Sexual Reproduction: Types of Fertilization

Fertilization: Sperm and Egg
This a colored scanning electron micrograph (SEM) of sperm clustered around a human egg (ovum) during fertilization. The round egg (green) is seen on human tissue (brown). Sperm attached to its surface are hair-like structures (yellow); each sperm has a rounded head and a long tail. They are penetrating the thick spongy surface of the egg that attracts sperm and enables them to attach. Credit: K.H. KJELDSEN/Getty Images

In sexual reproduction, two parents donate genes to their young resulting in offspring with a mix of inherited genes. These genes are donated through a process called fertilization. In fertilization, male and female sex cells fuse to form a single cell called a zygote. The zygote grows and develops by mitosis into a fully functioning new individual.

There are two mechanisms by which fertilization can take place.

The first is external fertilization (the eggs are fertilized outside of the body), and the second is internal fertilization (the eggs are fertilized within the female reproductive tract). While fertilization is necessary for organisms that reproduce sexually, individuals that reproduce asexually do so without the need for fertilization. These organisms produce genetically identical copies of themselves through binary fission, budding, fragmentation, parthenogenesis, or other forms of asexual reproduction.

Gametes

In animals, sexual reproduction encompasses the fusion of two distinct gametes to form a zygote. Gametes are produced by a type of cell division called meiosis. Gametes are haploid (containing only one set of chromosomes), while the zygote is diploid (containing two sets of chromosomes). In most cases, the male gamete (the spermatozoan) is relatively motile and usually has a flagellum.

On the other hand, the female gamete (the ovum) is non-motile and relatively large in comparison to the male gamete.

In humans, gametes are produced in male and female gonads. Male gonads are testes and female gonads are ovaries. Gonads also produce sex hormones which are required for the development of primary and secondary reproductive organs and structures.

External Fertilization

External fertilization occurs mostly in wet environments and requires both the male and the female to release or broadcast their gametes into their surroundings (usually water). This process is also called spawning. An advantage of external fertilization is that it results in the production of a large number of offspring. One disadvantage is that environmental hazards, such as predators, greatly reduce the chance of surviving into adulthood. Amphibians, fish, and coral are examples of organisms that reproduce this way. Animals that reproduce by broadcast spawning do not typically care for their young after spawning. Other spawning animals provide varying degrees of protection and care for their eggs after fertilization. Some hide their eggs in the sand, while others carry them around in pouches or in their mouths. This extra care increases the animal's chances of survival.

Internal Fertilization

Animals that use internal fertilization specialize in the protection of the developing egg. For example, reptiles and birds secrete eggs that are covered by a protective shell that is resistant to water loss and damage. Mammals, with the exception of monotremes, take this idea of protection a step further by allowing the embryo to develop within the mother.

This extra protection increases the chances of survival because mom supplies everything that the embryo needs. In fact, most mammalian mothers continue to care for their young for several years after birth.

Male or Female

It is important to note that not all animals are strictly male or female. Animals such as sea anemones may have both male and female reproductive parts; they are known as hermaphrodites. It is possible for some hermaphrodites to self-fertilize, but most must find a mate to reproduce. Since both parties involved become fertilized, this process doubles the number of young that are produced. Hermaphroditism is a good solution to the scarcity of potential mates. Another solution is the ability to change sex from a male to a female (protandry) or from a female to a male (protogyny).

Certain fish, like wrasses, may change from female to male as they mature into adulthood.