Introduction
In our environment, organisms cannot live forever and that is why the continuity of the species relies on individuals reproducing. Living organisms on earth including humans, animals, plants, fungi, and microorganisms follow the law of nature called reproduction. Reproductions in a layman’s language can be described as a means to ensure the survival of the species. There are two types of reproduction in our environment; one is asexual reproduction and another one is sexual reproduction. There are so many unicellular Organisms such as protists and bacteria as well as some plants and some animals also reproduce asexually. Those Organisms that reproduce asexually are found in stable environments to which they are very well suited. Most multicellular organisms usually reproduce sexually. In sexual reproduction, it involves male and female gametes (sperms and eggs) to form a zygote.
Both methods of reproduction, sexual and asexual, have their advantages and disadvantages. Sexual reproduction is adopted by vertebrates however simpler organisms such as amoeba reproduce through asexual reproduction. Refer to the official website of Vedantu or download the app for a detailed and comprehensive explanation.
Asexual Reproduction
In asexual reproduction, organisms reproduce by mitotic divisions producing offspring that are identical to their parents- clones. It is a form of reproduction where gamete fusion or change in chromosome number is not required. In this form of reproduction, there are no sex cells or fertilization. Therefore, this process requires the involvement of a single parent. In multicellular Organisms, an ordinary body cell undergoes cellular division and as a result, a new individual will arise.
Some methods of sexual reproduction are as follow: -
Binary fission such as in Amoeba, bacteria
Fragmentation such as in Planaria
Spore formation such as in ferns
Budding such as in hydra
Vegetative propagation such as in the onion
A common feature of all these methods is that the offspring in asexual reproduction is a direct clone of the parent. The aim of the process of reproduction is to propagate one’s own genes. Evolutionarily speaking, this form of reproduction suits these species better. Asexual reproduction is a quick and simple process for simpler organisms. In this type of reproduction, the genes of parents do not get diluted by those of other individuals. This method of reproduction also helps to produce twice as fast as one that reproduces sexually.
Let us learn about these methods in detail.
Types of Asexual Reproduction in Animals
There are several ways by which Organisms may reproduce asexually
Fission Reproduction
In fission reproduction, a single cell is divided into two equal halves.
Fission is one of the common ways of asexual reproduction among unicellular Organisms such as bacteria.
After the mitotic division of the nucleus, fission occurs in which a new organism is formed by splitting the parent cell into two equally sized daughter cells.
It occurs when the environmental conditions are good, such as plenty of food, the right temperature, water, etc. Fission is a very effective way of producing many offspring. Example - Amoeba, Euglena
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Asexual Budding
Except for the division of cytoplasm which is not equal, Budding is a little bit similar to fission.
The new individuals arise from a bud or an outgrown from the parent.
In some small multicellular animals, this type of asexual reproduction is observed.
Example - In yeasts, the cell does not divide equally into two halves; instead, there is a large parental cell and a smaller daughter cell.
Budding of Hydra
Many hydrae reproduce asexually by producing buds in the body wall. Budding in hydra occurs when they are well-fed which results in a new bud or sub bud formation.
On a specific side, the repeated cell division leads to the development of bud as an outgrowth. The development of buds occurs when it is fully matured. After maturation, it detaches from the main body and becomes a new independent individual.
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Fragmentation
In some multicellular organisms, Fragmentation occurs. In this mode of reproduction, a new organism is formed by each part of the organism which is broken or split into two or more parts.
This fragmentation reproduction is common in marine worms, starfish (echinoderms).
Parthenogenesis
Parthenogenesis is considered one of the main types of asexual reproduction where the female eggs are developed without fertilization. Pathogenesis is observed to occur naturally in invertebrates, along with some amphibians, fishes, reptiles as well.
Asexual Reproduction in Plants
Fragmentation
There are some green plants that are sophisticated in this asexual reproduction. Offspring may be produced by runners, bulbs, tubers, or rhizomes.
Spore Formation
Mostly, Fungi develop the spores. These spores are contained within a structure that is popularly known as sporangium.
The spores are formed by the budding. Example - Ferns
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Vegetative Reproduction
There are some plants, which are capable of vegetative reproduction. The vegetative reproduction is the system to separate parts of one plant and to form a new independent plant.
Vegetative reproduction may arise from many parts of plants including the underground stems and the leaves.
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Parthenogenesis
In some plants, fruits are developed from the ovary without fertilization. This type of fruit is called parthenocarpy and process formation of fruit is called pathogenesis.
Sexual Reproduction
Sexual reproduction involves two individuals and the combination of their reproductive cells from two individuals to form a third unique offspring. While reproducing sexually, the offspring produces with a different combination of genes. If one compares sexual and asexual reproduction, it will show that the method of sexual reproduction is more complex in nature. Involvement of gametes—that consist of half the number of chromosomes compared to all other cells in the organism, in sexual reproduction. Gametes are produced by a process called meiosis which generates haploid cells from diploid cells. This is when crossing over and recombination of genes. Switching from one chromosome to another ensures that genes will remain active in a given population. The period of gestation is also an important part of sexual reproduction. This is the time when the foetus gradually develops inside the mother’s womb.
The cells of multicellular organisms are divided into two broad categories, and these are
1. Somatic Cells
Somatic cells are all cells of the body with the exception of the germs cells.
2. Germ Cells
Germ cells are the cells that give rise to gametes. The gametes are cells that combine in sexual reproduction to form a new organism.
Process of Sexual Reproduction
The process of Sexual Reproduction is mitotic divisions of germ cells that produce haploid gametes (ova and sperm).
The sperm and ova meet with each other to form a diploid (two sets of chromosomes) zygote (single cell).
Then zygote divides by mitotic divisions.
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Sexual Reproduction in Animals
The reproductive system is made of the primary and secondary sex organs.
In animals, primary sex organs that produce the gametes include ovaries in females and testes in the male.
The secondary sex organs contain various glands, ducts, and chambers which provide storage and play important roles in the development of gametes and give protection to the primary sex organ.
FAQs on Sexual and Asexual Reproduction
1. What is the fundamental difference between asexual and sexual reproduction?
The fundamental difference lies in the number of parents involved and the genetic makeup of the offspring. Asexual reproduction involves a single parent and produces offspring that are genetically identical clones of the parent. In contrast, sexual reproduction involves two parents who contribute genetic material (gametes) to produce a unique, genetically diverse offspring.
2. What are the common types of asexual reproduction seen in different organisms?
Asexual reproduction occurs through several methods, depending on the organism. The most common types include:
- Fission: A parent cell splits into two or more identical daughter cells. Example: Amoeba, bacteria.
- Budding: A new individual develops from an outgrowth or bud on the parent's body. Example: Hydra, yeast.
- Fragmentation: The parent's body breaks into distinct pieces, each of which can grow into a new organism. Example: Planaria, starfish.
- Spore Formation: The parent produces numerous microscopic spores which can grow into new individuals under favourable conditions. Example: Fungi (like Rhizopus), ferns.
- Vegetative Propagation: New plants grow from vegetative parts of the parent plant like roots, stems, or leaves, not from seeds. Example: Potato (from tubers), rose (from stem cuttings).
3. How does sexual reproduction introduce genetic variation, and why is this so important?
Sexual reproduction creates genetic variation through two main processes during meiosis: crossing over (exchange of genetic material between homologous chromosomes) and the independent assortment of chromosomes. This shuffling of genes, combined with the fusion of gametes from two different parents, ensures that every offspring is genetically unique. This variation is crucial for the survival and evolution of a species, as it allows populations to adapt to changing environments, resist diseases, and avoid the accumulation of harmful mutations.
4. What is parthenogenesis and how does it differ from other forms of reproduction?
Parthenogenesis is a unique form of asexual reproduction where an embryo develops from an unfertilised egg. It is sometimes called 'virgin birth'. While it's a type of asexual reproduction (only one parent), it involves an egg cell (a gamete), which is typically associated with sexual reproduction. This makes it distinct from methods like fission or budding. Parthenogenesis occurs naturally in some insects (like aphids and bees), reptiles, and fishes.
5. If asexual reproduction is faster and requires less energy, why did sexual reproduction evolve and become so widespread?
While asexual reproduction is efficient for rapidly increasing population size in stable environments, its major drawback is the lack of genetic diversity. All offspring are clones, making the entire population vulnerable to the same diseases or environmental changes. Sexual reproduction, despite being slower and more energy-intensive, provides a significant long-term evolutionary advantage. The genetic variation it produces increases the probability that some individuals within a population will have traits that allow them to survive and reproduce in the face of new challenges, ensuring the species' continued existence.
6. How does vegetative propagation in plants benefit agriculture?
Vegetative propagation is highly beneficial in agriculture and horticulture for several reasons:
- Genetic Uniformity: It produces plants that are genetically identical to the parent, ensuring desirable traits like fruit flavour, flower colour, or disease resistance are preserved.
- Faster Growth: Plants grown from vegetative parts often mature and bear fruit much faster than those grown from seeds.
- Propagation of Seedless Varieties: It is the only way to reproduce plants that do not produce viable seeds, such as certain varieties of bananas and grapes.
7. What is the specific role of gametes and the zygote in the process of sexual reproduction?
Gametes are specialised haploid reproductive cells (containing half the number of chromosomes), such as sperm in males and ova (eggs) in females. Their role is to carry genetic information from each parent. The process culminates in fertilisation, where a male gamete fuses with a female gamete to form a zygote. The zygote is a single diploid cell (containing a full set of chromosomes) that is the first cell of a new, unique individual. It then undergoes repeated mitotic divisions to develop into an embryo.
8. Can an organism that reproduces asexually be considered 'immortal'?
In a sense, single-celled organisms that reproduce asexually through binary fission, like an Amoeba, can be considered biologically immortal. When the parent cell divides, it doesn't die or leave a corpse; its entire being is split to form two new daughter cells. Therefore, its lineage continues indefinitely without a natural death of the original individual from old age. However, these organisms are not immortal in the absolute sense, as they can still be killed by external factors like predators, changes in environment, or lack of nutrients.
