex and XX combination
within a diploid organism produces females. In his experiments, Bridges crossed
a triploid female (3A + XXX) with a diploid male (2A + XY), the result of this
cross can be seen as
Fig. Classical cross of a
triploid (3A+XXX) female fly and a diploid (2A+XY) male fly (Drosophila)
The presence
of triploid inter sexes in the above experiment is a proof that autosomes also
carry genes for sex determination. The intersexes, super males and super
females were obtained,where interpreted by bridges in the form genic balance
theory of sex determination. According to this theory, ratio between the number
of X–chromosomes and number of complete sets of autosomes will determine sex.
If the ratio between X and A is 1.0, it will be female.
If 0.5 it will be male, ifmore then 1, it will be super female, if less then
0.5, it will be super male and Ifbetween 0.5 to 1, it will be inter sex.
Table Different doses of X-chromosomes and autosome
sets and their effect on sex determination in Drosophila.
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Phenotypes
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Number of chromosomes (X)
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Number of autosomes
(A sets)
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Sex index =
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Super
female
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3
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2
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1.5
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Normal
female
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1)
Tetraploid
2)
Triploid
3)
Diploid
4)
Haploid
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4
3
2
1
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4
3
2
1
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1.0
1.0
1.0
1.0
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Intersex
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2
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3
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0.67
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Normal
male
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1
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2
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0.50
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Super
male
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1
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3
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0.33
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b) Gynandromorphs in Drosophila.
In Drosophila, occasionally, flies are obtained with one half having
male characteristics and another half having female characteristics
Concept of sex determination as developed for Drosophila
is verified by the occasional occurrence of gynandromorphs which are individual
in which part of the body expressed male characters, whereas other parts
express female characters. In a way, gynandromorphs represent one kind of
mosaic or an organism made up of tissues of male and female genotypes. For
example, bilateral gynandromorphs of Drosophila are male on one side (either
right or left) and female on the other. It results due to the loss of an X-chromosome
in a particular cell during development i.e. when the pair of sex X-chromosome
fails to separate during gametogenesis one X and is lost forever. If this event
happens during first cleavage of mitotic division or the zygote, then one of
the two blastomeres will have AAXX chromosomal complement and the other will
have AAXO. The portion of the body developing from AAXX blastomeres will be
normal female and the portion developing from the AAXO blastomeres will be
male. The cytological examination of gynandromorphs suggested that Y-chromosome
does not play any role in the determination of sex in Drosophila. , as shown
below
Fig.7.5:Gynandromorph
of Drosophila, in which right half is male and left half is female
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Fig. The
loss of an X chromosome during mitosis in a 2A+XX cell and formation of two
types of cells – XX and XO
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iii) Haplodiploidy Mechanism:
Another
mechanism of sex determination is male haploidy or haplodiploidy. This
mechanism is particularly common in hymenopteran insects, e.g., honey bee,
wasp, ants, etc.
The pattern is
more common in those insects, in which parthenogenesis is generally seen. In
these insects, normally, three types of individuals are seen -
a) Diploid queen:
Fully functional females developed from fertilized eggs.
b) Diploid workers:Underdeveloped or non-functional females developed from fertilized eggs.
Actually here,
the fertilized egg gives the females in general. Out of which, the female which
fed on Royal jelly (a specialized food) becomes queen and rest are known as
workers.
The workers
have specific function of protection of colony, while queen involved in egg
production mainly. The workers are unable to perform fertilization and egg
laying as in them, the organs of ovipositionare mo6dified to sting apparatus.
c) Drones:
Functional males are developed parthenogenetically from unfertilized
haploid eggs. Drones are haploid organism. These males
are short lived and normally die after fertilization.
As males are
developed parthenogenetically, hence, we can say that in these mechanisms,
parthenogenesis (i.e., development of eggs to adult without fertilization)
plays a major role in determination of the sex. The sex chromosomes have no
identity here. In certain extreme cases,
males are altogether absent and females are produced by parthenogenesis. These
females are always diploid. e.g., Lacertasexicota. In Habrobraconjuglandis (a
parasitic wasp also called Braconhebetor) the sex determination mechanism is
similar to wasps and bees, i.e., unfertilized eggs develop into males and
fertilized eggs develop into females.
The difference
lies in the fact that some fertilized eggs also develop into males. This is due
to the presence of a number of alleles promoting maleness and femaleness, e.g.,
XA, XB, XC and XD, etc. The diploid
males have homozygous condition of these alleles, while diploid females have
heterozygous condition.
Honeybee or Diploid-haploid method of sex determination
Fig.
Diploid-Haploid Sex determination system
iv) Single gene effects
or single gene control of sex
In certain organisms, for example Chlamydomonas,
Neurospora, yeast, Asparagus, maize, Drosophila, etc.,
individual single genes are found to be responsible for the determination or
expression of sex, following cases exemplified the single gene control of sex:
a) Monogenic sex determination in Drosophila: In Drosophila, a
transformer gene (tra) has been recognized which when present in
homozygous condition (tra/ tra) transforms a female fly into a sterile male,
but, it does not act upon normal male individuals. Thus, a XX female with tra/ tra
genotype will be a sterile male, but, a XY male with tra/ tra genotype will
still be a normal male fly.
b) Sex reversal gene (Sxr) in mammals: Recently a sex reversal gene
(Sxr) has been discovered in human beings, so that in the presence of this gene
XX female individuals may become male. Such cases of sex reversal are also
reported in goat and mice. Mice also contain two other genes Tdyand
Tda-1 which interact to cause sex reversal in XY male individuals to
transform them into females.
c) Complementary sex factors: Besides the haploidy mechanism of
sex determination, two hymenopterans insects – Bracon hebetor (parasitic
wasp) and honeybees are known to produce males by homozygocity at a single gene
locus.
d) Sex-determination in Asparagus:Asparagus is a dioecious plant, however,
sometimes the female flowers bear rudimentary anthers and the male flowers bear
rudimentary pistils. Thus, sometime it may happen that a rare male flower with
poorly develop pistil may set seeds. In one of the experiments when the seeds
of such a rare male flower were raised into plants, then, the male and female
plants were found to be present in 3: 1 ratio. When the male plants raised thus
were used to pollinate the female flowers on female plants, only two third of
them showed segregation indicating that the sex is controlled by a single gene.
B) Metabolically controlled sex
determining mechanism:
Certain workers have seen the possibility of sex
determination in the phenomenon of metabolism. Cruw suggested that sex is a physiological
equitable division between anabolic and catabolic individuals. A.F. Shull and
D.D. Whitney have shown that by increasing metabolic rate in rotifers the
occurrence of male individuals increase than females.
C) Hormonal and Environmental Control of Sex
Determination:
Besides above
written mechanisms of genetic control, hormonal and environmental role is also
seen in determining the sex. The sex determination theories (chromosomal theory
and genic balance) apply to lower animals but in higher vertebrates, the
hormones especially the gonadal hormones (secondary products of sex
chromosomes) may considerably alter the sexual characters.
In a
particular sex, a particular hormone produced by endocrine glands (gonads for
normal development. The intentional or unintentional removal of such a gland
results into changes in characteristic.
For example,
in fowl, if the ovary from a genetic female is removed (intentionally or
accidentally it results into the development of male comb along with male
plumage and in extreme cases testes will also appear (findings of crene).
The above case
of sex reversal was explained by assuming that as soon as the ovary was removed
or destroyed, the ovarian hormones were stopped. After sometime, the dormant
testes present as rudiments in almost all the female birds start functioning.
The male
hormones were produced, which helped in the development of secondary sexual
characters and sperm formation in extreme cases. Cases of such sex reversal are
observed in some fishes, amphibians, birds and even in mammals.
In Bonelia viridis, all the larvae are
genetically and cytologically similar. If a particular larva settles near
proboscis of an adult female, becomes a male individual.
If it
develops free in water, it becomes a female this is called environmental
control of sex determination.
