Reproduction Organisms

Transcription

1
Reproduction
in Organisms
In The Snapshot
Asexual Reproduction
Sexual Reproduction
Reproduction is the characteristic feature of all living organisms
by which they produce young ones of their own species. It is means
of multiplication and perpetuation of the species because the older
individuals of each species undergo senescence and die.
It is a vital process without which species cannot survive for long.
It enables the continuity of species generation after generation.
It maintains life on earth and allow species to live indefinitely.
There are several factors which determine how an organism
reproduces like the organisms habitat, its internal physiology and
environmental conditions.
Reproduction is divided into following two types based on the
number of parents involved in the process
1. Asexual reproduction It involves only one parent and no
involvement of gametes.
2. Sexual reproduction It involves both male and female
parents. Both contributes their part in the form of gametes
called sexual gametes or male and female gametes.
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Exploring BIOLOGY • Vol 2
Asexual Reproduction
Production of offspring by single parent without the formation or fusion of gametes is called asexual
reproduction. It involves only one parent. Thus, process like meiosis, gamete production, fertilisation and
parthenogenesis, etc., does not occur. Since asexual reproduction does not involves meiosis and fusion of gametes,
the offsprings produced are genetically similar to their parents. Further, they do not show any variations due to
absence of crossing over and recombination. These offsprings are referred to as clones. Genetic variation arises in
them only as a result of random mutation.
Asexual reproduction occurs most commonly in unicellular or cellular organism like monerans and protists,
and in higher plants and animals with relatively simple body organisations. It is very quick mode of reproduction
and therefore used by plant breeders for cloning. While in animals and other simple organisms the term asexual
reproduction is used ambiguously, but in plants the term vegetative propagation is preferred.
Modes of Asexual Reproduction
It involves various methods such as binary fission, sporulation, budding, gemmae and gemmules, fragmentation
and vegetative reproduction both in plants as well as animals. Various types of asexual reproduction may be
catagorised on the conditions in which organism is reproducing. In favourable condition reproduction by binary
fission, budding (gemmae) and gemmules are favored. But sporulation generally occurs during unfavourable
condition.
The various modes of asexual reproduction are as follows
Binary Fission
Binary fission is the process of division of parent into two daughter cells of equal size. During binary fission, the cell
elongates and its nucleus divides into two daughter nuclei. A transverse wall is formed in the centre of the parent
cell dividing it into two daughter cells, which later on separate and lead independent lives. Bacteria divides rapidly
by binary fission.Binary fission is also the characteristic feature of some yeasts, e.g., Schizosaccharomyces pombe.
Cleavage
(a)
Fig. 1.1
(b)
Dividing
nucleus
Daughter cells
(c)
Fission in Schizosaccharomyces pombe
Binary fission is the common method of asexual reproduction in Amoeba. It includes all the stages of
karyokinesis of mitosis (prophase, metaphase, anaphase, telophase) along with cytokinesis. A very interesting
feature seen in karyokinesis is the existence of multiple spindle which is reduced to unipolar spindle at the end of
the anaphase stage.
In case of Amoeba, the parent divides into offsprings. Thus, there exists a continuity of life and we can say that
Amoeba is potentially immortal. The various phases of binary fission are associated with the changes in the
shape of the animal and is seen under favourable conditions of food and water.
(i) In the prophase stage, animal withdraws its normal pseudopodia and becomes somewhat rounded and
small pseudopodia are seen all around. The cell organelles and honeycomb lattice disappear and cytoplasm
loses its transparency. The chromidia appear as duplicated chromosomes in the nucleus.
(ii) In the metaphase stage, the chromosomes are arranged over metaphase plate and formation of multipolar
spindles takes place.
(iii) In the anaphase stage, daughter chromosomes move towards opposite poles and construction of nuclear
membrane begins from the middle. Nuclear spindle becomes unipolar from the multipolar condition of
metaphase and the pseudopodia become larger and irregular in shape.
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Reproduction in Organisms
(iv) In the telophase stage, pseudopodia assume the normal shape. The body first elongates and then constricts in
the middle thus, resulting into two daughter amoebae.
This condition is exactly opposite to prophase, i.e., involves reappearance of organelles which were
disappeared in prophase.
Plasmalemma
Chromosomes
Fragmenting
honey comb
lattice
Chromatin
Honey comb
lattice
Multipolar
nuclear
spindle
Nucleoli
disintegrating
Nuclear
membrane
Interphase
Daughter chromosomes
(chromatids)
Prophase
Early anaphase
Metaphase
Daughter
nuclei
Mid anaphase
Tripolar
nuclear
spindle
Late anaphase
Fig. 1.2
Unipolar
nuclear
spindle
Daughter
amoebae
Telophase
Cytokinesis
Amoeba : Stages in binary fission
Types of Binary Fission
Various types of binary fission are as follows
(a) Transverse binary fission The fission in which plane of cytoplasmic division coincides with the transverse of
individual.
(b) Longitudinal binary fission The fission in which plane of cytoplasmic division coincides with the
longitudinal plane of individual.
(c) Irregular binary fission Plane is not fixed.
Multiple Fission
In this type of fission cell divides several times by mitosis producing several nuclei and then multiple daughter
cells. It is a common method of reproduction in many protists, algae, protozoans and sporozoans.
Many daughter
cells produced by
Nucleus
multiple fission
Cytoplasm
Daughter cells
release
Cell (schizont)
Fig. 1.3 Multiple fission in Plasmodium
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Budding
Gemmule
In this process, the parent cell produces one or more bud
like protuberances which detach from parent cell and
grow into new individual. Such type of reproduction is
very common in budding yeast (Saccharomyces) where
chain of buds may produce pseudomycelium.
Reproduction by gemmules also known as propagules
is a charateristic of plants as well as animals. These
arise as modified branches and are rich in food and
germinate into new plant on detachment from the
parent.
During bud formation, the cell wall of the parent cell
bulges out in the form of an outgrowth. The parent
nucleus divides into two daughter nuclei, one of which
migrate into the bud. As the bud enlarges and is fully
formed, it is separated from the parent cell and leads an
independent life.
Developing bud
Gemmae
Gemma cup
Daughter cell
Dividing
nucleus
Mother
cell
(a)
Fig. 1.6
Fig. 1.4 Budding in Saccharomyces
Budding also occurs in Hydra where a bud arises from
the side of this animal and later on detach and gives rise
to a new individual.
Tentacles
Mouth
A gemmae cup contains about 15-20 gemmae. On
detachment from the parent thallus, they find a
suitable substratum to germinate and give rise to a
new plant.
Batteries of nematocysts
In bryophytes, the gemmae are common in
Hepaticopsida, less common in Anthocerotopsida
and approximately not present in Bryopsida.
Knob like tentacles
In case of invertebrates, all freshwater and some
marine sponges have a regular and specialised mode of
asexual reproduction called as gemmule formation.
Bud
Gemmules are normally seen in unfavourable
conditions and help in passing over these conditions. A
fully grown gemmule is a small hard ball consisting of
an internal mass of food laden archaeocytes,
surrounded by a resistant chitinous covering.
Ovary
Bud
(a)
In case of Marchantia, the gemmae are multicellular,
green bodies of definite form, which develop in a
gemma cup. These may be unicellular, bicellular or
multicellular at the time of release.
In case of algae, the gemmae occurs in Sphacelaria.
Body
Pedal disc
Marchantia : (a) A thallus with gemma cups
(b) Gemmae cup
Hypostome
Testes
Ovum
(b)
Contracted
body
(b)
Fig. 1.5 (a) Expanded body of Hydra with bud and gonads
(b) Contracted body bearing bud
The structure contains a single opening called as
micropyle. The covering of gemmule is two layered
with monaxon spicules in between.
The gemmules are commonly seen in autumn when cold
and food scarcity is evident. The archaeocytes present
inside the gemmule are called histoblasts. As the
sponge body dies in autumn, the histoblast cells
present inside hatch from micropyle in the next
summer when the conditions become favourable again.
Micropyle
Outer covering
Inner covering
Archaeocytes
Spicules
Fig. 1.7 A gemmule of sponges
Thus, resulting into new generation of sponges. This
generation then dies in autumn again. Thus, an
alternation of generation seems to be present in these
forms.
Sporulation
The process of formation of spores is known as
sporulation. It is a mode of reproduction in monera,
protista, algae and fungi. Different organism produces
different types of spores. Each spore, later on
germinates to produce a new individual.
In plants, micropores and in some other cases
megaspore are formed from all four products of meiosis.
Different types of spores present in plants are as follows
(i) Zoospores In
Chlorophyceae
and
Phaeophyceae, motile and flagellated zoospores
are produced from zoosporangia during favourable
conditions. Zoospores may be biflagellate as in
Chlamydomonas and Ulothrix quadriflagellate
as in Ulothrix and Ectocarpus. The zoospores of
Vaucheria are multinucleate known as synzoospores.
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(ii) Aplanospores During unfavourable conditions,
some unicellular, uninucleate, non-motile and
thin-walled spores are produced, which are called
aplanospores, e.g., Spirogyra and Ulothrix.
(iii) Statospores These are thick-walled spores
found in diatoms. Their wall may be smooth or
variously ornamented.
(iv) Neutral spores In certain algae, the protoplast
of vegetative cells directly function as spores and
these are called as neutral spores, e.g.,
Asterocystis and Ectocarpus.
(v) Carpospores Spores produced directly in the
carpogonium of gonimoblast filament are called as
carpospores. These carpospores are common in
Polysiphonia, Batrachospermum and other red
algae.
(vi) Tetraspores In brown algae, non-motile spores
known as tetraspores are produced within the
specialised cells known as the tetrasporangia.
Tetraspores are produced on the diploid plant and
are haploid.
(vii) Hormospores Multicellular spores like bodies
having thick cell walls formed in some
cyanophycean algae are called as hormospores.
These are somewhat similar to hormogonia but
are surrounded by a stratified wall. Hormospores
are usually formed in series. usually in Westiella.
(viii) Chlamydospores These are thick-walled,
non-motile spores commonly found in Rhizopus.
(ix) Sporangiospores These
are
non-motile
spores produced inside the sporangium as in
Rhizopus and Mucor, etc.
(x) Conidia These are thick-walled, non-motile
spores produced in chains over the conidiophores.
These are commonly produced by Penicillium and
Aspergillus.
(xi) Oidia These are thin-walled, non-motile spores
borne on a hypha oidiophores, e.g., Rhizopus.
(xii) Soredia Soredia are special kind of spores
produced in the sporangia of lichens.
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In Amoeba, sporulation is seen during adverse conditions without encystation. In this process, the nuclear
membrane disappears and the nucleoplasm breaks into several small chromatin blocks or fragments in cytoplasm.
Each of these become a complete nucleus after acquiring the nuclear membrane.
Conidia
Conidiospore
angium
Sporangiospore
Zoospores
Sporangium
Zoosporangium
Conidiophore
Zoospores
Sporangiospores
Conidia
Conidiosporangia
Basidium
Chlamydospores Gemmae
Fig. 1.8
Arthrospores Uredospore Ascospores Basidiospores
(Oidia)
Various types of spores found in algae and fungi
It is further protected by the formation of a spore case resulting into about 200 spores in the parent body. These
are liberated and remain dormant throughout the adverse conditions. Each of these spores produces a small
Amoeba on the approach of favourable conditions.
Quick Digest
Daughter nuclei
Spores are classified on basis of producing structure, like
urediospore (uredium in smuts), teliospore (telium in rusts) and
zygospore (zygosporangium in zygomycetes)
< Parasitic fungal spores are divided into two internal spores
(germinate within the host) and external spores (environmental
spores released by one host to infest other hosts).
< Spores are also classified as
(i) Monolete spore A single line on the spore indicating the axis
on which the mother spore was split into four along a vertical
axis.
(ii) Trilete spore All four spores share a common origin and are in
contact with each other and on separation each spore shows
three lines radiating from a center pole.
<
Disintegrating
nucleus
Spores
Chromatin
block
Each spore
develops to
new Amoeba
Amoeba under
unfavourable conditions
Fig 1.9 Sporulation in Amoeba
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Fragmentation
It is the process of breaking down of parent individual into small pieces or fragments accidentally or through external
force. Each piece or fragment develops into a new individual.
This type of reproduction is very common in lower forms, under unfavourable conditions. In plant group, it occurs
commonly in algae, fungi, bryophytes and pteridophytes. Among animals asexual reproduction by fission or
fragmentation is common in Protists. Many flatworms can construct into two halves, each of which can become a
new individual.
(i) (ii)
Fragments
(i)
Decayed portion
(a)
(b)
(ii)
(c)
Fig. 1.10 Fragmentation in Marchantia : (a) Parent thallus (b) Fragmentation (c) Daughter thallai
Vegetative Propagation
In higher plants specially angiosperm, a very important type of asexual reproduction is found called as vegetative
reproduction. It is the process of formation of new plants from detached vegetative parts or propagules of the parent
plant. It is of two types, i.e., natural and artificial vegetative propagation.
Natural Vegetative Propagation in Higher Plants
In this type of propagation, a fragment of the mother plant functions as a propagule to form an independent plant
under suitable conditions. The modes of natural propagation in higher plants are
(a) Roots Tap root branches can develop adventitious buds and form new plants in guava, popular and
Dalbergia. Fleshy adventitious roots also take part in vegetative propagation as in sweet potato, dahlia and
Asparagus.
Young plant
Fleshy root
Sprouting bud
Fig. 1.11 Sprouted root of sweet potato bearing young plants
(b) Underground stems Various underground stems are given below

Buds present inside the bulbs sprout to form new plants in onion, garlic and Narcissus.

Corms bear buds for the growth of daughter plants, e.g., Crocus, Colocasia, Freesia.
Node
Bud
Daughter
corms
Parent corm
(a)
Roots
(b)
Fig. 1.12 Corm : (a) Crocus (b) Colocasia
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Tubers are modified underground stem branches having several buds. Each eye of the potato tuber is a bud
which grows into a new potato plant, when planted with a portion of the swollen tuber.
The potato crop is raised by tubers and not by seeds.
Aerial shoot
Young leaves
Scale leaf
Branch
Aerial shoot
Rhizome
Petiole
Root
Branch
Rhizome
Roots
Roots
Bud
(a)
(b)
(c)
Fig. 1.13 Rhizomes : (a) Monopodial rhizome of Saccharum (b) Sympodial rhizome of
ginger (Zingiber officinalis) (c) Rootstock rhizome of fern Dryopteris
(c) Subaerial stems (Creeping stems) Runners, stolons and offsets are meant for forming new crowns at their
tips as well as in nodes in the case of former two. Breaking of these horizontal stems convert the different
crowns into independent plants, e.g., Eichhornia, Pistia (offsets), grass (runners) and strawberry (stolons).
Leaf
Stolon
Bud
Scale leaf
Node
Adventitious roots
Fig. 1.14 Stolon of strawberry
Parent plant
Runner growing from axillary
bud in axis of scale leaf
New plants growing
from axillary buds
Apical bud
Runner
Tap root
Lateral roots
Scale leaf
Adventitious roots
Fig. 1.15 A strawberry runner
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Aerial shoots Segments of Opuntia and other
cacti produce new plants after falling on the
ground.
Leaves Injured leaves of Begonia develop new
plants when come in contact with the soil while in
case of Bryophyllum and Kalanchoe uniniored
leaves do so and form buds present in its marginal
notches.
In Bryophyllum, marginal buds form plantlets
while they remains attached in the plants.
Another example of vegetative propagation by
leaves in Adiantum caudatum (walking fern).
Colocasia and Gladiolus), bulb e.g., garlic and
onion), part of rhizome (e.g., banana and ginger),
stem tuber (e.g., potato) and bulbil (e.g.,
pineapple).
Underground
stem branches
Scar
(heel end)
Tubers
(a)
Scale
Leaf
Apex
(rose end)
(b)
Bud
(c)
Fig. 1.18 Solanum tuberosum :
(a) Base of potato plant showing development of
stem tubers (b) A stem tuber (c) An eye
Adventitious bud
Petiole
Fig. 1.16 A leaf of Bryophyllum showing formation
of new plants from marginal adventitious buds

Eyes
Bulbils
These are fleshy buds produced in the
axil of foliage leaves in place of axillary buds. They
grow to form new plants when shed and fall on the
ground, e.g., Oxalis and Allium sativum.
In century plant (Agave sp.), the floral buds are
modified into bulbils which grow into new plants
when shed from the mother plant.
Bulbil
Fleshy
root
Bulbil
Stem
(b) Cuttings When a small piece of any plant organ
(stem, root or leaf) used for propagation it is called
as cutting. Sansevieria is propagated by leaf
cuttings. Root cuttings are used in case of
blackberry, raspberry, tamarind and lemon.
Stem cuttings (20-30 cm segments of one year old
stem) are employed in case of rose, Clerodendron,
Durantia, Citrus, tea, coffee, Bougainvillea and
cocoa. Rooting of stem cuttings is hastened by
dipping in NAA or IBA. Leaf and stem cuttings are
sown laterally with morphologically upper side
upward.
(c) Layering The middle part of a soft basal branch
is defoliated, slightly injured (tongueing, notching
or ringing) and pegged in the soil to develop
adventitious roots.
Later on the branch or layer is separated and
planted, e.g., cherry, jasmine and grapevine.
In serpentine layering, the branch is pegged at
several places so as to form many plants.
Leaf
(a)
(b)
(c)
Fig. 1.17 Bulbils : (a) Dioscorea (b) Oxalis
(c) Agave sp.

Turions These are special types of fleshy buds
that develop in aquatic plants, e.g., Potamogeton,
Utricularia, etc., for vegetative propagation.
Bud
New plant
Artificial Method of Vegetative
Propagation
Artificial method of vegetative propagation is as follow
(a) Use of special vegetative parts Many
plants are multiplied vegetatively by using their
specialised vegetative structures like root tubers
(e.g., sweet potato and dahlia), corm (e.g., Crocus,
Inter
node
Adventitious roots
Node
(a)
(b)
Fig. 1.19 (a) A portion of sugarcane stem having buds
(b) A bud growing into new plant
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In mound layering, the shoot is pruned and lower part is covered by soil when a number of new shoots develop,
e.g., gooseberry and currant. In air layering (goottee), about 3.5 cm ring of bark is removed from near the base
of an aerial shoot. It is covered by a thick plaster of grafting clay (hay, cowdung, clay and water) with small
quantity of root promoting hormone and wrapped in polythene.
After 2-3 months, roots appear and the shoot is removed below the bandaged region to be used for planting,
e.g., litchi, pomegranate, guava, orange and lemon. Plants having scattered vascular bundles like sugarcane
cannot be propagated by air layering (gootee). This is because vascular bundles may be harmed in this method
and water and food supply will be affected.
Branch
g
Ton
ng
uei
Ste
m
Ringing
Soil
Adventitious
roots
Polythene
covering
Roots
Branch
Stem
Notching
Soil
Grafting
clay
Ring of
bark removed
Roots
(a)
(b)
Fig. 1.20 Vegetative propagation by (a) Mound layering (b) Air layering (Gootee)
(d) Grafting It is the most common method of artificial vegetative propagation. In this method, parts of two plants
are joined in such a way that they grow as one plant. Grafting is done between the two closely related
dicotyledonous plants having vascular cambia. The rooted supporting portion of one plant called stock is joined
with a twig of another plant called scion.
Generally, the root stock belongs to a wild variety which is resistant to diseases and pests and possess an efficient
root system for the absorption of water and minerals on the other hand the scion is derived from the plant
possessing better characters. For example, a scion of Dussehri mango is grafted on the stock of Desi mango.
Similarly, scion of high quality roses are grafted on wild rose root stocks.
Grafting is of four types

Tongue or whip grafting Both the stock and scion are cut obliquely at about the same angle.

Wedge grafting A V-shaped notch is made on stock and wedge-shaped cut is made on the scion.

Crown grafting Several scions having wedge-shaped cut are grafted on the slits at the top of stock.

Side grafting Single scion having wedge-shaped cut is inserted in a lateral slit of the stock.
In case of tongue and wedge grafting, the scion and stock have almost the same diameter, whereas in case of
crown and side grafting, the stock has more diameter than the scion. Examples of the plants propagated by
grafting are mango, roses, apple, rubber, citrus, pear, plum and peach, etc.
Scion
Scion
Bud scion
inserted in
bark of stock
Scion
Bud
T-shaped slit
(d)
Bark of scion
(a)
Stock
Stock
(b)
(c)
(e)
Fig. 1.21 (a-c) Bud grafting, (d-f) Crown or wedge grafting
(f)
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(e) Bud grafting It is also known as budding. This
method is similar to grafting except that scion in
this case consists of a bud along with a small
portion of bark having intact cambium. The living
tissue of scion bud is inserted into an incision made
in the stock so that it reaches upto the cambium of
stock.
The bud portion should remain exposed outside in
the air. The joint is thoroughly sealed using
bandage. After 3-5 weeks, the bud begins to grow. As
soon as the bud sprouts, the stock is cut above the
level of graft, e.g., rose, apple, peach, etc.
Importance of Vegetative Propagation
Importance of vegetative propagation is given below
(a) Vegetative propagation is the only method of
reproduction for the plants which have lost their
capacity to produce seeds, e.g., banana, seedless
grapes, oranges, rose, jasmine, figs, pineapple,
carnation, etc.
(b) Plants which produce small quantities of viable
seeds (e.g., Cynodon and and Dactylon) are mostly
propagated vegetatively.
(c) The vegetative reproduction methods are more rapid,
easier and less expensive as compared to others.
(d) The great advantage of vegetative propagation is
that plant biotype (original plant) can be retained
and multiplied indefinitely without any change or
variation.
(e) Effects of physico-chemical treatments and
environmental
variations
can
be
studied
conveniently in those plants which propagate
vegetatively.
Advantages of Asexual
Reproduction
Asexual reproduction in a commom method of
reproduction. The best part of asexual reproduction is
that it is very simple. It is a common method of
reproduction in normal/favourable days.
The main advantage of asexual reproduction is the
addition of individuals to the population, resulting in
increase in population size. For example, bacteria can
generate a population of billions in 1-2 days from one
original cell.
Disadvantages of Asexual
Reproduction
Some disadvantages of asexual reproduction are as
follows
1. No genetic variations occur among the
offsprings.
2. If spores are produced, many will fail to find a
suitable place for germination and so energy
and material used in their manufacture are
wasted.
3. If an organism spreads in one area, it may
result in overcrowding and exhaustion of
nutrients.
Practice Test
1
1. Match the following columns.
A. Plasmogamy
1. Layering
B. Bulbils
2. Sporulation
C. Gootee
3. Fusion of protoplasts
D. Conidia
4. Fleshy buds
2. Name any two spores by which fungi reproduce.
3. List one of the method of natural vegetative
propagation.
4. Differentiate between wedge and whip grafting.
5. Mention two dissimilarities between a zoospore and a
conidia and at least one feature common to both
structure.
(NCERT Exemplar)
6. Mention the characteristic feature of binary fission in
Amoeba.
7. Match the following columns.
A. Aplanospore
1. Polysiphonia
B. Carpospore
2. Lichens
C. Hormospore
3. Ulothrix
D. Soredia
4. Westiella
12
Sexual Reproduction
Sexual reproduction involves the production of male gametes and female gametes (by meiosis) either by the same
individual (referred to as bisexual) or by different individuals of opposite sex (referred to as unisexual). During the
process of sexual reproduction the male and female gametes fuse to form the zygote, which develops into a new
organisms by the process of mitosis.
As the individuals produced here involve meiosis and gametic fusion, they exhibit genetic variation and difference
from either of their two parents or amongst themselves. Although sexual reproduction is an elaborate, complex and
slow process as compared to asexual reproduction, but still it plays a very important role in evolution of species.
Modes of Sexual Reproduction
Various modes of sexual reproduction are as follows
Syngamy
It is the complete and permanent fusion of male and female gametes to form the zygote.
Strasburger discovered syngamy. Syngamy is of two types
(i) Exogamy also known as cross fertilisation. It involves the fusion of two gametes, produced by different
parents. Thus, it is biparental, e.g., rabbit and other unisexual animals.
(ii) Endogamy also known as self fertilisation. It involves the fusion of two gametes of the same parent. Thus, it
is uniparental, e.g., tapeworm and other hermaphrodites.
On the basis of structure of fusing gametes, syngamy is of four types
(i) Isogamy Fusion of morphologically similar gametes, e.g., Monocystis.
(ii) Anisogamy Fusion of two dissimilar gamete, e.g., frog, rabbit and mammals.
(iii) Oogamy Fusion of Large non-motile female gamete (egg) and small motile male gamete antherozoid, e.g., red
algae.
(iv) Hologamy
Fusion of two organisms which act as gamete, e.g., yeasts.
Conjugation
It involves temporary union of two parents of the same species which exchange their male pronuclei and then
separate. It is some what like cross fertilisation and found in Paramecium bacteria and other ciliates. It provides
genetic variability among the organisms of the same species.
Conjugation in Bacteria
In case of bacteria, the sex organs are not formed. There is no fusion of gametes and production of zygote. Genetic
recombination in bacteria takes place by conjugation. Conjugation was discovered by Lederberg and Tatum
(1946) in E. coli.
13
It occurs between two sexually different strains of bacteria. One act as donor of genes (male) and other as recipient
(female). Both are haploid.
F-factor divide in two
Factor Sex pilus
Conjugation
bridge
DNA
F+
(a)
F+
(c)
F–
F+
F+
F+
–
F
(b)
(d)
F+
Fig. 1.22 Conjugation between F + male and F - female of E.coli (The F - is converted to F + )
The donor possess sex factor or fertility factor which is a small genetic particle of circular DNA. Sex factor
determines the formation of sex pilli which forms a conjugation bridge between the donor and recipient. Two
bacteria, i.e., F + and F - come close to each other.
The F factor divides into two out of which one remains in the donor cell and the other move to the recipient through
the conjugation bridge. As a result, the F - cell now becomes F + cell. Thus, a conjugation between F + and F - strains
always yield F + progeny.
Conjugation in Paramecium
It is the process, which occurs frequently between binary fission and is necessary for the continued vitality of
species. The complete process of conjugation in Paramecium is given below.
Conjugant
a
1
Two daughter Disintegrating
micronuclei macronucleus
Conjugant
b
Zygote
nucleus
Four
daughter
nuclei
Exconjugant b
8
7
Eight
daughter
nuclei
Four macronuclei
9
10
Stationary
gamete
nucleus
Migratory gamete
nucleus
Three disintegrating Zygote 6
5 nucleus
4 nuclei
3
2
Two
daughter
nuclei
Exconjugant a
Four daughter
micronuclei
Two
Micronucleus
daughter
dividing
paramecia
mitotically
Four
daughter
paramecia
Three disintegrating nuclei
11
12
Fig. 1.23 Stages of conjugation
13
14
14
Sexual Reproduction in Algae
Depending upon the structure and behaviour of gametes, three
different modes of sexual reproduction are involved in algae.
These are of following types
(i) Isogamy It occurs
in Ulothrix, Chlamydomonas,
Spirogyra, Oedogonium, etc.
(ii) Anisogamy It occurs in Chlamydomonas.
(iii) Oogamy It occurs in Chlamydomonas, Fucus, Chara,
Volvox, etc.
Sexual Reproduction in Fungi
Asci are produced in fruiting bodies called
ascocarps, which are mainly of three types
based on their shape and structure. These are
as follows
(i) Cleistothecium It is completely
closed and spherical body, opening by
the breakage of outer wall.
(ii) Perithecium It is a flask-shaped
body having a terminal opening or
ostiole.
(iii) Apothecium It
is
cup-shaped
fruiting body with a wide mouth.
Sexual reproduction involves plasmogamy (fusion of protoplasts
of male and female cells), karyogamy (fusion of male and female
nuclei forming zygotic nucleus) and subsequently meiosis to
convert diploid structure to haploid spores. Gametes may be
isogametes as in Rhizopus and Mucor. They are labelled as
plus (+) and as minus ( -) and form a zygospore on fusion. Different
types of gametes are produced in heterogametic fungi species.
In Basidiomycetes, basidiospores are
produced on the tip of the basidium. At the
apex of basidium, four haploid basidiospores
are produced, which on germination produce
monokaryotic mycelium either of (+) or ( -)
strain. In Deuteromycetes (fungi Imperfecti),
sexual reproduction is completely lacking.
Male gametes are produced in antheridium while female gametes
are produced in oogonium. Same mycelium can produce both male
and female sex organs (homothallic mycelium) or male and
female sex organs may be borne on different mycelia
(heterothallic mycelium). In some fungi, sex organs are not
developed and two vegetative cells fuse (somatogamy). Positive
and negative strains of two primary monokaryotic hyphae fuse
and form secondary dikaryotic hypha. In the members of
Ascomycetes, the sexual spores are called ascospores, which
are formed in asci (singular ascus) by meiosis.
Sexual Reproduction in
Bryophytes
Planogametic copulation
Sperm
Egg
Isogamy
Anisogamy
Oogamy
Gametangial contact
Oogonium
Antheridium
Sex organs in bryophytes are multicellular
and are covered by multicellular sterile
jackets. The male sex organ antheridium
encloses a mass of androcytes. These
androcytes give rise to biflagellate, motile
antherozoids. The female sex organ is
the
archegonium
a
multicellular
flask-shaped structure differentiated into
swollen venter and elongated neck.
The venter encloses the egg or oosphere. On
maturity, the egg fuses with the antherozoid
and forms a diploid zygote. The zygote by
repeated cell divisions gives rise to embryo,
which forms a sporophytic plant.
Operculum
Neck cells
Antheridial
wall
Egg
+
–
(a)
Spermatization
Neck
canal cells
Male
gametangium
Gametangial copulation
(b)
Host cell
Venter wall
Somatogamy
Ventral
canal cell
Stalk
Fig.1.24
Neck
Female
gametangium
Spermatids
Receptive hypha
Cover cell
Hyphae of opposite
mating types
Different types of sexual reproduction in fungi
(a)
Venter
Egg
Stalk
(b)
Fig. 1.25 (a) Mixture antheridium ready
for dehiscence (b) A nearly mature
Sexual Reproduction in Higher
Plants
In higher plants production of male and female gametes
are common during sexual reproduction. The male and
female gametes are common during sexual
reproduction.
The male gamete or sperms are produced in pollen
grains and transferred to female gamete or egg via
process of pollination. The fusion of gametes or
syngamy occurs and zygote develops.
Deviations in Reproductive Strategies in
Animals
Though asexual and sexual reproduction are the
common methods of multiplication in animals, many
deviations are also observed in the reproductive
strategies of animals.
These are as follows
(a) Hermaphroditism Animals having both male
Deviations in Reproductive Strategies in
Plants
Deviations in reproductive strategies in plants are
(a) Parthenogenesis It is defined as
(b)
(c)
(d)
(e)
(f)
the
formation of an embryo from an unfertilised ‘egg
or female’ gamete.
Parthenocarpy It is defined as the production
of fruits without fertilisation.
Polyembryony The occurrence of more than
one embryo in a seed is called polyembryony.
Amphimixis It is actually the process of sexual
reproduction which involves meiosis and syngamy.
By meiosis, the diploid cells of the sporophyte give
rise to haploid gametophytes which produces male
and female gametes. Syngamy, e.g., fusion of
haploid gametes results in the restoration of the
diploid sporophytic generation.
Apomixis Generally, plants reproduce by
syngamy but sometimes involvement of usual
sexual gametes do not take place but offsprings
are formed due to a kind of asexual reproduction.
It is known as apomixis.
Apogamy Formation of a sporophyte directly
from gametophyte without meiosis and syngamy
is called as apogomy.
Sexual Reproduction
Animals
in
Higher
In higher animals syngomy is main mode of
reproduction. Male and females are generally different
and produce the male and female gametes separately.
Fusion of gametes develops zygote which produces the
young one.
15
(b)
(c)
(d)
(e)
(f)
and female sex organs in the same individual are
known as hermaphrodites and the condition is
called hermaphroditism. These are also known
as monoecious.
Self-fertilisation occurs in the tapeworm (Taenia)
but between sperms produced by the testes of one
strobilus and eggs of another one. Cross
fertilisation occurs in earthworm (Pheretima)
whereas the sperms of one individual fertilise eggs
of another.
Neotany The larva of anuran embryos fail to go
through the normal metamorphosis and sexual
maturity develops in the larval stage, e.g., axolotl
larva of necturans.
Paegogenesis It
is
also
known
as
reproduction by the child. It is the reproduction by
immature or larval animals caused by
acceleration of maturation. It occurs in very small
fishes.
Androgenesis It is the development of an
embryo with paternal (sperm) chromosomes only,
accomplished by removing or destroying egg
nucleus before syngamy.
Gynogenesis In some species of roundworms
(Rhabditis) and flatworms (Dugesia), the sperm
penetrates the egg and stimulates development,
but soon degenerates without having taken any
part in the formation of embryo. This phenomenon
is called gynogenesis or pseudogamy or
partial fertilisation.
Parthenogenesis It is the development of an
embryo from an unfertilised egg, or if a
spermatozoan does penetrate the egg, no union of
male and female pronuclei occurs. It occurs in
rotifers, gastropods lizards, crustaceans, etc. Male
drones in honeybee are naturally produced by this
process.
16
Other Modes of Sexual
Reproduction
Table 1.1 Differences between Asexual and Sexual
Reproduction
Various other modes of sexual reproduction are
(i) Paedogamy Fusion of young individuals.
(ii) Merogamy Fusion
of
small
and
morphologically dissimilar gametes.
(iii) Macrogamy Fusion of two macrogametes
takes place.
(iv) Microgamy Fusion of two micro gametes
takes place.
(v) Cytogamy Fusion of cytoplasm of two
individuals but no nuclear fusion.
(vi) Plasmogamy Fusion of related cytoplasm.
(vii) Karyogamy Fusion of nuclei of two gametes.
(viii) Automixis Fusion of gamete nuclei of the
same cell.
Advantages of Sexual
Reproduction
Sexual reproduction involves the union of two
genetically different organisms, and so there is
formation of an organism with new genetic make up
sharing the characters of both the parents. So,
sexual reproduction results in addition of variations
to the population which ultimately contribute to
evolution.
Asexual reproduction involves the
participation of single individual
parent.
Sexual reproduction involves
participation of two separate parents.
It generally occurs without the
formation of sex organs.
It usually requires the formation of
sex organs.
It does not involve meiosis or
reduction division.
It involves meiosis which occurs at
the time of sporogenesis in flowering
plants.
Asexual reproduction does not
involves sexual fusion or fusion of
two gametes, i.e., the zygotes are not
formed.
The sexual reproduction requires
fertilisation to take place between
two opposite gametes leading to the
production of a zygote.
The offsprings do not show variations The offsprings exhibit variations and
and are genetically similar to the
are different from either of the two
parent.
parents.
Practice Test
2
1. Briefly describe various types of ascocarps.
2. What is the role of sex pilli in conjugation?
3. What are the common sex organs of bryophytes?
4. Describe any two types of deviations in sexual reproduction in
higher plants and animals.
5. Differentiate between androgenesis and gynogenesis.
Capsule
ö
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The another name for grafting is patching.
Gametes are produced in gonads, which are mesodermal in origin (from nephrotomes or mesomeres).
Leeuwenhoek (1677) discovered spermatozoa (sperm) in human semen by self-designed microscope.
Karl Ernst von Baer (1792-1876) identified a mammalian ova first time in 1827.
Schleiden and Schwann (1838-39) established the cellular nature of sperm and ova.
Regner de Graaf (1672) discovered the follicles in human ova and thought them to be eggs that is why ovarian
follicles are called as Graafian follicles. Blakeslee in 1922 discovered haploids in Datura. The term
‘parthenocarps’ is given by Noll (1902).
Balasubramanyam and Rangaswamy (1959) obtained parthenocarpic fruits (seedless) of ‘Allahabad round’
variety of guava simply by treating emasculated flowers with an aqueous extract of pollen grains.