Reproduction is defined as a biological process in which an organism gives rise to young ones (offspring) similar to itself. The offspring grow, mature and in turn produce new offspring. Thus, there is a cycle of birth, growth and death. Reproduction enables the continuity of the species, generation after generation.
There is a large diversity in the biological world and each organism has evolved its own mechanism to multiply and produce offspring. The organism’s habitat, its internal physiology and several other factors are collectively responsible for how it reproduces. Based on whether there is participation of one organism or two in the process of reproduction, it is of two types.
When offspring is produced by a single parent with or without the involvement of gamete formation the reproduction is asexual. When two parents (opposite sex) participate in the reproductive process and also involve fusion of male and female gametes, it is called sexual reproduction.
Types Of Reproduction
(A) Asexual Reproduction – In this method, a single individual (parent) is capable of producing offspring. As a result, the offspring that are produced are not only identical to one another but are also exact copies of their parent. Are these offspring likely to be genetically identical or different? The term clone is used to describe such morphologically and genetically similar individuals.
Let us see how widespread asexual reproduction is, among different groups of organisms. Asexual reproduction is common among single-celled organisms and in plants and animals with relatively simple organisations. In Protists and Monerans, the organism or the parent cell divides into two to give rise to new individuals.
Thus,in these organisms cell division is itself a mode of reproduction. Many single-celled organisms reproduce by binary fission, where a cell divides into two halves and each rapidly grows into an adult (e.g., Amoeba, Paramecium). In yeast, the division is unequal and small buds are produced that remain attached initially to the parent cell which, eventually gets separated and mature into new yeast organisms (cells).
(B) Sexual Reproduction – Sexual reproduction involves formation of the male and female gametes, either by the same individual or by different individuals of the opposite sex. These gametes fuse to form the zygote which develops to form the new organism. It is an elaborate, complex and slow process as compared to asexual reproduction. Because of the fusion of male and female gametes sexual reproduction results in offspring that are not identical to the parents or amongst themselves.
A study of diverse organisms-plants, animals or fungi-show that though they differ so greatly in external morphology, internal structure and physiology, when it comes to sexual mode of reproduction surprisingly, they share a similar pattern. Let us first discuss what features are common to these diverse organisms.
All organisms have to reach a certain stage of growth and maturity in their life, before they can reproduce sexually. That period of growth is called the juvenile phase. It is known as vegetative phase in plants. This phase is of variable duration in different organisms.
Events In Sexual Reproduction
For convenience these sequential events may be grouped in three distinct stages namely, the pre-fertilisation, fertilisation and the post fertilisation.
(A) Pre-fertilisation Events – These include all the events of sexual reproduction prior to the fusion of gametes. The two main pre-fertilisation events are gametogenesis and gamete transfer.
- Gametogenesis – As you are already aware, gametogenesis refers to the process of formation of the two types of gametes – male and female. Gametes are haploid cells. In some algae the two gametes are so similar in appearance that it is not possible to categorise them into male and female gametes. They are hence called homogametes (isogametes). However, in a majority of sexually reproducing organisms the gametes produced are of two morphologically distinct types (heterogametes). In such organisms the male gamete is called the antherozoid or sperm and the female gamete is called the egg or ovum.
- Gamete Transfer – After their formation, male and female gametes must be physically brought together to facilitate fusion (fertilisation). Have you ever wondered how the gametes meet? In a majority of organisms, male gamete is motile and the female gamete is stationary. Exceptions are a few fungi and algae in which both types of gametes are motile. There is a need for a medium through which the male gametes move. In several simple plants like algae, bryophytes and pterídophytes, water is the medium through which this gamete transfer takes place. A large number of the male gametes, however, fail to reach the female gametes. To compensate this loss of male gametes during transport, the number of male gametes produced is several thousand times the number of female gametes produced.
(B) Fertilisation – The most vital event of sexual reproduction is perhaps the fusion of gametes. This process called syngamy results in the formation of a diploid zygote. The term fertilisation is also often used for this process. The terms syngamy and fertilisation are frequently used though, interchangeably.
(C) Post-fertilisation Events – Events in sexual reproduction after the formation of zygote are called Post-fertilisation Events. It includes The Zygote and Embryogenesis.