Reproduction in organisms

 1. Reproduction

n Reproduction is a biological process by which an organism produces another organism (offspring) similar to itself.

Function/Importance

n Reproduction is necessary for the continuity of the species.

n Sexual reproduction is responsible for variation in a population and its inheritance to future generations.

 

 

S. No.	Asexual reproduction	Sexual reproduction
(i)	Asexual reproduction does not involve formation or fusion of gametes.	Sexual reproduction involves formation and fusion of gametes.
(ii)	The offsprings formed by asexual reproduction are genetically identical to the parent and are referred to as clones.	The offsprings are not identical to the parents or to each other hence, it leads to variation in a population.
(iii)	It is a simple and fast process.	It is an elaborate, complex and slow process.
(iv)	It involves only mitosis.	It involves meiosis.
(v)	It is always uniparental (single parent).	It is usually biparental (two parents).
(vi)	It helps in maintaining same characters for generations.	It plays a vital role in evolution process.
(vii)	It is common among single-celled organisms, and in plants and animals with relatively simple organisation.	It is common in higher animals with complex organisation.

 

2. Asexual Reproduction

n Uniparental, with or without gamete formation.

n It is a condition where an offspring receives two copies of a chromosome or part of chromosome from one parent and no copies from the other parent.

n Offsprings are exact copies or clones of each other and the parent.

n Mitosis is the essential mode of asexual reproduction.

(i) Fission: The splitting of parental cell into two or more daughter cells.

(a) Binary fission: The splitting of a parental cell into two equal daughter cells, each of which rapidly grows into an adult, is called binary fission. It occurs in single-celled animals like bacteria and protozoans (e.g., Amoeba, Paramecium). It can be simple or irregular, longitudinal, oblique or transverse, depending on the plane of division.

(b) Multiple fission: The splitting of a parent cell into numerous daughter cells, each of which grow into an adult is called multiple fission, e.g., Plasmodium.

l Sporulation: During unfavourable conditions, organisms like Amoeba cover themselves with a three-layered hard covering or cyst. This is called encystation. On return of favourable conditions, it divides by multiple fission within the cyst and produces many Amoebae. This is called sporulation. The cyst bursts open and spores are liberated to develop into adults.

(ii) Fragmentation: It is a mode of asexual reproduction in which the parental body breaks into two or more fragments and each fragment grows into a new individual, e.g., Spirogyra.

(iii) Budding: It is a mode of asexual reproduction in which one or more outgrowths (buds) are produced which initially remain attached to the parent cell and eventually get separated from it to grow into a new individual, e.g., yeast.

(iv) Regeneration: It is a mode of asexual reproduction in which the lost part of an organism is repaired/reproduced by proliferation of cells, e.g., Planaria.

(v) Spore formation:

l Zoospores: These are endogenously produced unicellular, naked and motile spores with one or two flagella. Zoospores are produced in a sac-like structure called zoosporangium, e.g., Chlamydomonas.

l Conidia: Asexual non-motile spores cut off externally either singly (e.g., Phytophthora) or in chains (e.g., Penicillium) from the tip of a special hyphae called conidiophore.

l Gemmules: Internal asexual reproductive units or buds are called gemmules, e.g., sponges. These develop within the parental body and are released during germination.

(vi) Vegetative propagation: It is a mode of reproduction in which new plants are formed from vegetative parts (vegetative propagules) of the plant like root, stem, etc. It is very common in higher angiosperms.

Following are some units of vegetative propagation:

Bulbil	—	Agave
Bulbs	—	Onion, Garlic
Eyes on tuber	—	Potato
Leaf buds	—	Bryophyllum
Offset	—	Hyacinth and Pistia
Rhizome	—	Ginger
Runner	—	Oxalis
Sucker	—	Mint

3. Events in Sexual Reproduction

n Events in sexual reproduction may be grouped into three distinct stages as follows:

(i) Pre-fertilisation

(ii) Fertilisation (also known as syngamy)

(iii) Post-fertilisation

(i) Pre-fertilisation

n This includes formation of gametes (gametogenesis) and their transfer.

(a) Gametogenesis

n It involves formation of two haploid reproductive units called gametes.

n The formation of male gamete or male reproductive unit is called spermatogenesis.

n The formation of female gamete or female reproductive unit is called oogenesis.

n When male and female gametes are similar in appearance and it is not possible to differentiate between them, they are called homogametes or isogametes.

n When the male and female gametes are morphologically distinct, they are called heterogametes.

n Among heterogametes, the male reproductive unit is called antherozoid or sperm and female reproductive unit is called egg or ovum.

n A single organism bearing both male and female sex gametes is called homothallic or monoecious, e.g., coconut, Chara.

n Organisms in which different individuals carry male and female gametes are called heterothallic or dioecious, e.g., papaya, Marchantia.

n Unisexual male flowers bearing stamens are called staminate flowers.

n Unisexual female flowers bearing pistils are called pistillate flowers.

n The animals bearing both the sexes are called hermaphrodites, e.g., earthworm, sponge, tapeworm and leech.

Cell division during gamete formation

n Gametes are always haploid.

n Haploid organisms produce gametes by mitotic division. Diploid organisms undergo meiosis in specialised cells called meiocytes (gamete mother cell) to form gametes.

n At the end of both divisions, only one set of chromosomes get incorporated into each gamete.

(b) Gamete transfer

n After gamete formation, male and female gametes must be physically brought together to facilitate fusion (fertilisation).

n Male gametes are usually motile, whereas female gametes are static.

n In lower plants like some algae and fungi, both male and female gametes are motile.

n In simple plants like algae, bryophytes and pteridophytes, water acts as the medium for gamete transfer.

n In angiosperms, the pollen grains are transferred from anther of one flower to the stigma of another flower by pollinating agents and the process is called pollination.

(ii) Fertilisation or syngamy

n It is the fusion of male and female gametes to form a diploid cell called zygote.

n The phenomenon of development of female gamete directly into an individual without fertilisation is called parthenogenesis, e.g., rotifers, honeybees, lizards and birds.

n Fertilisation is of two types, i.e., external fertilisation and internal fertilisation.

S. No.	External fertilisation	Internal fertilisation
(i)	When fusion of gametes occurs outside the body of an organism, it is called external fertilisation.	When fusion of gametes occurs inside the body of an organism, it is called internal fertilisation.
(ii)	Large number of both male and female gametes are released into the surrounding medium (e.g., water) in order to enhance chances of syngamy.	The number of sperms produced are very large and number of eggs produced are less.
(iii)	The offsprings are extremely vulnerable to predators, threatening their survival.	The offsprings are well protected comparatively.
(iv)	Examples: Bony fishes, frogs and majority of algae.	Examples: Birds, mammals, etc.

(iii) Post-fertilisation events

These are the events that takes place after the formation of zygote. It includes development of zygote and embryo after fertilisation.

(a) Zygote development

n In external fertilisation, these zygote is formed in the external medium, whereas in internal fertilisation, zygote is formed inside the body of the organism.

n Further development of zygote depends upon the life cycle and environment.

n Zygote ensures continuity of species between organisms of one generation and the next.

n In algae and fungi, the zygote develops a thick wall to resist dessication and damage.

n In haplontic life cycle, the zygote undergoes reductional division (meiosis) to form a haploid organism.

n In diplontic life cycle, the zygote undergoes mitotic division to form a diploid organism.

(b) Embryogenesis

n The processes of development of embryo from the zygote is called embryogenesis.

n During embryogenesis, zygote undergoes

(i) cell division (mitosis) to increase cell number, and

(ii) cell differentiation to form specialised tissues and organs.

n Based on whether the zygote develops outside or inside the body of female parent, animals can be classified into oviparous and viviparous, respectively.

S. No.	Oviparous animals	Viviparous animals
(i)	Oviparous animals lay fertilised eggs.	Viviparous animals give birth to young ones.
(ii)	The fertilised eggs have calcareous shell and young ones hatch out after a period of incubation.	The fertilised egg (zygote) has no shell and develops into a young one inside the body of the female organism.
(iii)	Chances of survival of young one is less as the female lays egg in the environment surrounding.	Chances of survival of young one is more because of proper embryonic care and protection inside the mother’s body.
(iv)	For example, reptiles, birds, etc.	For example, majority of mammals including humans.

n In flowering plants, the zygote is formed inside the ovule, where the zygote develops into an embryo.

n The fertilised ovule develops into seed and ovary develops into fruit.

n The seed after dispersal in favourable condition germinates to produce new plants.

n The outermost protective covering of fruit is called pericarp or fruit wall.