Fertilisation

 The fusion of gamets is the most important event of sexual reproduction.

This process called syngamy results in the formation of a diploid zygote.

In some organisms like rotifers, honeybees and even some lizards and birds ,

the female gamete undergoes development to form new organisms without fertilisation.

This process is called parthenogenesis.

The reason behind syngamy

In most aquatic organisms, like 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 with external fertilisation show great synchrony between

the sexes and release a large number of gametes into the surrounding

medium like water in order to enhance the chances of syngamy.

For example in fishes and frogs a large number of offspring are produced inside the water.

A major disadvantage is that the offspring are extremely

vulnerable to predators threatening their survival up to adulthood.

In many terrestrial organisms, like 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 with internal fertilisation, egg is formed inside the female body where they fuse with the male gamete.

In organisms having internal fertilisation,

the male gamete is motile and has to reach the egg in order to fuse with it.

In these even though the number of sperms produced is very large,

there is a significant reduction in the number of eggs produced.

Whereas, in seed plants, the non-motile male gametes are carried to female gamete by pollen tubes.