Fertilisation

Fertilisation

The single most important event in sexual reproduction is the fusion of the male and female gametes. This fusion, called syngamy, produces a diploid zygote; the terms "syngamy" and "fertilisation" are used interchangeably for this process.

Parthenogenesis: An Exception

Not every organism strictly requires syngamy to reproduce. In a few groups — rotifers, honeybees, and even some lizards and birds such as the turkey — the female gamete can develop into a new organism on its own, without being fertilised. This exceptional phenomenon is called parthenogenesis.

External Fertilisation

In most aquatic organisms — many algae, fish and amphibians — syngamy takes place outside the body, in the surrounding water; this is called external fertilisation. Species that fertilise externally show strong synchrony between the sexes and release very large numbers of gametes into the water simultaneously, maximising the chance that sperm and egg will meet (this is why bony fish and frogs produce so many offspring at once). The major drawback of external fertilisation is that the resulting eggs and larvae develop exposed in the open water, making them highly vulnerable to predation, so relatively few survive to adulthood.

Internal Fertilisation …