Events in Sexual Reproduction > Pre-fertilization Events

All the events of sexual reproduction prior to the fusion of gametes are referred as prefertilization events.

Pre-fertilization events are all the events of sexual reproduction that occur before the fusion of gametes.

Main pre-fertilization events

The two main pre-fertilization events are:

• Gametogenesis
• Gamete transfer.

• Gametogenesis is the process of gamete formation in sexually reproducing organisms.
• Gametes are haploid structures.
• In lower organisms, male and female gametes may be morphologically similar; such gametes are called isogametes.
• In most sexually reproducing organisms, the gametes are morphologically distinct; such gametes are called heterogametes.

In heterogametic organisms:

• The male gamete is called an antherozoid or sperm.
• The female gamete is called an egg or an ovum.

• Gametes are haploid, but the parent body producing them may be haploid or diploid.
• In several organisms belonging to monera, fungi, algae and bryophytes, the plant body is haploid.
• In pteridophytes, gymnosperms, angiosperms and most animals, the parent body is diploid.
• A haploid parent produces gametes by mitotic division.
• A diploid organism produces haploid gametes by meiosis.
• During meiosis, the gamete mother cells undergo meiotic division.
• At the end of meiosis, only one set of chromosomes is incorporated into each gamete.

Plant species

Animal species

After the formation of male and female gametes, both must be physically brought together to facilitate fertilization. In a majority of organisms, the male gamete is motile and the female gamete is stationary.
Therefore, a medium is needed through which the male gametes move.

Gamete transfer in simple plants

• In several simple plants, such as algae, bryophytes, and pteridophytes, water is the medium through which gamete transfer occurs.
• A large number of male gametes fail to reach the female gametes.
• To compensate for this loss during transport, male gametes are produced in several thousand compared to the number of female gametes produced.
• In some fungi and algae, both male and female gametes are motile.

Gamete transfer in seed plants

• In seed plants, pollen grains are the carriers of male gametes and the ovule contains the egg.
• Pollen grains produced in anthers must be transferred to the stigma before fertilization can occur.
• This transfer is facilitated by a specialised event called pollination.

Pollination can be achieved by:

• Anemophily (air)
• Hydrophily (water)
• Entomophily (insects)
• Chiropterophily (bats)
• Ornithophily (birds).

Pollen grains germinate on the stigma.
Pollen tubes carrying the male gametes reach the ovule and discharge the male gametes near the egg.

Gamete transfer in animals

• In lower aquatic animals such as sponges and coelenterates, sperms are transferred with the help of water.
• In higher animals, a special copulatory organ, the penis, transfers the sperms directly into the female genital organ during copulation.

• Pre-fertilization events occur before fertilization and mainly include gametogenesis and gamete transfer.
• Gametogenesis produces haploid gametes, which may be isogametes or heterogametes.
• In haploid organisms, gametes are formed by mitosis; in diploid organisms, they are formed by meiosis.
• In lower plants and animals, gamete transfer often occurs through water.
• In seed plants, pollen grains carry male gametes to the ovule through pollination.
• In higher animals, sperms are transferred directly to the female body during copulation.