Reproduction: Biological process in which an
organism give rise to young one (offspring) similar to itself.
The offspring, grow, mature and in turn produce new
offspring.
Importance of reproduction: Ensures –
·
Survival
·
Continuity of race
·
Group immortality
Since by reproduction, the old ones are replaced by the
young ones, which can feed, grow and reproduce again.
·
Increase in population
·
Vehicle of organic evolution since it transmits
advantageous variations to offspring.
Lifespan: Period from birth to natural death of
an organism
Every organism as a specific average life span, ranging from
a day to 4000 years (Table 1). It can be
divided in to 4 stages:
Ø
Juvenility
Ø
Maturity
Ø
Aging and senescence
Ø
Death
S No.
|
Animal
|
Life Span
|
1
|
Butterfly
|
1-2 weeks
|
2
|
Housefly
|
1-4 months
|
3
|
Jelly fish
|
1 year
|
4
|
Rat
|
2-3 years
|
5
|
Squirrel
|
6-8 years
|
6
|
Dog
|
16-18 years
|
7
|
Leech
|
20 years
|
8
|
Horse
|
30-40 years
|
9
|
Ostrich
|
50 years
|
10
|
Man
|
60-80 years
|
11
|
Crocodile
|
60 years
|
12
|
Elephant
|
70-90 years
|
13
|
Tortoise
|
100-150 years
|
14
|
Parrot
|
150 years
|
With respect to reproduction, organisms show wide diversity.
Each organism has evolved its own method of reproduction dependent on its:
Ø
Habitat
Ø
Physiology
Ø
And other factors
Types of Reproduction
Broadly of 2 types:
I.
Asexual: offspring produced by single parent,
with or without gamete formation
II.
Sexual: two parents of opposite sex participate
in reproduction. Involves fusion of male and female gamete.
I.
ASEXUAL
REPRODUCTION
A single individual is capable of producing offspring. The
offspring being produced from single parent are identical and exact copies of
their parents. Such genetically identical and morphologically similar
individual are known as Clones.
[Apomixis: The formation
of new individual through asexual reproduction without formation and fusion of
gametes]
Occurrence of asexual reproduction:
§
Single celled organisms such as Protists and
Monerans, parent cell divides in to two, to give rise to new individuals. Thus
here, cell division is a method of reproduction
§
Plants and animals with relatively simple organization
such as Algae, fungi etc by zoospores, buds (in Hydra), conidia, gemmules (in Sponges) etc.
Development of an organism from non-sexual reproductive units
like buds, a fragment etc is called blastogenesis, while the asexual
reproductive body is called blastos.
Characteristics of asexual reproduction:
1)
Uniparental reproduction
2)
No fusion of gametes
3)
Somatogenic reproduction
4)
Only mitotic division, no meiosis
5)
No variations produced, no role in evolution
6)
Rapid method
7)
Found in lower organisms. Absent from higher
invertebrates and all vertebrates
Types of
Asexual Reproduction
1.
Fission:
Type of asexual reproduction in which a full grown
parent organism divides in to two or more than two daughter cells. Here the
reproductive unit is whole parental body.
Types of Fission:
On the basis of number of daughter cells produced, fission is of two types:
·
Binary Fission: 2
equal daughter cells produced. Found in Protists (Amoeba, Euglena), bacteria and green alga; and flat worms.
·
Multiple Fission:
more than 2 daughter cells produced. Found in Amoeba, Plasmodium, and some alga.
2.
Budding:
type of asexual reproduction in which one or more unicellular or multicellular
outgrowths, called buds are formed on the parent body. Each of these growths
enlarges and develops in to a new organism. Occurs in Sponges, Coelenterates (Hydra), and among fungi in Yeast.
In Yeast, an unequal cell division
occurs to produce small buds that remain attached initially to parent cell.
These separate later and mature in to new yeast organism.
3.
Fragmentation:
Parental body breaks in to two or more fragments accidentally and each body
fragment develops in to an organism. Found in some flat worms, sea anemones and
echinoderms.
4.
Zoospores:
Special asexual reproductive structures, that are microscopic, flagellated and
motile. Members of kingdom Fungi and simple plants such as Algae.
5.
Conidia:
Non motile spores produced exogenously by constrictions at the tips of special
hyphal branches known as condiophores. E.g. Penicillium
6.
Gemmules:
Produced on Sponges
Vegetative
reproduction: Asexual reproduction in plants is termed as
vegetative reproduction. The vegetative structures in plants such as runner, rhizome, sucker, tuber, offset,
bulb etc that are capable of giving rise to new offspring are known as vegetative propagules.
Plants may be propagated by vegetative propagules
naturally as well as artificially.
Natural
Methods:
A. Vegetative
Propagation by Roots: Roots of some plants develop adventitious buds o them
e.g. Dalbergia sisoo (Sheesham),
guava etc. These buds sprout under suitable conditions and form individual
plants.
B. Vegetative
Propagation by Stems:
a. Rhizome:
Rhizomes are underground swollen stems that form vegetative propagules. Old and
decaying branches cause isolation of newly formed branch that develop in to a
new plant. E.g. ginger, Saccharum,
turmeric etc.
b. Stem
Tuber: Found in potato and artichoke. Tubers are swollen apical parts of an
underground stem and bear a number of nodes or ‘eyes’. New plants are produced
from the buds growing on the eyes.
c. Runner:
aerial weak stems when come in contact with soil, give off adventitious roots
at the nodes. When older part of plant dies, the branches separate from the
parent plant and form independent plants.
C. Vegetative
Propagation by Leaves: Leaves of some plants bear adventitious buds e.g. Bryophyllum, Begonia etc. These buds are
borne in the notches of the margins of succulent leaves. When these nodes come
in contact with damp soil or water, they form roots and new plants.
This ability is exploited by
farmers and gardeners to produce new plants.
Artificial
Methods of Vegetative Reproduction
a.
Cutting
b.
Grafting
c.
Budding
d.
Layering
e.
Bulbil
Artificial methods of vegetative reproduction are used
by farmers and gardeners for:
·
Quick production of new plants
·
Combining good qualities of two
different varieties
Water
Hyacinth (Eicchornia): Terror of
Bengal
Aquatic plant which is the most invasive weeds found
growing wherever there is standing water. It drains oxygen from water which
leads to death of fishes.
The plant was introduced in India because of its beautiful
flowers and shapes of leaves. Since it can propagate vegetatively at a very
high rate and spread all over the water body in a short period of time, it is
very difficult to get rid of them.
Significance
of vegetative reproduction:
·
Characters of parent plants
preserved
·
Quicker
·
Easier and cheaper
·
It is the only method of
perpetuation and reproduction in plants that does not produces viable seeds.
·
Easier to get rid of pathogen from
any part of plant by vegetative reproduction
·
Method of multiplying plants which
have poor seed viability
·
Some methods like grafting allow
physical as well as physiological union of two different individuals to the
best of economic advantage.
In organisms with simple organisation such as algae,
fungi etc; asexual methods of reproduction are the preferred methods. Sexual
methods are restricted to adverse conditions. Higher plants undergo both
asexual and sexual reproduction while animals show only sexual reproduction.
Sexual
Reproduction
Involves the formation of male and female gametes;
either by the same individual or by different individuals of the opposite sex.
The gametes so formed fuse to form the zygote which develops in to a new
organism.
Because of fusion of male and female gametes, sexual
reproduction results in offspring that are not identical to the parent and
among themselves.
Despite the wide diversity of living organisms, it has
been observed that all living organisms share a similar pattern in terms of
sexual reproduction. The major characters of sexual reproduction are:
·
Involves 2 parents; biparental
·
Involves formation and fusion of
gametes
·
Involves meiosis during
gametogenesis and mitosis during development of zygote
·
Also known as germinal reproduction
as organisms are produced from germ cells
·
Offspring genetically different
from parent as variations appear due to new combinations of genes during
crossing over. Plays important role in evolution
·
Slow
·
Units of reproduction of haploid
gamete
·
Found in higher plant and organism
·
Also found in some protists e.g. Plasmodium, paramecium etc.
DIFFERENT
PHASES IN SEXUAL REPRODUCTION
Sexual
reproduction is an elaborate and slow process compared to asexual reproduction.
Since the organism must reach a certain stage of growth and maturity in their
life cycle, before they can produce sexually. That period of growth is known as
juvenile phase and is also known as vegetative phase in plants. The end of
juvenile or vegetative phase is known as reproductive phase. It can be
identified in plants by the appearance of flowers.
1. JUVENILE PHASE
It is a period of growth and maturity of an organism before it can
reproduce sexually. It is of varying duration in different plants. For e.g. it
is very small for annual plants, longer for biennial and longest for perennial
plants. Perennial plants have many years to complete their life cycle; while annuals
have a limited period of 1 year to do so.
Also, the
phases are clearly defined in annuals and biennials; but in perennial plants it
is difficult to define these phases. In plants where flowering occurs more than
once, i.e. seasonal flowering plants; the interflowering period is known as
mature phase.
2. REPRODUCTIVE PHASE
Period in which organism is capable of reproducing sexually
- Easily seen in higher plants
when they come to flower.
- Some plants flower seasonally
& some throughout the year
- Few plants exhibit unusual
flowering phenomenon
For e.g.;
·
Bamboo species
flower only once in their lifetime, produce large number of fruits and die.
·
Srobilanthus kunthiana (neelakurinji) found in hilly areas of Kerala, Karnataka & Tamil
Nadu flower once in 12 years. The latest being in September 2006.
- In animals juvenile phase is
followed by morphological and physiological changes prior to active
reproductive behavior.
- Reproductive phase is also of
variable duration
- Sexual reproduction is usually
seasonal for e.g. birds in nature lay eggs only seasonally. However, birds
living in captivity (as in poultry farms) can be made to lay eggs
throughout the year. This is not related to reproduction but for
commercial purposes.
- In placental mammals there is
occurrence of cyclical changes in the activities of ovaries &
accessory ducts as well as hormones.
- In non primate mammals like
dogs, rats etc these cyclical changes during reproduction is called oestrus
cycle – no bleeding occurs.
- In primate mammals like apes
& humans, these cycles are called menstrual cycles.
- On the basis of reproductive
activity these mammals are classified in to 2 groups:
ü Seasonal Breeders: mammals that
breed only during favorable seasons during their reproductive cycle
ü Continuous breeders: mammals that
are reproductively active throughout their reproductive cycles
3. SENESCENT
PHASE
- Marked by the end of
reproductive phase
- Many other changes also occur
in the body like slowing of metabolism.
- This leads to last phase of
life span, death
In both
plants and animals, transition from one phase to next occurs due to hormonal
changes. Interactions between hormones and certain environmental factors
regulate the reproductive process and the associated behaioural expressions of
the organisms.
EVENTS IN SEXUAL REPRODUCTION:
After
reaching maturity, all sexually reproducing organisms show similar events and
processes that are involved in sexual reproduction. Though the structures
involved in these processes are different. The events even though complex and
elaborate, follow the same sequence.
1. PRE-FERTILIZATION
EVENTS: Events occurring before gamete fusion. Involve 2 events: gametogenesis & gamete transfer
a) GAMETOGENESIS
- Process of formation of
gametes: male & female
- The gametes are usually haploid
- Gametes called homogametes /
isogametes when both have same appearance e.g. algae
- When gametes are morphologically
different, they are called heterogametes; male gamete known as antherozoid
/sperm, female is called ovum / egg.
Sexuality in organisms
The male and female gametes formed in sexually
reproducing organisms may be produced in same or 2 different individuals.
·
Plants may
have both male and female reproductive structures in same plants (Bisexual), or on different plants (unisexual).
·
Bisexual organisms are called as homothallic / monoecious whereas unisexual organisms called heterothallic / dioecious.
·
In flowering
plants the unisexual male flower is known as staminate, i.e. bearing stamens.
·
The female
flower bearing pistils is known as pistillate.
·
If both male
and female flowers on same individual: monoecious
·
If male and
female flowers on different individuals: dioecious
o
Among
animals, individuals having both male and female reproductive organs (bisexual) are earthworms, sponges,
tapeworm leeches. These are known as hermaphrodites.
o
Mostly
animals are unisexual
Cell
Division during gamete formation: Gametes are usually haploid, produced on
parent body that may be haploid or diploid.
ü On a haploid parent body as in
monera, alga, fungi and bryophytes; the gametes are produced by mitosis. Such organisms undergo gametic
fusion leading to formation of diploid zygote. The zygote undergoes meiosis or
reduction division and then form haploid plant body.
ü Organisms having diploid parent body
such as Pteridophytes, gymnosperms, angiosperms and most animals including
humans; the formation of haploid gamete involves meiosis. In these organisms specialized cell called meiocytes (gamete mother cells), undergo
meiosis. At the end of meiosis, one set of chromosome is present in each of the
gametes: n.
Gamete Transfer:
Bringing together male and female gametes to
facilitate fusion/fertilisation
·
In most of
the organisms, male gametes are motile & female gametes are stationary.
·
In algae,
bryophytes & pteridophytes water is the medium for gamete transfer.
·
A large
number of male gametes fail to reach the female gametes. In order to compensate
for this loss and to ensure fertilization; the number of male gametes produced
is several thousand times the number of female gametes.
·
In higher
plants, pollination is the method of gamete transfer as pollen grains are
carriers of male gametes; and ovule has the egg.
·
Pollen
grains are produced in anthers and need to be transferred to stigma for
fertilization.
·
In bisexual
plants, male and female reproductive parts on same individual, hence gamete
transfer is easier.
·
In unisexual
or dioecious plants; where male and female reproductive structures are located
on different individuals; a specific event called pollination is required. Pollination involves the transfer of
pollen grains to stigma.
2. SYNGAMY / FERTILIZATION:
- Most important event of
reproduction
- Syngamy/fertilisation results
in formation of a diploid zygote.
- In certain animals, female
gamete develops into the adult organism without fertilization; such a
process is called parthenogenesis - seen in rotifers,
lizards turkeys etc.
- Fertilisation may be external
or internal
EXTERNAL FERTILIZATION
Syngamy occurs outside the body of the organism
High level of synchrony between
male and female sexes
Large numbers of gametes are released in the surrounding medium.
Both male and female gametes are
motile
Number of offspring produced is
large
Ex. Bony fishes and Amphibians.
|
INTERNAL
FERTILIZATION
Syngamy occurs inside the body of the organism
High level
of synchrony not required
Numbers of ova produced are less, but large numbers of male gametes are
released and they travel towards the ovum.
Female
gametes are non motile, male may or may not be motile.
Number of
offspring produced is small
Ex. Birds and Mammals.
|
A major
disadvantage of external fertilization is that the offspring are highly
vulnerable to predators.
3. POST FERTILIZATION EVENTS:
- Formation of diploid zygote is
a universal events in all sexually reproducing organisms
- Zygote – vital link that ensures
continuity of species between organisms of one generation and the next.
Every living organism, including humans begin its life cycle as a single
cell zygote
- Development of zygote depends
on
- the type of life cycle of the
organisms.
- the environment it is exposed
to.
- In organisms undergoing
external fertilization, zygote is formed in external medium; while in
those with internal fertilization; the zygote is formed inside the body of
organism.
- Further development of zygote
varies among species:
ü In fungi and algae; zygote develops
a thick wall resistant to desiccation and damage. It undergoes a period of rest
before germination
ü In organism with haplontic cycle;
the zygote undergoes meiotic division; forms haploid spores; that develop in to
haploid individuals.
- Zygote develops into embryo. The
process of development of embryo from zygote is known as embryogenesis.
Embryogenesis involves:
- cell division (mitosis) to
increase the number of cells
- cell differentiation for
the formation of different kinds of tissues
The
different types of cells form undergo certain modifications to form specialized
tissues and organs that together form the complete organism.
- Based on whether the
development of zygote occurs inside or outside the body of the female
parent, animals are categorized into oviparous and viviparous.
OVIPAROUS
These animals lay eggs.
The fertilized eggs have a calcareous shell to protect them from harsh
environment.
|
VIVIPAROUS
These animals give birth to young ones
Embryo protected inside the mother’s body.
|
- In flowering plants the zygote
formed inside the ovule; zygote develops into embryo after which the ovule
becomes the seed and ovary becomes the fruit. The fruit
develops a thick pericarp that
is protective in function. The sepals, petals and stamens of the flower
wither and fall off. In some plants e.g. Brinjal, tomato etc, the sepals
remain attached and are known as persistent. After dispersal, seeds
germinate under favourable conditions.
Embryogenesis: Zygote Cell Division & Cell
Differentiation Embryo