Sunday, July 26, 2020

Reproduction in Organisms - Notes | Class 12 | Part 3: Events in Sexual Reproduction

1. REPRODUCTION IN ORGANISMS

EVENTS IN SEXUAL REPRODUCTION
3 stages: Pre-fertilisation, Fertilisation & Post-fertilisation events.
1. Pre-fertilisation Events
These are the events prior to the fusion of gametes.
They include gametogenesis and gamete transfer.

a. Gametogenesis
It is the formation of male and female gametes.
Gametes (haploid cells) are 2 types:
a. Homogametes (isogametes): Similar gametes. They cannot categorize into male & female gametes. E.g. Some algae like Cladophora.
b. Heterogametes: The male and female gametes are distinct types. Male gamete is called antherozoid (sperm) and female gamete is called egg (ovum). E.g. Fucus (an alga), Human beings etc.

Sexuality (bisexual or unisexual) in organisms:
a.  Bisexual: Male & female reproductive structures present in the same individual.

Bisexual plants: E.g. Hibiscus, Pisum.
In flowering plants, male flower is staminate (bears stamens) and female flower is pistillate (bears pistils).
If male & female flowers are present on same plant, it is called monoecious. E.g. Cucurbits, coconuts, Chara.

Bisexual animals (hermaphrodites): E.g. Earthworms, leech, sponge, tapeworm, etc.

b.  Unisexual: Male and female reproductive structures are present on different individuals.
If male & female flowers are present on different plants, it is called dioecious. E.g. papaya, date palm, Marchantia.

Unisexual animals: E.g. Cockroach, higher animals etc.
Fungi may be homothallic (bisexual) or heterothallic (unisexual).

Cell division during gamete formation:

- Many monerans, fungi, algae & bryophytes have haploid parental body. They produce haploid gametes by mitosis.
- Pteridophytes, gymnosperms, angiosperms & animals have diploid parental body. They produce 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
6
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
16
8

b. Gamete Transfer

    • Male gametes need a medium to move towards female gametes for fertilisation.
    • In most organisms, male gamete is motile and the female gamete is stationary. In some 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 carries the egg. Pollen grains are transferred to the stigma.
    • In bisexual self-fertilizing plants (e.g. peas), anthers & stigma are closely located for easy transfer of pollen grains.
    • In cross pollinating plants (including dioecious plants), pollination helps in transfer of pollen grains. 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.
    • In rotifers, honeybees, some lizards, birds (turkey) etc., 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. zygote is formed outside the body. E.g. most aquatic organisms (many algae, bony fishes etc.) and amphibians.

Such organisms show synchrony between the sexes and release large number of gametes into the surrounding medium to ensure syngamy.
Disadvantage: The offspring are extremely vulnerable to predators threatening their survival up to adulthood.

b. Internal fertilisation: Syngamy occurs inside the body of the organism. 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.
Zygote
  • 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.
  • Sexually reproducing organisms begin life as a zygote.
  • Zygote 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. Reptiles & birds lay fertilized eggs covered by a hard calcareous shell. After incubation, young ones hatch out.
b. Viviparous: Here, 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.
It shows proper care and protection. So the chances of survival of young ones are greater.

Embryogenesis in flowering plants (see next chapter)
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