Sexual Reproduction in Animals - Fertilisation in Organisms

Fertilisation in Organisms:

• Fertilisation involves events leading to the fusion of gametes and zygote formation.
• The process of fusion of haploid gametes to form a diploid zygote is known as syngamy.
• The terms syngamy and fertilisation are frequently used though, interchangeably.

What would happen if syngamy does not occur?

Parthenogenesis: The development of an egg into a complete individual without fertilization by a sperm is known as parthenogenesis. It is also known as virgin birth or virginal reproduction. It occurs in many invertebrates such as rotifers, insects, and arachnids and even in some vertebrates and some birds. Some creatures, such as rotifers, honeybees, and even some lizards and birds (turkey).

Where does syngamy occur?

1) External fertilisation:

• In most aquatic organisms, such as a majority of algae and fishes as well as amphibians, syngamy occurs in the external medium (water), i.e., outside the body of the organism. This type of gametic fusion is called external fertilisation.
• Organisms exhibiting external fertilisation show great synchrony between the sexes and release a large number of gametes into the surrounding medium (water) in order to enhance the chances of syngamy. This happens in the bony fishes and frogs where a large number of offspring are produced.
• A major disadvantage is that the offspring are extremely vulnerable to predators threatening their survival up to adulthood.

2) Internal Fertilisation: 

• In many terrestrial organisms, belonging to fungi, higher animals such as reptiles, birds, mammals and in a majority of plants (bryophytes, pteridophytes, gymnosperms and angiosperms), syngamy occurs inside the body of the organism, hence the process is called ‘internal fertilisation’.
• In all these organisms, egg is formed inside the female body where they fuse with the male gamete. In organisms exhibiting internal fertilisation, the male gamete is motile and has to reach the egg in order to fuse with it. In even though the number of sperms produced is very large, there is a significant reduction in the number of eggs produced.
• In seed plants, however, the non-motile male gametes are carried to female gamete by pollen tubes.

Read More: Parthenogenesis