CBSE CLASS 12 BIOLOGY CHAAPTER 1 REPRODUCTION IN ORGANISM
PART-1 ASEXUAL REPRODUCTION
INTRODUCTION
“Reproduction is a crucial biological process that enables living organisms to produce offspring that inherit their genetic traits. It serves as a mechanism for the continuation and proliferation of a species, as individuals of a species may eventually age and pass away.”
“Reproduction is characterized by several fundamental processes, including the replication of DNA, cell division (either mitotic, meiotic or both), the formation of reproductive structures, and the subsequent development of these structures into offspring.
These processes can be broadly categorized into two types: asexual reproduction and sexual reproduction.”
Asexual reproduction
“In asexual reproduction, new individuals are generated through mechanisms other than the fusion of gametes. This means that meiotic division and the fusion of gametes are not involved in the production of offspring, and the resulting individuals are genetically identical or nearly identical to the parent organism.”
“Asexual reproduction, also known as apomixis, was named by Winkler.
Apomixis is characterized by the rapid multiplication and reproduction of genetically identical or nearly identical plants from a single parent. The population produced from a single individual in apomixis is referred to as a “clone,” and each member of the clone is known as a ramet. Asexual reproduction results in the multiplication of the species without introducing any variation, and therefore does not play a role in evolution. This method of reproduction is most advantageous in a stable and favorable environment, as it perpetuates successful genotypes with precision. The rate of reproduction is generally faster in asexual reproduction than in sexual reproduction.”
Basic characteristics of asexual reproduction
“The basic characteristics of asexual reproduction include the production of offspring by a single parent, making it a uniparental process. Unlike sexual reproduction, gametes are not formed in asexual reproduction, and cell divisions are only mitotic. The resulting offspring are usually genetically identical to the parent, with variability, if any, being limited to mutations. Asexual reproduction allows for rapid multiplication, and offspring are often produced in large numbers. In some organisms, such as Protists and Monerans, the parent cell divides into two to give rise to new individuals, making cell division itself a mode of reproduction. In other organisms, such as members of the fungi kingdom and simple plants like algae, asexual reproduction occurs through specialized structures such as zoospores, conidia (Penicillium), buds (Hydra), or gemmules (sponge).”
Asexual reproduction in plants
“Asexual reproduction in plants can occur through two main methods in flowering plants: agamospermy and vegetative propagation.”
Agamospermy
“Agamospermy is a type of asexual reproduction in plants that involves the formation of an embryo without fertilization and meiotic division. This means that plants belonging to this category propagate through seeds, but the embryo formation does not involve meiosis and syngamy. There are three main types of agamospermy, which include diplospory, adventive embryony, and apospory.”
Diplospory
“Diplospory is a type of agamospermy where the archesporium differentiates to form a megaspore mother cell, but instead of undergoing meiosis, the megaspore mother cell directly gives rise to an embryo sac. This embryo sac is diploid, and a diploid embryo is formed without fertilization from the diploid egg of this embryo sac. Diplospory is also known as diploid parthenogenesis and is found in plants such as Parthenium and Taraxacum.”
“Parthenogenesis is a type of asexual reproduction where a haploid egg cell of the female gametophyte is responsible for forming a haploid embryo without fertilization.
Apogamy is another type of asexual reproduction where any haploid cell of the female gametophyte except the egg cell is responsible for forming a haploid embryo without fertilization. It should be noted that while parthenogenesis and apogamy are types of asexual reproduction, they are not included in agamospermy.”
“Adventive embryony is another type of agamospermy where an embryo is formed from any diploid cells (such as nucellus or integuments) of the ovule except for the embryo sac. This diploid cell behaves like a zygote and gives rise to an adventive embryo. Adventive embryos are derived from nucellus in plants such as Citrus, Mangifera, Opuntia, Mamillaria, and from integuments in plants such as Spiranthus australis.”
“Apospory is another type of agamospermy, where the embryo sac or female gametophyte is directly formed from any diploid cell of the sporophyte, except the megaspore mother cell, without undergoing meiosis. In this process, the gametophyte always remains diploid. Apospory was discovered by Rosenberg in Heiarcium plants and is also found in plants such as Ranunculus and Rubus.”
Natural methods of vegetative propagation include the following:
- Stolons or runners: These are horizontal stems that grow along the surface of the soil. At certain intervals, buds develop on them that develop into new plants. Examples include strawberry and mint.
- Rhizomes: These are underground stems that grow horizontally. At certain intervals, buds develop on them that grow into new plants. Examples include ginger and turmeric.
- Tubers: These are modified underground stems that store food. They can develop buds that grow into new plants. Examples include potato and yam.
- Bulbs: These are underground storage structures made up of layers of fleshy leaves. The buds that develop between these layers can grow into new plants. Examples include onion and garlic.
- Corms – Corms are compact, thick, underground, vertical stems surrounded by scaly leaves. Gladiolus, Crocus, Colocasia, etc. can be propagated through corms.
- Bulbs – Bulbs are underground buds surrounded by fleshy scale leaves. Onion, Garlic, Tulip, etc. can be propagated through bulbs.
- Stolons or runners – These are specialized horizontal stems, which grow above the ground and produce adventitious roots at their nodes. New plants arise from these roots. Examples include Mint, Strawberry, and Grass.
- Creepers : In some plants, the stem branches at the nodes, which grow horizontally along the ground and after covering some distance, give rise to adventitious roots at the nodes. Sweet potato, Watermelon, Cucumber, etc. can be propagated through creepers.
- Leaves : In some plants, leaves are capable of vegetative propagation. For example, Bryophyllum leaves have buds in their notches, which can grow into new plants when the leaves fall on the ground.
Natural vegetative propagation is also seen in some trees, where adventitious roots arise from the stem and grow down to the soil, forming new trees. This is called layering. Examples include banyan, rubber tree, and strawberry tree.
- Corm – Corms are short, swollen underground stems which look like short, upright rhizomes and consist of a stout, solid, fleshy underground stem growing in the vertical direction. Gladiolus, Colocasia, Crocus, Alocasia and Amorpho-phallus, etc.
- Tubers – New plants develop from buds and roots growing at the nodes in the stem tubers. Potato, Helianthus tuberosus, etc.
- Sucker – Mint and Chrysanthemum.
- Creepers – In creeping stem of the plants, adventitious root are developed from the nodes to form an aerial shoots such as –
- Runners – Cynodon, Oxalis and Centella
- Stolon = Fragaria (Strawberry) & Vallisneria
- Offset – Pistia, Eichhornia (Water hyacinth), etc.
- Aerial stem – Opuntia
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- Leaves – Some plants produce adventitious buds on their leaves, e.g., Bryophyllum, Begonia, Strepto-carpus, Saintpaulia. These bud remains dormant, when the leaves are attached with plants but after separation, when it comes in contact with moist soil, develops into new plantlets [buds] which form new plants.
- In Kalanchoe plant, whole portion of the leaf blade regenerates a new plant.
- In some of the plants, fleshy axillary buds that develop from axis of leaves are called bulbils.
Example- Dioscorea, Oxalis, Dentaria, Globba, Agave, Lilium
- There are special type of fleshy buds that develop in aquatic plant which are called turions. E.g., Potamogeton, Utricularia.
Artificial vegetative propagation
Artificial vegetative propagation is a process where plants are propagated using various man-made methods for commercial purposes. Horticulturists and gardeners use techniques like cutting, grafting, layering, and micropropagation for artificial vegetative propagation.
Cutting involves taking a portion of the root, stem, or leaf and using it for propagation. In some cases, the stem cuttings are treated with rooting hormones like IBA, IAA, or NAA to encourage the development of adventitious roots. Sugarcane is an example of a plant that can be propagated through stem cuttings.
Grafting is the most common method of vegetative propagation. In this process, parts of two plants are joined together in such a way that they grow as one plant. Grafting is done between two closely related dicotyledonous plants that have vascular cambium. The rooted supported portion of one plant, called stock, is joined with a twig of another plant, called scion. Generally, the rootstock belongs to a wild variety that is resistant to disease and pests, while the scion is derived from the plant possessing better characters. The stock and scion cuts are marked in an oblique manner so that they fix with each other. This joint is covered with clay or a layer of wax. Within a few days, the tissues of stock and scion combine together to form a new plant. Examples of plants that can be propagated through grafting include mangoes, roses, oranges, seedless grapes, guavas, apples, and pears.
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DEMERITS OF VEGETATIVE PROPAGATION
One of the main disadvantages of vegetative propagation is that it leads to the loss of genetic diversity since the offspring are genetically identical to the parent plant. This can make the plant population more susceptible to diseases and pests.
Another disadvantage is that vegetative propagation is often more expensive and time-consuming than seed propagation, especially when micropropagation techniques are used.
Additionally, not all plant species can be easily propagated through vegetative means, and some may not respond well to the different techniques used in artificial vegetative propagation.
Finally, if a plant is already infected with a disease or virus, vegetative propagation can spread that disease to the new plant.
SIGNIFICANCE OF VEGETATIVE PROPAGATION
Vegetative propagation has significant importance in horticulture and agriculture for various reasons, including:
- Production of plants: Vegetative propagation allows for the production of a large number of plants in a short period of time. This can be helpful in situations where large quantities of plants are needed for commercial or landscaping purposes.
- Preservation of genetic traits: Vegetative propagation allows for the preservation of desirable genetic traits in a plant, such as disease resistance, fruit quality, and yield. By cloning a plant, the exact same genetic makeup is retained in each propagated plant, ensuring consistency in the crop.
- Seedless plants: Vegetative propagation is the only method of reproduction for seedless plants such as bananas, pineapples, and seedless grapes. This is important for commercial production of these crops.
- Reduced cost and time: Vegetative propagation is often faster and less expensive than seed propagation. This can be particularly advantageous in situations where time and resources are limited.
- Rejuvenation of old or weak plants: Vegetative propagation can be used to rejuvenate old or weak plants, resulting in new and healthier plants.
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Overall, vegetative propagation is a valuable tool in plant production, allowing for the preservation of desirable genetic traits and the efficient production of large numbers of plants.
ASEXUAL REPRODUCTION IN ANIMALS
Binary fission is the division of the parent into two small, nearly equal daughter individuals.
During binary fission nuclear divisions or karyokinesis, always followed by division of cytoplasm or cytokinesis.
Examples – Protozoans (Amoeba, Euglena, etc.) Bacteria and Planarians.
There are three types of binary fission : simple, transverse and longitudinal.
Asexual reproduction is found in various organisms, including some lower and higher organisms. Some examples of organisms that reproduce asexually are bacteria, yeast, sponges, sea anemones, flatworms, and some species of fish and lizards.
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Asexual reproduction can occur through various mechanisms such as fission (binary and multiple fission), budding, fragmentation, and the production of cysts and spores.
Binary fission in Amoeba is a type of asexual reproduction where the parent cell divides into two daughter cells. The process starts with the replication of the nucleus, followed by the elongation of the cell and the division of the cytoplasm. The cytoplasmic division is completed by the formation of a transverse furrow that eventually separates the two daughter cells. Each daughter cell receives a copy of the genetic material and inherits the same characteristics as the parent cell. This process of binary fission allows Amoeba to reproduce quickly and efficiently in favorable environmental conditions, and can lead to the formation of large populations in a short period of time.
Transverse binary fission: If the plane of cytoplasmic division passes across the body at right angles to its long axis, the fission is called transverse binary fission. Example – Paramecium.
Longitudinal binary fission: If the plane of cytoplasmic division passes lengthwise or longitudinally through the body, the fission is called longitudinal binary fission. Example – Euglena.
Multiple fission is a type of asexual reproduction that results in the formation of multiple daughter cells simultaneously. This process involves repeated nuclear division (karyokinesis) without cytokinesis, resulting in the formation of many nuclei within the parent cell. The cytoplasm then divides into many fragments, each with a nucleus, and the fragments separate to form individual daughter cells.
Multiple fission is observed in many unicellular organisms, including protozoans such as Plasmodium (causes malaria), Amoeba, and Monocystis (parasitic flatworm). In Plasmodium, multiple fission is a crucial part of its life cycle, occurring in the liver and red blood cells of the host. It allows for the production of numerous infectious forms (merozoites) that can infect new red blood cells and continue the cycle of infection.
NOTE
Plasmotomy is a form of asexual reproduction found in certain multinucleate protozoans where the cytoplasm divides into several small, multinucleate daughters without any accompanying nuclear division. The daughter cells then grow and regain the normal number of nuclei by undergoing nuclear divisions. Opalina and Pelomyxa are two examples of organisms that undergo plasmotomy.
Budding
Budding is a form of asexual reproduction where a new individual arises from a small outgrowth or projection called a bud on the parent body. The new individual is formed by mitosis. Budding is a common method of reproduction in certain lower animals and some protozoans. Examples of animals that reproduce through budding include sponges (Scypha), coelenterates (Hydra), annelids (Chaetopterus), and tunicates (Salpa). There are two types of budding: exogenous and endogenous.
Exogenous or external budding involves the development of a small outgrowth on the parent’s body, which then separates to become a new individual. An example of this type of budding is Hydra.
In endogenous or internal budding, the offspring is formed inside the parent organism and is released as a separate entity after development. Gemmules are formed in freshwater and marine sponges as a means of survival during adverse conditions and are capable of developing into a new individual under favourable conditions. Examples of organisms that exhibit endogenous budding include freshwater sponges (Spongilla) and marine sponges (Sycon).
Fragmentation
Fragmentation is the breaking up of the parent’s body into two or more fragments, each of which can grow into a new individual. It occurs in some lower organisms such as algae (e.g., Spirogyra) and certain animals like flatworms (e.g., Microstomum). In fragmentation, the body of the parent organism breaks into several pieces and each fragment can regenerate into a complete new organism.
Cysts and spores
Cysts and spores are two types of specialized structures that some organisms use for asexual reproduction and survival under unfavorable conditions.
Cysts are a type of protective covering that some protozoans, such as amoeba and paramecium, form around themselves in response to unfavorable environmental conditions. Inside the cyst, the protozoan is in a dormant state and can survive for extended periods until favorable conditions return. When conditions improve, the cyst ruptures, and the protozoan emerges to resume its normal life cycle.
Spores are tiny, specialized structures produced by various organisms such as fungi, algae, and bacteria. In sporulation, which occurs in many organisms, asexual reproduction occurs by the division of the nucleus into several daughter nuclei, and each daughter nucleus gets enclosed by a small amount of cytoplasm to form a spore. Spores are usually protected by a thick wall and can remain dormant for long periods. When conditions are favorable, spores germinate, and new individuals grow.
In summary, both cysts and spores are essential mechanisms used by some organisms to ensure their survival and asexual reproduction in unfavorable conditions.