UNIT IV REPRODUCTION
KEY CONCEPTS
Biology in
essence is the story of life on earth.
Reproduction is a
vital process without which species cannot survive for long.
Each individual
leaves its progeny by asexual or sexual means.
Sexual mode of reproduction
enables creation of new variants, so that survival advantage is enhanced.
CHAPTER 1 REPRODUCTION IN ORGANISMS
LIFE SPAN
Each and every
organism can live only for a certain period of time. The period from birth to
the natural death of an organism represents its life span.
(1)
Life spans of various animals may be as short as a few days or as
long as a few thousand years.
(2)
Life spans of organisms are not necessarily correlated with their
sizes. A mango tree has a much shorter life span as compared to a peepal tree.
(3)
No individual is immortal, except single-celled organisms.
REPRODUCTION IN ORGANISMS
Reproduction is defined as a biological process
in which an organism gives rise to young ones (offspring) similar to itself.
(1)
The offspring grow, mature and in turn produce new offspring.
(2)
Thus, there is a cycle of birth, growth and death.
(3)
Reproduction enables the continuity of the species, generation
after generation.
(4)
There is a large diversity in the biological world and each
organism has evolved its own mechanism to multiply and produce offspring.
(5)
The organism’s habitat, its internal physiology and several other
factors are collectively responsible for how it reproduces.
TYPES OF REPRODUCTION
Based on whether
there is participation of one organism or two in the process of reproduction,
it is of two types.
(1) When offspring is
produced by a single parent with or without the involvement of gamete
formation, the reproduction is asexual.
(2)
When two parents (opposite sex) participate in the
reproductive process and also involve fusion of male and female gametes,
it is called sexual reproduction.
ASEXUAL REPRODUCTION
In this method,
a single individual (parent) is capable of producing offspring. As a result,
the offspring that are produced are not only identical to one another but are
also exact copies of their parent.
(1)
The term clone is used to describe such morphologically and genetically similar
individuals.
(2)
Asexual reproduction is common among single-celled organisms, and
in plants and animals with relatively simple organisations.
(3) DIFFERENT METHODS OF
ASEXUAL REPRODUCTION
(A)
CELL DIVISION
In Protists and
Monerans, the organism or the parent cell divides into two to give rise to new
individuals Thus, in these organisms cell division is itself a mode of
reproduction.
(B) BINARY FISSION
Many single-celled
organisms reproduce by binary fission, where a cell divides into two
halves and each rapidly grows into an adult (e.g., Amoeba, Paramecium).
(C) BUDDING
In yeast, the
division is unequal and small buds are produced that remain attached
initially to the parent cell which, eventually gets separated and mature into
new yeast organisms (cells).
(D) SPORE FORMATION
Members of the
Kingdom Fungi and simple plants such as algae reproduce through special asexual
reproductive structures. The most common of these structures are zoospores that usually are microscopic motile structures. Other common asexual
reproductive structures are conidia (Penicillium), buds (Hydra) and gemmules (sponge).
(4)
While in animals and other simple organisms the term asexual is used unambiguously,
in plants, the term vegetative reproduction is frequently used.
(5)
In plants, the units of vegetative
propagation such as runner, rhizome, sucker, tuber, offset,
bulb are all capable of giving rise to
new offspring These structures are called vegetative
propagules. Obviously,
since the formation of these structures does not involve two parents, the
process involved is asexual.
(6)
The sites of origin of the new plantlets in the plants invariably
arise from the nodes present in the modified stems of potato, sugarcane,
banana, ginger, dahlia etc. When the nodes come in contact with damp soil or water, they produce roots
and new plants. Similarly, adventitious buds arise from the notches present at
margins of leaves of Bryophyllum.
(7)
It is interesting to note that asexual reproduction is the common
method of reproduction in organisms that have a relatively simple organisation,
like algae and fungi and that they shift to sexual method of reproduction just
before the onset of adverse conditions.
(8) Asexual (vegetative) as
well as sexual modes of reproduction are exhibited by the higher plants.
On the other hand, only sexual mode of
reproduction is present in most of the animals.
SEXUAL REPRODUCTION
Sexual
reproduction involves formation of the male and female gametes, either by the
same individual or by different individuals of the opposite sex.
(1)
These gametes fuse to form the zygote which develops to form the new
organism.
(2)
It is an elaborate, complex and slow process as compared to asexual
reproduction.
(3)
Because of the fusion of male and female gametes, sexual
reproduction results in offspring that are not identical to the parents or
amongst themselves.
(4)
All organisms have to reach a certain stage of growth and maturity
in their life, before they can reproduce sexually. That period of growth is called
the juvenile phase. It is known as vegetative phase in plants. This phase is of variable durations in different organisms.
(5)
The end of juvenile/vegetative phase which marks the
beginning of the reproductive phase can be seen easily in
the higher plants when they come to flower.
(6)
Plants–the annual and biennial types, show clear cut vegetative, reproductive
and senescent phases, but in the perennial species it is very difficult to
clearly define these phases.
(7)
A few plants exhibit unusual flowering phenomenon; some of them
such as bamboo species flower only once in their life time, generally after
50-100 years, produce large number of fruits and die.
Another plant, Strobilanthus kunthiana (neelakuranji),
flowers once in 12 years.
(8)
In animals, the juvenile phase is followed by morphological and
physiological changes prior to active reproductive behaviour.
(9)
The reproductive phase is also of variable duration in different
organisms. Birds living in nature lay eggs only seasonally. The females of
placental mammals exhibit cyclical changes in the activities of ovaries and accessory
ducts as well as hormones during the reproductive phase.
(10) In non-primate mammals
like cows, sheep, rats, deers, dogs, tiger, etc., such cyclical changes during
reproduction are called oestrus cycle where as in primates ( monkeys, apes, and humans ) it is called menstrual cycle.
Many mammals,
especially those living in natural, wild conditions exhibit such cycles only
during favourable seasons in their reproductive phase and are therefore called seasonal breeders.
Many other
mammals are reproductively active throughout their reproductive phase and hence
are called continuous breeders.
(11) The end of reproductive phase can be
considered as one of the parameters of senescence or old age.
(12) There are concomitant
changes in the body (like slowing of metabolism, etc.) during this last phase
of life span.
(13) Old age ultimately leads
to death.
(14) In both plants and
animals, hormones are responsible for the transitions between the three phases.
(15) Interaction between hormones
and certain environmental factors regulate the reproductive processes and the associated
behavioural expressions of organisms.
Events in sexual reproduction
(1) The events of sexual reproduction though elaborate and complex,
follow a regular sequence.
(2) Sexual reproduction is characterised by the fusion (or
fertilisation) of the male and female gametes, the formation of zygote and
embryogenesis.
(3) For convenience these sequential events may be grouped into three
distinct stages namely, the pre fertilisation, fertilisation and the post-fertilisation
events.
Pre-fertilisation Events
These include
all the events of sexual reproduction prior to the fusion of gametes. The two
main pre-fertilisation events are gametogenesis and gamete transfer.
Gametogenesis
(1) Gametogenesis refers to the process of formation of the two types of gametes –
male and female.
(2) Gametes are haploid cells.
(3)
In some algae the two gametes are so similar in appearance that it is
not possible to categorise them into male and female gametes. They are hence
called homogametes (isogametes)
(4)
However, in a majority of sexually reproducing organisms the gametes produced
are of two morphologically distinct types (heterogametes).
(5) In such
organisms the male gamete is called the antherozoid or sperm and the female gamete is called
the egg or ovum
Sexuality in organisms
Sexual
reproduction in organisms generally involves the fusion of gametes from two
different individuals. But this is not always true.
Sexuality in plants
Plants may have
both male and female reproductive structures in the same plant (bisexual) or on
different plants (unisexual). In several fungi and plants, terms such as homothallic and monoecious are used to denote the bisexual condition and heterothallic and dioecious are the terms used to describe unisexual condition. In flowering
plants, the unisexual male flower is staminate, i.e., bearing stamens, while the female is pistillate or bearing pistils. In some flowering plants, both male and female
flowers may be present on the same individual (monoecious) or on separate
individuals (dioecious). Some examples of monoecious plants are cucurbits and
coconuts and of dioecious plants are papaya and date palm.
Sexuality in
animals
Earthworms, sponge,
tapeworm and leech, are typical examples
of bisexual animals that possess both male and female reproductive organs, are hermaphrodites. Cockroach is an
example of a unisexual species.
Cell division during gamete formation
(1)
Gametes in all heterogametic species are of two types namely, male and female. Gametes are haploid though
the parent plant body from which they arise may be either haploid or diploid. A
haploid parent produces gametes by mitotic division.
(2)
Several organisms belonging to monera, fungi, algae and bryophytes have
haploid plant body, but in organisms belonging to pteridophytes, gymnosperms,
angiosperms and most of the animals including human beings, the parental body
is diploid. It is obvious that meiosis, the reduction division, has to occur if
a diploid body has to produce haploid gametes.
(3)
In diploid organisms, specialised cells called meiocytes (gamete mother cell)
undergo meiosis. At the end of meiosis, only one set of chromosomes gets
incorporated into each gamete.
Gamete
Transfer
After their
formation, male and female gametes must be physically brought together to
facilitate fusion (fertilisation).
(1)
In a majority of organisms, male gamete is motile and the female
gamete is stationary. Exceptions are a few fungi and algae in which both types
of gametes are motile.
(2)
There is a need for a medium through which the male gametes move.
(3)
In several simple plants like algae, bryophytes and pteridophytes,
water is the medium through which this gamete transfer takes place. A large
number of the male gametes, however, fail to reach the female gametes. To
compensate this loss of male gametes during transport, the number of male
gametes produced is several thousand times the number of female gametes
produced.
(4)
In seed plants, pollen grains are the carriers of male gametes and ovule
have the egg. Pollen grains produced in anthers therefore, have to be
transferred to the stigma before it can lead to fertilisation.
(5)
In bisexual, self-fertilising plants, e.g., peas, transfer of
pollen grains to the stigma is relatively easy as anthers and stigma are
located close to each other; pollen grains soon after they are shed, come in contact
with the stigma.
(6)
But in cross pollinating plants (including dioecious plants), a specialised
event called pollination facilitates transfer of
pollen grains to the stigma. Pollen grains germinate on the stigma and the
pollen tubes carrying the male gametes reach the ovule and discharge male
gametes near the egg.
(7)
In dioecious animals, since male and female gametes are formed in
different individuals, the organism must evolve a special mechanism for gamete transfer.
Successful transfer and coming together of gametes is essential for the most
critical event in sexual reproduction, the fertilisation.
Fertilisation
The most vital
event of sexual reproduction is perhaps the fusion of gametes. This process
called syngamy results in the
formation of a diploid zygote. The term fertilisation is also often used for this process.
parthenogenesis
In some organisms
like rotifers, honeybees and even some lizards and birds (turkey), the female
gamete undergoes development to form new organisms without fertilisation. This
phenomenon is called parthenogenesis.
TYPES OF FERTILISATION
(a)
External fertilisation
In most aquatic organisms,
such as a majority of algae and fishes as well as amphibians, syngamy occurs in
the external medium (water), i.e., outside the body of the organism. This type
of gametic fusion is called external fertilisation.
(1)
Organisms exhibiting external fertilisation show great synchrony
between the sexes and release a large number of gametes into the surrounding medium
(water) in order to enhance the chances of syngamy.
(2)
This happens in the bony fishes and frogs where a large number of
offspring are produced.
(3)
A major disadvantage is that the offspring are extremely vulnerable
to predators threatening their survival up to adulthood.
(b)
Internal fertilisation
In many
terrestrial organisms, belonging to fungi, higher animals such as reptiles,
birds, mammals and in a majority of plants (bryophytes, pteridophytes,
gymnosperms and angiosperms), syngamy occurs inside the body of the organism,
hence the process is called internal fertilisation.
(1)
In all these organisms, egg is formed inside the female body where
they fuse with the male gamete.
In organisms
exhibiting internal fertilisation, the male gamete is motile and has to reach
the egg in order to fuse with it.
(2)
In these even though the number of sperms produced is very large,
there is a significant reduction in the number of eggs produced.
(3)
In seed plants, however, the non-motile male gametes are carried to
female gamete by pollen tubes.
Post-fertilisation Events
Events in sexual
reproduction after the formation of zygote are called post-fertilisation events.
The Zygote
(1)
Formation of the diploid zygote is universal in all sexually
reproducing organisms.
(2)
In organisms with external fertilisation, zygote is formed in the
external medium (usually water),
whereas in those
exhibiting internal fertilisation, zygote is formed inside the body of the
organism.
(3)
Further development of the zygote depends on the type of life cycle
the organism has and the environment it is exposed to.
(4)
In organisms belonging to fungi and algae, zygote develops a thick
wall that is resistant to dessication and damage. It undergoes a period of rest
before germination.
(5)
In organisms with haplontic life cycle zygote divides by meiosis to
form haploid spores that grow into haploid individuals.
(6)
Zygote is the vital link that ensures continuity of species between
organisms of one generation and the next.
(7)
Every sexually reproducing
organism, including human beings begin life as a single cell–the zygote.
Embryogenesis
Embryogenesis refers to the process
of development of embryo from the zygote. During embryogenesis, zygote undergoes cell division (mitosis) and cell differentiation.
(1)
While cell divisions increase the number of cells in the developing
embryo; cell differentiation helps groups of cells to undergo certain modifications
to form specialised tissues and organs to form an organism.
(2)
Animals are categorised into oviparous and viviparous based on whether the development of the zygote takes place outside
the body of the female parent or inside, i.e., whether they lay
fertilised/unfertilised eggs or give birth to young ones.
(3)
In oviparous animals like reptiles and birds, the fertilised eggs
covered by hard calcareous shell are laid in a safe place in the environment; after a period of
incubation young ones hatch out.
(4)
In viviparous animals (majority of mammals including human beings),
the zygote develops into a young one inside the body of the female organism.
After attaining a certain stage of growth, the young ones are delivered out of
the body of the female organism.
(5)
Because of proper embryonic care and protection, the chances of
survival of young ones is greater in viviparous organisms.
(6)
In flowering plants, the zygote is formed inside the ovule. After fertilisation
the sepals, petals and stamens of the flower wither and fall off.
(7)
The pistil however, remains attached to the plant. The zygote
develops into the embryo and the ovules develop into the seed. The ovary develops into the fruit which develops a thick wall called pericarp that is protective in function. After dispersal, seeds germinate
under favourable conditions to produce new plants.
SUMMARY
Reproduction
enables a species to live generation after generation.
Reproduction in
organisms can be broadly classified into asexual and sexual reproduction.
Asexual reproduction
Asexual
reproduction does not involve the formation or fusion of gametes.
It is common in
organisms that have a relatively simple organisation such as the fungi, algae
and some invertebrate animals.
The offspring
formed by asexual reproduction are identical and are referred to as clones.
Zoospores,
conidia, etc., are the most common asexual structures formed in several algae
and fungi. Budding and gemmule formation are the common asexual methods seen in
animals.
Prokaryotes and
unicellular organisms reproduce asexually by cell division or binary fission of
the parent cell.
In several
aquatic and terrestrial species of angiosperms, structures such as runners, rhizomes,
suckers, tubers, offsets, etc., are capable of giving rise to new offspring.
This method of
asexual reproduction is generally referred to as vegetative propagation.
Sexual reproduction
Sexual reproduction
involves the formation and fusion of gametes.
It is a complex
and slower process as compared to asexual reproduction.
Most of the
higher animals reproduce almost entirely by sexual method.
Events of sexual
reproduction may be categorised into pre-fertilisation, fertilisation and
post-fertilisation events.
Pre-fertilisation
events include gametogenesis and gamete transfer while post-fertilisation
events include the formation of zygote and embryogenesis.
Organisms may be
bisexual or unisexual.
Sexuality in
plants is varied, particularly in angiosperms, due to the production of diverse
types of flowers.
Plants are
defined as monoecious and dioecious.
Flowers may be
bisexual or unisexual flowers.
Gametes are haploid
in nature and usually a direct product of meiotic division except in haploid
organisms where gametes are formed by mitosis.
Transfer of male
gametes is an essential event in sexual reproduction. It is relatively easy in
bisexual organisms. In unisexual animals it occurs by copulation or
simultaneous release.
In angiosperms,
a special process called pollination ensures transfer of pollen grains which carry
the pollen grains to the stigma.
Syngamy
(fertilisation) occurs between the male and female gametes.
Syngamy may
occur either externally, outside the body of organisms or internally, inside
the body. Syngamy leads to formation of a specialized cell called zygote.
The process of
development of embryo from the zygote is called embryogenesis.
In animals, the
zygote starts developing soon after its formation.
Animals may be
either oviparous or viviparous.
Embryonal protection
and care are better in viviparous organisms.
In flowering
plants, after fertilisation, ovary develops into fruit and ovules mature into
seeds. Inside the mature seed is the progenitor of the next generation, the
embryo.
EXERCISES
1. Why is
reproduction essential for organisms?
2. Which is a
better mode of reproduction: sexual or asexual? Why?
3. Why is the
offspring formed by asexual reproduction referred to as clone?
4. Offspring
formed due to sexual reproduction have better chances of
survival. Why?
Is this statement always true?
5. How does the
progeny formed from asexual reproduction differ from
those formed by
sexual reproduction?
6. Distinguish
between asexual and sexual reproduction. Why is vegetative
reproduction
also considered as a type of asexual reproduction?
7. What is
vegetative propagation? Give two suitable examples.
8. Define
(a) Juvenile
phase,
(b) Reproductive
phase,
(c) Senescent
phase.
9. Higher
organisms have resorted to sexual reproduction in spite of its
complexity. Why?
10. Explain why
meiosis and gametogenesis are always interlinked?
11. Identify
each part in a flowering plant and write whether it is haploid
(n) or diploid
(2n).
(a) Ovary
———————————
(b) Anther
———————————
(c) Egg
———————————
(d) Pollen
———————————
(e) Male gamete
———————————
(f ) Zygote
———————————
12. Define
external fertilisation. Mention its disadvantages.
13.
Differentiate between a zoospore and a zygote.
14. Differentiate
between gametogenesis from embryogenesis.
15. Describe the
post-fertilisation changes in a flower.
16. What is a
bisexual flower? Collect five bisexual flowers from your
neighbourhood
and with the help of your teacher find out their common
and scientific
names.
17. Examine a
few flowers of any cucurbit plant and try to identify the
staminate and
pistillate flowers. Do you know any other plant that
bears unisexual
flowers?
18. Why are
offspring of oviparous animals at a greater risk as compared
to offspring of
viviparous animals?
Happy Learning
Yours
Praveen Kumar
Praveen Kumar
