Reproduction is a biological process. It is seen in both plants and animals (unicellular and multicellular). It is the process of formation of a new offspring or organism similar to their parents.
It is an important characteristic of all living organisms because it shows the difference between living and non-living organisms. The living organism has the capacity of reproduction, but the non-living organism has no ability for reproduction.
Reproduction enables the continuity of species generation; that is, it ensures the passage of genes from one generation to another generation.
Types of Reproduction in Animals
On the basis of the involvement of a number of individuals participate in reproduction, there are two types of reproduction.
- Asexual Reproduction
- Sexual reproduction
In both types of reproduction, there are some common features like RNA synthesis, protein synthesis, DNA replication, Cell division, the formation of reproductive unites or bodies, and the development of new offspring or individuals.
1. Asexual Reproduction
Asexual reproduction is the process of formation of a new individual without the involvement of gamete formation or fusion of gametes.
In asexual reproduction, the participation of two organisms is not required.
In asexual reproduction, the nucleus of somatic cells or body cells is divide, either by amitosis pr mitosis. This type of reproduction is also known as somatogenic or blastogenic reproduction.
Asexual reproduction is the primary form of reproduction in most of the lower animals, such as protozoans, sponges, coelenterates, etc.
In prokaryotes, there is no formation or fusion of gametes take place, but during lateral gene transfer, conjugation, transformation, and transduction are similar to sexual reproduction in the sense of genetic recombination in meiosis.
Types of Asexual reproduction
Based on the division of cells, there are six different types of asexual reproduction.
- Binary fission
- Multi fission
1) Binary fission
In Binary fission, the parents are divides into two daughter cells, which are morphologically and genetically identical with their parents.
It is the most common method of reproduction in most of the unicellular organism, under the favourable condition of the environment.
During binary fission, the nucleus is divide into two parts, followed by the division of the cytoplasm.
There are three different types of binary fission based on the plane of division.
- Simple binary fission
- Transverse binary fission
- Longitudinal binary fission
a) Simple Binary Fission
In simple binary fission, the plane of division of the organism is difficult to ascertain and take place in the irregular shaped organism like Amoeba.
b) Transverse Binary Fission
In Transverse binary fission, the plane of division of the organism is at the right angle to the long axis like paramecium and planaria.
c) Longitudinal Binary Fission
In this type of binary fission, the plane of division of the organism is parallel to the long axis like vorticella, euglena, and in some corals.
In binary fission, the organelles of the parent’s body divide to produce two genetically identical daughter organisms or one daughter individual retain them, and others must develop new organelles.
2) Multi Fission
In this process of asexual reproduction, the parent nucleus repeatedly divides to form a large number of daughter nuclei.
This followed the division of cytoplasm according to the number of nuclei, and each part enclosing one nucleus.
It results in the formation of the numbers of daughter cells from a single parent at the same time.
The multiple fission usually takes place in Amoeba, plasmodium, monocytes, algae, and fungi.
Gemmulation is a type of asexual reproduction that mostly takes place in freshwater sponges and some marine sponges. Within the parent body of sponges, buds are formed called gemmules or internal buds or endogenous buds.
Gemmulation in Marine water sponges
In some marine water sponges, gemmulation starts when a small group of cells (archaeocytes) become laden with reserve food granules and become isolated at the internal surface of a sponge.
A protective covering covers over each one mass is called as gemmule.
The gemmules are expelled from the adult sponge. After degeneration of the parent sponge due to drought or extreme temperature, the gemmules are liberated and germinated to an adult sponge.
Gemmulation in freshwater sponges
In freshwater sponge, the gemmules consist of a mass of archeocytes that are laden with reserve food granules and are protected by a protective membrane formed by archeocyte cells.
The gemmules of freshwater sponges help them to survive in unfavourable conditions.
In cold regions, gemmulation occurs in winter, and the inactive gemmules hibernate.
In the worm region, gemmulation occurs in summer, and the inactive gemmules hibernate.
And in spring or autumn (favourable condition), the gemmules germinate. Their archeocytes emerge through an opening called the micropyle.
Budding is a process of formation of buds (outgrowth) on the parental body. The buds may be unicellular or multicellular. One or more buds are produced from a single parent body.
The buds, which are much smaller than the parent, develop into its full size either after detachment from the parents or before detachment being attached to its parent body.
Budding is external or exogenous in Hydra and internal or endogenous in Acinate.
The process of formation of new individuals from the fragments of the parent is known as fragmentation.
When the body of Hydra is cut into many fragments, each fragment develops into a new individual.
The process by which organisms develop or generate their lost or worn out parts is called regeneration.
It is highly developed in lower animals like protozoans, sponges, planarians, coelenterates, echinoderms, etc.
Advantages of Asexual reproduction
- Mating is not required.
- A large no of individuals is produced from a single individual within a short period of time.
- The daughter’s offsprings are genetically identical to the parents.
- Asexual reproduction takes place only through simple mitotic division.
- It also helps the animals to tide over unfavourable environmental conditions.
Disadvantages of Asexual reproduction
- Continous binary fission for several generations makes the daughter individuals genetically weak and requires rejuvenation.
- The organism which is produced by asexual reproduction is generally less adaptive to changing the environment.
- Since the daughter individuals are similar to the parent, there is no genetic variation in the offspring and hence not contribute to Speciation.
2. Sexual Reproduction
Sexual reproduction is commonly seen in multicellular organisms. During sexual reproduction, there is an involvement of two different sex is takes place (male and female), and these two opposite sexes are produced two different sex cell known as a gamete.
The males produce male gametes or sperm, and the female produces female gametes or ova. These two gametes are fuse to form a zygote which grows into a new individual.
The sperms are produced from the male sex organ testis, and the ova are produced from the female sex organ ovaries.
In some animals like an earthworm, both the sex organs testes and ovary are present; these are called hermaphrodite animals.
During the process of formation of sperm in testis and ova in ovaries, meiosis or reduction division takes place in which haploid gametes are formed from diploid cells.
The process of fusion of male gamete and female gamete is known as fertilization.
During fertilization, haploid (n) motile male gamete fuses with a haploid (n), non-motile female gamete to form a diploid (2n) zygote, that gives rise to a new individual.
Fertilization may be external or internal.
If a species releases eggs into the environment and after that, the male fertilizes them is called external fertilization.
If the fertilization takes place within the body of a female by the male gamete is known as internal fertilization.
A zygote is the diploid (2n) cell consists of 46 chromosomes (23 pair), 23 from male gamete, and 23 from female gamete.
Advantages of sexual reproduction
- The offsprings are adapt themselves to the changing environmental conditions.
- The formation of gametes by meiosis and their fusion during fertilization produce reshuffling of genes and variation in the offspring.
- Variation in the offspring helps them in natural selection and evolution.
- The genetic variation takes place in sexual reproduction. This leads to the evolution of species from time to time.
- The offsprings are genetically different from parents which help the future evolution of the organism.
Disadvantage of sexual reproduction
- During the mating, two partners are required.
- The formation of a new individual is slow.
- Some variations may have undesirable qualities.
|Sexual Reproduction||Asexual Reproduction|
|It require both male and female for reproduction.||Only one individual is required for reproduction.|
|In sexual reproduction, the cells are divide by meiosis.||In asexual reproduction, cells are divide either mitotically or amitotically.|
|The offsprings are genetically and morphologically, similar to their parents.||The offsprings are genetically and morphologically different from their parents.|
|This process does not contribute to the process of evolution and Speciation.||The recombination of chromosomes in meiosis induce variations that favor organic evolution.|