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

Events in Sexual Reproduction

Three stages: Pre-fertilisation, Fertilisation, and Post-fertilisation events.

1. Pre-fertilisation Events

These are the events prior to the fusion of gametes, including gametogenesis and gamete transfer.

a. Gametogenesis

It is the formation of male and female gametes. Gametes (haploid cells) are two types:

1. Homogametes (Isogametes): Similar gametes that cannot be categorized into male and female. E.g., some algae like Cladophora.
2. Heterogametes: 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 and female reproductive structures are 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 and female flowers are present on the 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 and 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, and bryophytes have haploid parental body. They produce haploid gametes by mitosis.
• Pteridophytes, gymnosperms, angiosperms, and animals have diploid parental body. They produce haploid gametes by meiosis of meiocytes (gamete mother cell).

Name of Organism	Chromosome Number in Meiocytes (2n)	Chromosome Number 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 fertilization.
• In most organisms, the 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, and pteridophytes), gamete transfer takes place through a water medium. To compensate for the loss of male gametes during transport, a large number of male gametes is produced.



• In seed plants, pollen grains (in anthers) carry male gametes, and the ovule carries the egg. Pollen grains are transferred to the stigma.
• In bisexual self-fertilizing plants (e.g., peas), anthers and stigma are closely located for easy transfer of pollen grains.
• In cross-pollinating plants (including dioecious plants), pollination helps in the 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, fertilization helps with the 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., the female gamete develops into new organisms without fertilization. This is called parthenogenesis.

Types of Fertilization

• External Fertilisation: Syngamy occurs in the external medium (water), i.e., the 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 a 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.
• Internal Fertilisation: Syngamy occurs inside the body of the organism. E.g., terrestrial organisms, belonging to fungi, animals (reptiles, birds, mammals), and plants (bryophytes, pteridophytes, gymnosperms, and angiosperms).
  • In this, a non-motile egg is formed inside the female body, where the motile male gamete reaches and fuses.
  • In seed plants, non-motile male gametes are carried to the female gamete by pollen tubes.
  • A large number of sperms are produced, but the number of eggs is very low.

3. Post-fertilisation Events

These are the events after the formation of the zygote.

Zygote

• Development of the zygote depends on the type of life cycle of the organism and the nature of the environment.
• In fungi and algae, the zygote develops a thick wall that is resistant to desiccation and damage. It undergoes a period of rest before germination.
• In organisms with a haplontic life cycle, the zygote divides by meiosis into haploid spores that grow into haploid individuals.
• Sexually reproducing organisms begin life as a zygote.
• The zygote is the vital link between organisms of one generation and the next.

Embryogenesis

• It is the development of the embryo from the zygote.
• During embryogenesis, the 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 the place of zygote development, animals are two types:
  1. Oviparous: Animals lay fertilized/unfertilized eggs. E.g., reptiles and birds lay fertilized eggs covered by a hard calcareous shell. After incubation, young ones hatch out.
  2. Viviparous: The zygote develops into a young one inside the female body. Later, the young ones are delivered out of the body. E.g., most 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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1. Asexual Reproduction 2. Sexual Reproduction 3. Events in Sexual Reproduction

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