Sex determination in animals and plants - Mechanisms

1. SEX
DETERMINATION
IN ANIMALS AND
PLANTS
SAKEENA ASMI
MAHATMA GANDHI
UNIVERSITY

2. SEXUALITY
• Biologically sex is an aggregate of those morphological,
physiological and behavioral qualities that differentiate the
organisms producing eggs from those organisms producing sperm.
• The organism producing eggs are known as female and those
producing sperm are known as males.
• The sexes behave as a Mendalian character.
• Its inheritance follows laws of seggregation.
• The various genetically controlled sex determination mechanisms
are have been classified into following categories:
1. Chromosome mechanism.
2. Male haploidy or Haplo-diploidy mechanism.
3. Genic balance mechanism.
4. Single gene effect.
5. Cytoplasmic sex determination.

3. CHROMOSOME THOERY OF SEX DETERMINATION
• In majority of diploid sexual animals, along with autosomes, a pair
of sex chromosomes specialized for sex determination are found.
• These are represented by X and Y.
1. Sex chromosomes and Autosomes
• The X chromosome was first observed by German biologist,
HENKING in 1891 during the spermatogenesis in male bug and was
described as X-body.
• The chromosome theory of sex determination was worked out by
E.B WILSON and STEVENS (1902-1905).
• They named the X(female determining gene) and Y(male
determining gene) chromosome as Sex chromosomes or allosomes
and other chromosomes of the cell as autosomes(carries genes for
somatic characters).

4. 2. Types of Chromosomal Mechanisms of Sex determination.
• In primitive forms sex chromosomes(X and Y chromosomes) are not
identified.
• The genes determining the sex seen to be located on certain
autosomes.
• This is regarded as most primitive type of sex determination.
A) XX-XY type or Lygaeus type
• XX-XY type of sex determination mechanism was first studied in the
milk weed bug, Lygaeus turticus by WILSON and STEVENS.
• Therefore it is called as Lygaeus type.
• There are two different patterns of sex determination in Lygaeus
type:-
(i) Female homogametic XX and male heterogametic XY
- The homogametic sex (XX) is female and produces ova all of
one type ie having X chromosome.
- The male is heterogametic (XY) that produces sperma of 2
types. 50% possess X chromosome and other 50% Y.

5. Example 1: Drosophila – In Drosophila total number of chromosome is
8 of which 6 are autosomes, common to both male and female. The
fourth pair is of sex chromosomes. In male this is represented by
XY(6+XY) and in female XX(6+XX).
MALE FEMALE
XY XX
Sperms OvaX
XY
X
X
X X
X
Y
Y
XY
XX XX
XX – female – 50%
XY - male – 50%

6. Example 2: Man – In case of man total number of chromosomes is 23
pairs or 46.
In male (man) 44 + XY
In female(woman) 44 + XX
The sperm produced by male are of 2 types : 22 + X, 22 + Y whereas
the ova all have 22 + X chromosome.

7. Example 3: Melandrium – In Melandrium (the garden pink) a variety of
garden flower, sex is determined by a pair of XY chromosome just as in
animals. In Melandrium the Y chromosome longer than X hromosome.
MODIFICATIONS of XX-XY mechanism
• X and Y chromosome attached to a pair of autosomes – In this type
X and Y are attached to a pair of autosomes and during maturation
follow the autosomes.
• X or Y complex – In certain organisms either X or Y chromosome
gets broken into 2 or more fragments forming X- complex or Y-
complex. In Tenodera, Mantis, Stegomantis X chromosome is
broken into 2 fragments, therefore the female possess X1X1 X2X2
and male X1X2Y.

8. (ii) Female heterogametic and male homogametic
- In fowl, other birds and some fishes, certain moths and butterflies
the female sex is heterogametic with X and Y chromosome often
represented by Z and W.
- The male sex is homogametic having XX or ZZ chromosome.

9. B) XX-XO type or Protenor type
• MCCLUNG in male squash bug (Anasa) observed 10 pairs of
chromosomes and an unpaired chromosome.
• The females have 11 pairs of chromosomes (22).
• Thus all the eggs carry a set of 11 chromosomes but the sperm are
of 2 types: 50% with 11 chromosomes and other 50% with 10
chromosomes.
• Fertilization of an egg with sperm carrying 11 chromosome results
in female, while its fertilization by a sperm with 10 chromosomes
produces male.
• It is said to be evolved by the loss of Y chromosome.
Female: 22 chromosomes - 20 + XX
Male: 21 chromosomes - 20 + X
10 + X
10 + X
10 + X
10

10. The XO method is found in Orthopterans and Heteropterans.

11. MALE HAPLOIDY OR HAPLO-DIPLOIDY MECHANISM
(Hymenopteran type)
• In honey bee, wasp, ants and certain other hymenopterans
parthenogenesis is widespread. There are 3 types of individuals:
(i) Diploid queens – These develop from fertilized eggs and are
fully developed functional females.
(ii) Diploid workers – These also develop from fertilized eggs but
are under-developed non-functional females which are unable
to produce ova.
(iii) Haploid drones or males – These develop
parthenogenetically from the haploid unfertilized eggs and
are functional males.

13. Sex determination in Habrobracon
• In Habrobracon, the sex determination mechanism is similar to
wasp and bees but some fertilized eggs also develop into males.
• This is on account of the presence of a number of alleles
promoting malesness and femaleness.
• These alleles are Xa, Xb, Xc, and Xd etc.
• The diploid males are homozygous for these alleles being Xa/Xa,
Xb/Xb, Xc/Xc or Xd/Xd.
• The haploid males have only one allele they could be Xa, Xb, Xc or
Xd.
• The females are heterozygous being either Xa/Xb, Xc/Xa, Xd/Xa,
Xb/Xc or Xb/Xd etc.
• It means males produce only one type of sperms whereas females
produce 2 types of eggs.

14. Quantitative or Ratio Theory of Sex Determination
• In XX-XY system of sex determination X chromosome possess genes
for femaleness and Y chromosome possess genes for maleness.
• But PATTERSON discovered that in Drosophila Y chromosome is
heterochromatic and plays no significant role in sex determination,
the ratio between the X and the autosome governs the development
of male or female sex.
• Single doses of X chromosome in a diploid organism produces male
whereas 2X chromosomes produce female.
• C.B BRIDGES worked out ratio theory of sex determination in
Drosophila.
• According to this theory the ratio of chromosomes to autosomes is
the determining factor for sex.
• If a complete haploid set of autosome is designated by A then 2A:X
will give rise to male and 2A:2X to female.

15. 1. Intersexes in Drosophila and Ratio Theory Of Sex Determination
Bridges hypothesis was supported by studies of flies arising after
abnormal distribution of chromosomes on account of non-
disjunction.
(i) Non-disjunction of X chromosome
- Due to abnormal meiosis during oogenesis both the X
chromosome fail to separate and move to one pole of meiotic
spindle.
- Thus few eggs are formed with single autosomal genome but
with 2 X chromosome(AXX) and others with single autosomal
genome but no sex chromosome(A).
- When such abnormal eggs are fertilized with normal sperm, the
following results are obtained :-
AAXXY – Female
AAXXX – Super female
AAX – Sterile male
AAY – Nonviable

17. (ii) Non-disjunction of complete set of chromosome
- The ratio theory of sex determination is further supported by
another instance of abnormal meiosis where along with X
chromosomes autosomes also fail to separate and move to the
same pole of the spindle.
- Therefore, these abnormal eggs possess AAXX.
- When such diploid eggs are fertilized with normal sperms the
following combinations are produced.
AAAXXX – Female
AAAXXY – Intersex
(iii) Triploid Intersexes and balance theory
- The triploid flies with (3A*3X) are much like the normal diploid
females both in appearance as well as in fertility.
- On mating to diploid males their progeny consisted of the
following types :-

18. AAAXXX - Triploid females AAAXXY - Intersexes
AAXX - Diploid females AAXY - Normal males
AAXXY - Diploid females AAXXX - Super females
AAAXX - Intersexes AAAXY - Super males

19. 2. Gynandromorphs in Drosophila and Ratio Theory of Sex
Determination
• In Drosophila, occasionally flies are obtained in which part of the
body exhibits female characters and the other part exhibits male
characters.
• Such flies are known as Gynandromorphs.
• These are formed due to misdivision of chromosomes and start as
female with 2A+2X chromosomes.
• One of the X chromosome is lost during the division of the cell with
the result that one of the daughter cells possess 2A+2X
chromosomes and the other 2A+X.
• If this event happens during first zygotic division, 2 blastomeres
with unequal number of X chromosomes are formed.

20. • The balstomere with 2A+AX chromosomes develops into female
half, while the second with 2A+X chromosomes produces male half
and the resultant fly is bilateral gynandromorph.
• The occurrence of gynandromorphs clearly indicate that the
number of X chromosomes determines the sex of the individual.

21. Nondisjunction of of X chromosome in Drosophila as a
proof of Chromosome Theory of Sex determination.
1. Primary Nondisjunction
- Normally during gamete formation the homologous chromosomes
of each pair are equally distributed in the daughter cells(disjunction)
- While working on Drosophila, BRIDGES found that rarely chromos-
omes of X pair fail to separate and both the X chromosomes are
transmitted to one of the daughter cells.
- Therefore some ova contain 2X, while some others are without any
X chromosome.
- The failure of separation of homologues of a pair is called non-
disjunction.
- It was first described by BRIDGES in 1916 in a paper entitled
“Nondisjunction as a proof of chromosome theory of heredity”.

22. • According to Morgan’s Theory of sex linked inheritance, white eyed
female Drosophila when crossed with red eyed males are expected
to produce all females red eyed and male flies white eyed.
• But BRIDGES noticed that about one fly in 3000 flies of F1
generation exhibits unexpected eye colour ie white eyed females
among the red eyed females and red eyed males among the white
eyed males.
• BRIDGES explained that the white eyed exceptional females must
have received both the X chromosomes from mother.
• Similarly the red eyed exceptional males must have received their X
chromosome from father.
• This could happen only when the 2 X chromosomes of mother
carrying white eyed gene have failed to separate during gamete
formation and are passed together into 1 gamete.
• In male this could be explained only if the sperm with X
chromosome from the father had fertilized the ovum without X
chromosome. The zygote thus possess only 1 X chromosome and
that too from father. Therefore exhibits red eye colour in male.

24. 2. Secondary Nondisjunction
- The exceptional females with XXY chromosomes produce 4 types
of eggs :-
(i) Eggs with one C chromosome
(ii) Eggs with XY chromosomes
(iii) Eggs with XX chromosome
(iv) Eggs with Y chromosome alone.
Fertilized by normal spermatozoa of red eyed male, the eggs produce
8 type of zygotes but not in equal frequency. These are :-
(i) Red eyed female-XX ( v) White eyed male-XY
(ii) Red eyed female-XXY (vi) Supermale-XYY
(iii) Superfemale-XXX (vii) White eyed female-XXY
(iv) Red eyed male-XY (viii) YY - Dies

26. GENIC BALACE THEORY
• Based upon the observations of ratio theory BRIDGES put forward
genic balance theory in which he suggested that every individual
whether male or female possess in its genotype genes for both
male and female characteristics.
• Which sex will actually develop is decided by the preponderance of
that type of genes.
• If there is excess of female determining genes a female develops
and vice versa.
• The sex chromosomes and autosomes are mere vehicles of genes.
• In Drosophila the X chromosome carries more genes that incline
the development towards femaleness and the autosome possess
genes which incline the development towards maleness.
• Therefore the deciding factor is the ratio between the number of X
chromosomes to autosomes.
• But it is not a universal mechanism because COSSWIG has shown
that in Platypoecilus the sex determining factor is the ratio of
autosomes to Y chromosome.

27. SINGLE GENE EFFECT
In some organisms like Neurospora, Chlamydomonas, Yeast,
Asparagus, Drosophila, maize and several fishes single gene is
responsible for the expression of sex.
1. Monofactorial Sex Determination in Asparagus
In Asparagus, the sex is determined by a single pair of gene. The
maleness is dominant over femaleness and the male plants are
ordinarily heterozygous.
2. Monofactorial Sex Determination in maize
- In maize the gene for tassel seed(male inflorescence is tassel-ts)
converts the male inflorescence (tassel) into seed bearing
inflorescence.
- The gene for silkless (sk) causes the absence of silk (female
inflorescence). Therefore, a plant sk/sk is found to be male plant
and a plant ts/ts a female plant.

28. 3. Monofactorial Sex Determination in Drosophila
- In Drosophila a transformer gene (tra) when present in homozygous
condition (tra/tra) converts a female into sterile male, but has no
influence on normal male. Thus XX female with tra/tra genotype is a
sterile male while a XY male with tra/tra genes is a normal female.
- By using these genes it is possible to convert maize from a
monoecious to dioecious form.

29. CYTOPLASMIC SEX DETERMINATION
• In bacteria , the sexuality is controlled by cytoplasmic factors or F-
factor or fertility factor.
• LEDERBERG and TATUM (1947) found that the male or donor cells
possess a fertility factor and are designated as F+ cells while the
female or recipient cells are F-.
• During conjugation the F factor may be transferred to recipient cells
converting them into F+ donor cells.
SEX DIFFERENTIATION
1. Hormone and sex differentiation
• In higher vertebrates and under certain conditions in invertebrates,
the embryo develops some characters of the opposite sex together
with the characters of its own sex chromosome.
• It means the sex changes under certain circumstances.
• This is due to the hormone secreted by the gonads of that animal.

30. • The theory is based upon the observations of CREW in chicks.
• He found that a hen which laid fertile eggs, accidentally lost its
ovary, stopped laying eggs and develop male comb, male plumage
and became a cock.
• It finally functioned as male and became father of two chickens.
There are several examples of hormonal control of sex differentiation.
Some are given below:-
(i) Sex reversal
• As soon as the ovary was removed or destroyed the ovarian
hormones were stopped and after sometime the dormant testes
present as rudiments in almost all the female birds starts
functioning.
• The male hormones were produced and resulted in the
appearance of male secondary sexual characters and the formation
of sperm.

31. • Cases of sex reversal have been observed in fishes, amphibians,
birds and even in mammals.
• Cases of sex reversal in man have been found due to the
irregularities in the functioning of adrenal and pituitary.
(ii) Free martinism
• Found in cattles.
• LILLIE and others found that where twins of opposite sex (one male
and one female) are born, the male is normal but female is sterile
with many male characteristics.
• Such sterile females are known as free martins.
• The scientific explanation for the formation of free martins is the
effect of hormones of the male sex on female.
• In cattle the foetal membranes of the twins are fused in such a
manner that they have a common circulation of blood.
• The female hormone is produced at a slightly later stage in the
development.

32. • But since the twins have a common circulation and blood passes
from one twin into the body of another twin, the male hormone
which is produced slightly in advance of female hormone enters the
body off female twin, it is differentiated in the guidance of male
hormone.
• As a result the developing female is sterile.
(iii) Intersexes
• While working on Lymantria (gypsy moth) GOLDSCHMIDT observed
the following facts:-
• When Lymantria dispar (European race) males are crossed with
normal, L.japonica females, the males and females of the progeny
are all normal, but when the sexes were interchanged, the females
were all intersexes, whereas males were all normal.

33. • To account for these observations GOLDSCHMIDT presumed that
the factors for maleness are present on X chromosome and for
femaleness on Y chromosome or in the cytoplasm.
• The sex factors are weak in European strain and strong in Japanese
strain.
• Accordingly, if a European female (XY) is crossed to japonica male
(XX), the females produced are intersexes because X chromosome
from japonica race with its maleness is not completely dominated
by the relatively weak femaleness (Y) from European females.
• On contrary males receiving one strong X from Japanese race and
other weak X from European race result in the formation of
normal males.

34. 2. Environmental factors in sex determination
• In certain animals the environment plays an important role in the
differentiation of sex or in the expression of genes for male or
female sex.
• Most interesting case is the differentiation of sex in Bonellia.
• In this worm the larvae are potentially hermaphrodite.
• But if a single worm is retarded from the egg in isolation, it
invariably develops into a female.
• On the other hand, if newly hatched worms are released in water
containing mature females, young worms which attach to the
substratum develops into females, while others which attach
themselves to the proboscis of females differentiate into males
which migrate down to the nephridium of females and stay as
parasites.

35. SEX DETERMINATION IN DIOECIOUS PLANTS
• Most of the dioecious plants exhibit XX-XY type of sex determination
where the presence of Y chromosome determines the development
of male sex. Eg: Coccinia indica & Melandrium album
• XO and ZW method of sex determination have also been discovered
in certain cases.
MASTER GENE CONCEPT OF SEX DETERMINATION
• An international team of scientists (Dr. DAVID C PAGE) and others at
Whitehead Institute for Biochemical Research and the Massachusette
Institute of Technology in Cambridge very in 1987 have come to the
following conclusion about sec determination in man.
• The foetus having a master gene TDF (testes determining factor)
develops testes and grows into male.
• When this gene is absent the foetus develops ovaries and grows into
females.
• This gene is present on Y chromosome and is believed to act as a
biological switch, turning other genes on and off.

36. THANK
YOU