Reproduction (Asexual and Sexual) :Definition, types
What is the significance of reproduction in living organisms? Give an account of various types of asexual reproduction? Describe different types of sexual reproduction?
In this article we will discuss about the Reproduction (asexual & sexual) :-(1) Definition, (2) types of reproduction, (3) Binary fission, (4) Multiple fission, (5) Budding, (6) Sexual reproduction, (7) Syngamy, (8) Conjugation, (9) Automixis, (10) Parthenogenesis.
Reproduction is one of the fundamental characteristic of living organisms. It is the ability of an organism to produce new individuals. It enables an organism to preserve its own kind. An organism continues to exist only by means of reproduction. Reproduction involves the transmission of genetic material from the parents to the progeny. As old organisms are replaced by young ones, reproduction ensures continuity of the species, generation after generation. Thus, reproduction is indispensable for the preservation of the species.
There are a number of different reproductive strategies available in nature. Different animals. reproduce by different methods for the continuity of their race. In general, the less specialized an organism is, the simpler is its method of reproduction. In higher multicellular organisms, the process has become more complicated. So the process of reproduction is in relation with the grade of structural organization and the environment in which the animal lives. There is a tendency of increased efficiency of reproductive processes from lower to higher animals.
There are two basic types of reproduction observed among animals: Asexual and Sexual.
Asexual reproduction is a primitive form of reproduction of new individuals or offsprings from a single parent. The offsprings are identical to each other and often referred to as clone. Therefore, asexual reproduction can be described as a method of copying. Genetic variation is absent in asexual reproduction except in random mutation in certain individuals. Asexual reproduction is a characteristic feature of lower organisms. The sex-cells or gametes are not involved in this process. The whole body behaves as a reproductive unit as seen in case of Amoeba.
The common methods of asexual reproduction are:-
- Binary fission
- Multiple fission
Most of the unicellular organisms multiply by fission. It is the division of the parent cell in to two or more identical daughter cells. Binary fission is common in bacteria and some protozoans like Parmaecium, Amoeba etc. Amoeba is an eukaryotic, unicellular organism. Binary fission is a simple and common method of reproduction in Amoeba. It results in the division of the parent body into two daughter cells which are identical to each other and to the parent cell. Binary fission occurs during favourable temperature (about 25° C) and abundance of food. The animal feeds and continues to grow till it attains maximum limit of its size. The body assumes a spherical shape by withdrawing all its pseudopodia. The animal stops feeding. Replication of DNA precedes binary fission. The nucleus becomes elongated and the nucleolus disintegrates. The process of binary fission is a mitotic event; division of the nucleus preceeds division of the cytoplasm. The nuclear division includes the following four stages.
In the beginning of prophase, the nuclear membrane disappears. The chromatin reticulum condenses into 500-600 chromidia or duplicated chromosomes. A nuclear spindle develops from the nuclear sap.
In metaphase, the free chromosomes migrate towards the equatorial region of the nuclear spindle. Each duplicated chromosome splits up into two chromatids.
In anaphase, the chromatids are pulled towards the opposite poles of the spindle. The chromatids (now called chromosomes) at the poles are surrounded by a newly formed nuclear membrane.
The nuclear membrane gradually elongates. A constriction appears in the middle which divides the nucleus into two daughter nuclei.
Division of the nucleus is followed by the division of the parent body. A constriction furrow appears in the middle of the body wall which progressively deepens and finally cuts the parent body into two daughter cells or daughter amoebae. The daughter individuals are identical to each other and to the parent Amoeba.
The whole process of binary fission is completed in about 30 minutes. It has been observed that the whole process takes less time at a higher temperature (more than 25° C) and more time at a lower temperature. One of the daughter Amoeba retains the old contractile vacuole while the other Amoeba forms a new one. The young amoebae develop pseudopodia, start feeding and lead an independent life. After reaching a size peculiar to the species, it begins to divide. The process of fission is continuous. There in no old age or senescence in Amoeba. Therefore, Amoeba in considered potentially immortal. Other protozoans like paramecium also multiply by binary fission. Binary fission takes place in some platyhelminthes such as planaria.
In this type of reproduction the nucleus undergoes repeated division followed by the division of the cytoplasm. Amoeba reproduces by this method during unfavourable environmental conditions such as drought and extreme cold. First, the body becomes round by withdrawing all its pseudopodia. Next, it secretes a protective covering called ‘cyst around the body. The cyst is three-layered and made of chitin. Slowing down of metabolic activities makes the animal inactive. The nucleus inside the cyst undergoes repeated division to form about 500-600 daughter nuclei. Next, the cytoplasm segregates and each daughter nucleus is enclosed by a small mass of cytoplasm. A large number of daughter individuals called amoeba or pseudopodiospores are formed in this manner. At the approach of favourable condition, the cyst wall absorbs water to break open and release the pseudopodiospores. These daughter amoebae start feeding and soon grow into adult amoebae.
In Plasmodium, multiple fission occurs by a process called schizogony. The parasite enters into the liver cells of the host and starts feeding on the contents of the cytoplasm. It grows in to a large rounded structure called schizont. The nucleus of the schizont divides into numerous nuclei. Each nucleus gathers a little mass of cytoplasm around it. The schizont finally breaks up releasing a large number of daughter individuals.
Occassionally the body of Hydra splits in a transverse or longitudinal plane. This process is also called fission. This may occur due to the presence of matured buds on the parent body or to assume a normal shape from an abnormal position.
Budding is a simple and common mode of asexual reproduction. It is a process by which a new individual is formed as an outgrowth of the parent body. It is the most common method of asexual reproduction in Hydra. In Hydra budding usually occurs during the winter season under the prevailing favourable conditions such as abundance of food and water.
Only a healthy and full-grown Hydra undergoes budding. In the beginning, a slight bulge appears in the ‘budding zone’ located at the base or at the middle of the column. The budding zone has the greatest concentration of interstitial cells and acts as an embryonic reserve. Later, the bulge pushes out a small, hollow outgrowth containing epithelio-muscular cells of the epidermis, nutritive-muscular cells of the gastrodermis and an extension of the coelenteron. Soon the outgrowth extends outward as a diverticulum of the coelenteron. New cells are formed and are rearranged around the cavity of the bud which becomes dauble-layered with an outer epidermis and inner gastrodermis. The cells from both the layers secrete a non-cellular layer between them called the mesogloea. The free distal end of the bud becomes conical to form the hypostome. A small, star shaped mouth opens at the tip of the hypostome. A ring of blunt tentacles appears at the base of the hypostome around mouth. Within a period of 48 hrs to 96 hrs, the bud becomes fully grown with a stalk, hypostome, tentacles and mouth. A constriction appears at the base of the bud which eventually gets separated form the parent body and falls off into water to lead an independent life. The buds are miniature, but identical copies of the parents. Often the bud develops secondary buds while still attached to the parent body. Under highly favourable conditions, several buds may develop simultaneously on the parent body. Unicellular fungi such as yeast also reproduce by budding.
Sexual reproduction: Sexual reproduction is of wide spread occurrence in nature. Most of the higher animals reproduce by this method. It requires the fusion of male gamete with female gamete to form the zygote. The gametes are structurally and genetically distinct. The two types of gametes are obtained from different parents. Sexual reproduction involves the participation of both the parents. Therefore, it is biparental in nature. However, some animals produce both the gametes. They are called hermaphrodites. Sexual reproduction ensures structural, physiological and genetic variation in the offspring which provides raw materials for evolution. Genetic recombination is a prelude to sexual reproduction. The increased phenotypic variations that appear in the offspring due to mixing of genes help them to adapt successfully to the changing environment which may lead to the formation of a new species.
Sexual reproduction occurs in the following ways.
Complete and permanent fusion of male and female gametes results in syngamy. The following types of syngamy takes place in living organisms.
Male and female gametes are produced by the same organism or the same cell. The gametes fuse to form zygote. Example, Paramecium.
The two gametes undergoing fusion are structurally and physiologically identical.
The male and female gametes are produced by different parents which fuse to form zygote. Hologamy:
The matured organisms behave as gametes and fuse to form new individuals.
The gametes are small and morphologically distinct.
The union of young individual simmediately after their formation by the division of adult parents.
The male and female gametes are vary widely in shape, size and behavior. The male gametes (microgametes) are very small and active while the female gametes are very large (macrogametes) and passive. Anisogamy takes place in higher animals.
The union of large gametes (macrogametes) as seen in protozoans.
The union of small gametes (microgametes) as seen in protozoans.
It is a temporary union of individuals of the same species for the exchange of nuclear material. Example, Bacteria.
This is the union of gametic nuclei from the same cell.
The parthenogenesis (Gr., parthenos=virgin, genesis= birth) is the special type of sexual reproduction. In parthenogenesis, the eggs of an organism develop into the young individual without the fertilization of the eggs by the sperms. The parthenogenesis occurs in certain insects (wasps and bees, etc.) and rotifers.