- Reproduction is a process in which an organism gives rise to young ones (offspring) similar to itself.
-
The period from birth to the
natural death of an organism represents its life span.
-
No individual is immortal, except
unicellular organisms. There is no natural death in unicellular organisms.
Life spans of a few organisms
Organism
|
Lifespan
|
Organism
|
Lifespan
|
Elephant
|
50-70
yrs
|
Parrot
|
140
yrs
|
Rose
|
Crocodile
|
60
yrs
|
|
Dog
|
22
years
|
Horse
|
40-50
yrs
|
Butterfly
|
1-2
weeks
|
Fruit
fly
|
2
weeks
|
Crow
|
15
yrs
|
Tortoise
|
100-150
yrs
|
Banana
tree
|
Rice
plant
|
||
Cow
|
22
yrs
|
Banyan
tree
|
-
Reproduction enables the
continuity of the species, generation after generation.
-
Based on the number of participants,
reproduction is 2 types: Asexual reproduction & Sexual reproduction
ASEXUAL REPRODUCTION
-
It is the production of offspring
by a single parent.
-
The offspring are identical to
one another and to their parent. Such morphologically and genetically similar
individuals are known as clone.
-
Asexual reproduction is found in unicellular
organisms, and in simple plants and animals.
Types of asexual reproduction
o Fission: In this, the
parent cell divides (cell division) into two or more individuals. E.g.
Protists and Monerans. Fission is 2 types:
§ Binary
fission: It
is the division of parent cell into two individuals. E.g., Amoeba, Paramecium.
§ Multiple
fission: It
is the division of parent cell into many individuals. E.g. Plasmodium,
Amoeba.
o Budding: In this, a
small bud appears and grows in the parent body. After maturation, it is detached
from the parent body to form new individual. E.g. Hydra, Sponge, Yeast etc.
Other asexual reproductive structures: E.g. zoospores (microscopic motile structures in some algae and
protists), conidia (Penicillium) and gemmules (sponge).
o Vegetative
propagation: In
plants, vegetative propagules (the units of vegetative propagation such
as runner, rhizome, sucker, tuber, offset and bulb) are capable of
giving rise to new offspring.
Examples for vegetative
propagation:
-
Emergence of small plants from
the buds (‘eyes’) of the potato tuber, from the rhizomes of banana and ginger. They
arise from the nodes of modified stems. When the nodes come in contact
with damp soil or water, they produce roots and new plants.
-
Adventitious buds arise from the
notches present at margins of leaves of Bryophyllum.
Asexual
reproduction is the common method in simple organisms like algae and fungi. During
adverse conditions, they can shift to sexual method.
Higher
plants exhibit both asexual (vegetative) and sexual modes of reproduction. But
most of the animals show only sexual reproduction.
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.
-
It is an elaborate, complex and
slow process as compared to asexual reproduction.
-
It results in offspring that are
not identical to the parents or amongst themselves.
-
The period of growth to reach in maturity
for sexual reproduction is called the juvenile phase. It is known as vegetative
phase in plants.
-
In higher plants, the flowering
indicates the end of vegetative phase (beginning of the reproductive phase).
-
Annual & biennial type plants show
clear cut vegetative, reproductive and senescent phases, but in perennial
species it is very difficult to identify these phases.
-
Some plants exhibit unusual flowering
phenomenon.
E.g. Bamboo species flower only once in their lifetime
(after 50-100 years), produce large number of fruits and die.
Strobilanthus
kunthiana flowers
once in 12 years.
-
In animals, juvenile phase is
followed by morphological and physiological changes prior to active
reproductive behaviour.
-
Birds living in nature lay eggs
only seasonally. However, birds in captivity (e.g. poultry) can be made to lay
eggs throughout the year.
-
The females of placental mammals
exhibit cyclical changes in the activities of ovaries, accessory ducts and hormones
during the reproductive phase. It is called oestrus cycle in non-primates
(cows, sheep, rat, deer, dog, tiger etc.) and menstrual cycle in primates
(monkeys, apes and humans).
-
Seasonal breeders: The mammals (living
in natural conditions) exhibiting reproductive cycles only during favourable
seasons.
-
Continuous breeders: The mammals
those are reprodu-ctively active throughout their reproductive phase.
Senescence (old age):
-
It is the last phase of life span
and end of reproductive phase.
-
During this, concomitant changes
in the body (slowing of metabolism etc.) occur. It ultimately leads to death.
In plants & animals, hormones are responsible
for transition between juvenile, reproductive & senescence phases. Interaction
between hormones and environmental factors regulate the reproductive processes
and the associated behavioural expressions of organisms.
Events in sexual reproduction
3
stages:
Pre-fertilisation, fertilisation & post-fertilisation events.
1. Pre-fertilisation Events
These
are all the events prior to the fusion of gametes.
They
include gametogenesis and gamete transfer.
a. Gametogenesis
-
It is the process of formation of
male and female gametes (haploid cells).
-
In some algae, all gametes are
similar and cannot categorize into male and female gametes. They are called homogametes
(isogametes).
-
In others, the male and female gametes
are distinct types (heterogametes). Male gamete is called the antherozoid
(sperm) and female gamete is called the egg (ovum).
Sexuality in organisms:
-
Plants may be bisexual
(i.e. monoecious- male & female reproductive structures in the same
plant) or unisexual (i.e. dioecious- male and female reproductive
structures on different plants).
-
In dioecious (unisexual) flowering
plants, the male flower is staminate (bearing stamens) while the female
is pistillate (bearing pistils). E.g. papaya and date palm.
-
In monoecious
flowering plants, male & female flowers are present on same individual.
E.g. Cucurbits & coconuts.
-
Fungi may be homothallic (bisexual)
or heterothallic (unisexual).
-
Bisexual
animals (hermaphrodites): Earthworms, leech, sponge,
tapeworm, etc.
-
Unisexual animals:
Cockroach, higher animals etc.
Cell division during gamete formation:
-
Haploid parental body (many
monera, fungi, algae and bryophytes) produces haploid gametes by mitosis.
-
Diploid parental body (pteridophytes,
gymnosperms, angiosperms & animals) produces haploid gametes by meiosis of
meiocytes (gamete mother cell).
Name
of organism
|
Chromosome
number
|
|
In
meiocytes (2n)
|
In
gametes (n)
|
|
Human being
|
46
|
23
|
Housefly
|
12
|
24
|
Rat
|
42
|
21
|
Dog
|
78
|
39
|
Cat
|
38
|
19
|
Fruit fly
|
8
|
4
|
Ophioglossum
|
1260
|
630
|
Apple
|
34
|
17
|
Rice
|
24
|
12
|
Maize
|
20
|
10
|
Potato
|
48
|
24
|
Butterfly
|
380
|
190
|
Onion
|
32
|
16
|
b. Gamete Transfer
-
Male gametes need a medium to
move towards female gametes for fertilisation.
-
In a majority of organisms, male gamete
is motile and the female gamete is stationary. In a few fungi and algae both
types of gametes are motile.
-
In simple plants (algae,
bryophytes & pteridophytes), gamete transfer takes place through water medium.
To compensate the loss of male gametes during transport, large number of male
gametes is produced.
-
In seed plants, pollen grains (in
anthers) carry male gametes and ovule has the egg. Pollen grains are transferred
to the stigma.
-
In bisexual, self-fertilizing plants,
(e.g. peas) transfer of pollen grains to the stigma is easy as anthers and
stigma are located close to each other.
-
In cross pollinating plants
(including dioecious plants), pollination helps in 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.
-
In dioecious animals, the
fertilisation helps for successful transfer and coming together of gametes.
2. Fertilisation (syngamy)
It
is the fusion of gametes to form a diploid zygote.
Organisms
like rotifers, honeybees, some lizards and birds (turkey), the female gamete develops
to new organisms without fertilisation. This is called parthenogenesis.
Types of fertilization:
a.
External
fertilisation:
Syngamy occurs in the external medium (water), i.e. outside the body of the
organism. E.g. most aquatic organisms (many algae, bony fishes etc) and
amphibians.
Such
organisms show synchrony between the sexes and release a large number of
gametes into the surrounding medium in order to enhance the chances of syngamy.
Disadvantage: The offspring
are extremely vulnerable to predators threatening their survival up to
adulthood.
b.
Internal
fertilisation:
Syngamy occurs inside the body. E.g. terrestrial organisms, belonging to fungi,
animals (reptiles, birds, mammals) & plants (bryophytes, pteridophytes,
gymnosperms & angiosperms).
In
this, non-motile egg is formed inside the female body to where motile male
gamete reaches and fuses.
In
seed plants, the non-motile male gametes are carried to female gamete by pollen
tubes.
There
is large number of sperms produced but the number of eggs is very low.
3. Post-fertilisation Events
These
are the events after the formation of zygote.
The Zygote
-
In organisms with external
fertilisation, zygote is formed in the external medium. In organisms with internal
fertilisation, zygote is formed inside body.
-
Further development of the zygote
depends on the type of life cycle of the organism and the nature of
environment.
-
In fungi and algae, zygote
develops a thick wall that is resistant to desiccation and damage. It undergoes
a period of rest before germination.
-
In organisms with haplontic life
cycle, zygote divides by meiosis into haploid spores that grow into haploid
individuals.
-
Every sexually reproducing
organism begins life as a zygote. It is the vital link between organisms of one
generation and the next.
Embryogenesis
-
It is the development of embryo
from the zygote.
-
During embryogenesis, zygote
undergoes cell division (mitosis) and cell differentiation.
-
Cell divisions increase the
number of cells in the embryo. Cell differentiation causes the modifications of
groups of cells into various tissues and organs to form an organism.
Based
on place of zygote development animals are 2 types:
a.
Oviparous: Here, animals lay fertilized/unfertilized eggs.
E.g.
In reptiles & birds, the fertilized eggs covered by hard calcareous
shell are laid in a safe place. After incubation young ones hatch out.
b.
Viviparous: Here, the
zygote develops into a young one inside the female body. Later, the young ones
are delivered out of the body. E.g. most of mammals.
Because of proper care and protection, the chances of
survival of young ones are greater in viviparous animals.
Embryogenesis
in flowering plants (see next chapter)
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