Reproduction

Reproduction is the biological process by which new "offspring" individual organisms are produced from their "parents". Reproduction is a fundamental feature of all known life; each individual organism exists as the result of reproduction. The known methods of reproduction are broadly grouped into two main types: sexual and asexual.

In asexual reproduction, an individual can reproduce without involvement with another individual of that species. The division of a bacterial cell into two daughter cells is an example of asexual reproduction. Asexual reproduction is not, however, limited to single-celled organisms. Most plants have the ability to reproduce asexually.

Sexual reproduction typically requires the involvement of two individuals or gamates, one each from opposite type of sex.

Asexual reproduction

Asexual reproduction is the process by which an organism creates a genetically-similar or identical copy of itself without a contribution of genetic material from another individual. Bacteria divide asexually via binary fission; viruses take control of host cells to produce more viruses; Hydras (invertebrates of the order Hydroidea) and yeasts are able to reproduce by budding. These organisms often do not possess different sexes, and they are capable of "splitting" themselves into two or more individuals. On the other hand, some of these species that are capable of reproducing asexually, like hydra, yeast (See Mating of yeasts] and jellyfish, may also reproduce sexually. For instance, most plants are capable of vegetative reproduction—reproduction without seeds or spores—but can also reproduce sexually. Likewise, bacteria may exchange genetic information by conjugation. Other ways of asexual reproduction include parthenogenesis, fragmentation and spore formation that involves only mitosis. Parthenogenesis (from the Greek παρθένος parthenos, "virgin", + γένεσις genesis, "creation") is the growth and development of embryo or seed without fertilization by a male. Parthenogenesis occurs naturally in some species, including lower plants (where it is called apomixis), invertebrates (e.g. water fleas, aphids, some bees and parasitic wasps), and vertebrates (e.g. some reptiles fish, and, very rarely, birds and sharks]). It is sometimes also used to describe reproduction modes in hermaphroditic species which can self-fertilize.

Binary Fission

Binary fission involves mitosis only and hence the resultant individuals are genetically identical to each other and to the parent.

Binary Fission in Amoeba

It is the simplest and most common method of asexual reproduction. The whole parental body acts as the reproductive unit. The nucleus of the unicellular parent organism divides into two. This is followed by the division of the cytoplasm and 2 daughter cells of almost equal size are formed. The daughter cells grow in size and then divide again.

Examples: Seen in euglena, amoeba, paramoecium.

Based on the plane of cytoplasmic division binary fission is of 3 types, namely:

a) Simple binary fission

b) Transverse binary fission

c) Longitudinal binary fission

Multiple Fission

In some organisms the nucleus of the parent divides into many daughter nuclei by repeated divisions (amitosis). This is followed by the division of the cytoplasm into several parts with each part enclosing one nucleus. So a number of daughter cells are formed from a single parent at the same time. This kind of fission is known as multiple fission.

A - C Cyst Formation and Multiple Fission in AmoebaD - F Multiple Fission in Malarial Parasite

Example: Seen in Plasmodium (malarial parasite) where it is known as schizogony or sporulation, amoeba.

During unfavourable conditions, amoeba withdraws its pseudopodia, becomes almost round and secretes a three-layered hard covering called cyst around itself. This phenomenon is called encystation and lasts till the favourable conditions set in. On the onset of favourable conditions, the encysted amoeba divides by multiple fission to produce a large number of minute pseudopodiospores. At this point, the cyst bursts open and the spores are liberated into the surrounding medium. Each pseudopodiospore develops into an amoeba. This entire process is termed as sporulation

Budding

Here one or more outgrowths of reproductive units called buds are formed on the parental body. Each bud consists of a small group of cells surrounded by the epithelium.

A bud develops as a small outgrowth on the parents body. Each bud enlarges, develops the characteristics of the parent organism. A bud may become separated from the parent body and then develop into a new individual, or it may separate only after the completion of development. In some cases the buds never separate and as a result, colonies of interconnected individuals are formed.

Examples: Hydra, sponges, some tunicates.

Internal buds

In case of sponges, the parent releases a specialised mass of cells enclosed in a common opaque envelope, called as the gemmule. Gemmules are thought to be internal buds, which on germination give rise to an offspring.

Budding in Hydra and Sponge