Contents
The study of human anatomy and physiology is one of the crucial Biology Topics for medical professionals and researchers.
Asexual Reproduction in Organisms – Characteristics, Advantages, and Disadvantages
Reproduction in Plants
Like animals, plants also reproduce both asexually and sexually. Asexual reproduction in plants is either by fission, budding, fragmentation and regeneration, spore formation or by vegetative propagation or vegetative reproduction of plant parts. Sexual reproduction is by fusion of male and female gametes and it occurs in flowering plants.
Asexual Reproduction in Organisms
The process by which reproduction takes place without the formation of gametes and produces offspring simply by body cell division or by spore formation is called asexual reproduction.
Production of offspring by a single parent without the formation and fusion of gametes is called asexual reproduction. Meiosis does not occur in asexual reproduction. All divisions are mitotic. All the individuals formed through asexual reproduction from a parent are genetically similar to one another as well as their parent. A morphologically and genetically similar individual is called a clone. Asexual reproduction is common among lower groups of organisms – simple plants and simple animals. It is absent in higher invertebrates and vertebrates.
Characteristics of Asexual Reproduction
- A single parent is involved in reproduction.
- Gametes are not formed in asexual reproduction.
- Fertilization does not occur.
- No meiosis, only mitotic cell division takes place.
- Daughter individuals are genetically identical to their parents.
- Multiplication occurs in a rapid manner.
Occurrence of Asexual Reproduction
Asexual reproduction occurs usually in lower individuals, especially unicellular organisms (Bacteria, Yeast, Amoeba, Paramoecium, Euglena, etc.). It is absent in higher invertebrates and vertebrates.
Types of Asexual Reproduction
In the asexual reproduction method, certain body cells of the parent organism undergo repeated mitotic cell divisions to form two (or more) new organisms of the same kind. Asexual reproduction takes place through six different methods. These are:
- Fission
- Budding
- Spore formation
- Regeneration
- Fragmentation
- Vegetative propagation
We will now describe all these methods of asexual reproduction in detail, one by one. Let us start with fission.
1. Fission – Definition, Examples, Diagrams
Many single-celled organisms like protozoa and bacteria just split (or break) into two identical halves during cell division, leading to the creation of new organisms. This is called fission. In biology, fission is the process of reproduction in unicellular organisms such as protozoa (like Amoeba, Paramecium, Leishmania, etc.) and many bacteria. In the process of fission, a unicellular organism splits (or divides) to form two (or more) new organisms. Fission is of two types: binary fission and multiple fission, depending on whether the parent organism splits to form two new organisms or more than two organisms. This is a mode of asexual reproduction in which the body of the parent individual divides into two or more equal-sized daughters. It can occur by binary fission, multiple fission, and plasmotomy.
A. Binary Fission
Binary fission is an asexual method of reproduction of organisms. In binary fission, the parent organism splits (or divides) to form two new organisms. When this happens, the parent organism ceases to exist and two new organisms come into existence. Unicellular organisms like Amoeba, Paramecium, Leishmania, bacteria, etc., reproduce by binary fission. This is described below.
Amoeba reproduces by binary fission by dividing its body into two parts. This happens as follows: When the Amoeba cell has reached its maximum size of growth, then first the nucleus of Amoeba lengthens and divides into two parts. After that, the cytoplasm of the Amoeba divides into two parts, one part around each nucleus. In this way, one parent Amoeba divides to form two smaller Amoebae (called daughter Amoebae). And we say that one Amoeba produces two Amoebae. The reproduction in Amoeba by binary fission is shown in Figure.
Amoeba reproducing by binary fission.
The two daughter Amoebae produced here grow to their full size by eating food and then divide again to produce four Amoebae, and so on. In unicellular organisms such as Amoeba, the splitting of the parent cell during fission (or cell division) can take place in any plane.
Paramecium is a unicellular animal having short thread-like structures called cilia over its surface (see Figure). Paramecium also reproduces by the method of binary fission. A fully grown Paramecium divides its body into two parts to form two smaller Paramecia. This happens by the division of the nucleus followed by the division of cytoplasm.
Leishmania is a unicellular animal (which is a protozoan) (see Figure). It is a parasite that causes the disease known as kala-azar (or black fever). Kala-azar is also known as leishmaniasis. Leishmania has a greater degree of organization in its body, having a whip-like structure called flagellum at its one end (see Figure). Leishmania reproduces by the process of binary fission. In Leishmania, the splitting of the parent cell during fission (or cell division) takes place in a definite plane (longitudinally) with respect to the flagellum at its end. In this respect, Leishmania differs from Amoeba (in which fission can take place in any plane).
From the above discussion, we conclude that simple animals like Amoeba, Paramecium, and Leishmania reproduce by binary fission. The microorganisms like bacteria also reproduce by the method of binary fission. Please note that the word ‘binary’ means ‘two’ and the word ‘fission’ means ‘splitting’. So, the term ‘binary fission’ means ‘splitting into two’.
We can observe the binary fission of Amoeba or Paramecium under a microscope. This can be done as follows: Collect some water from a pond or any other stagnant water body (especially where weeds, hay, and husk are dumped). Put a few drops of this pond water on a clean slide and observe first under low magnification and then under high magnification of the microscope. We will see the Amoeba or Paramecium dividing (or reproducing) by the method of binary fission.
(a) An Amoeba dividing to give two Amoebae
(b) A Paramecium dividing to give two Paramecia
Amoeba and Paramecium reproduce by binary fission (as seen under the high magnification of a microscope).
The term ‘multiple’ means ‘many’ or ‘several’. So, multiple fission means ‘splitting into many’ or ‘splitting into several’.
It is the division of the parent individual into two almost equal halves each of which functions as an independent daughter individual. Binary fission usually involves mitosis.
Depending upon the plane of division, binary fission is of the following types:
(i) Irregular binary fission: Division can occur through any plane, e.g., Amoeba.
(ii) Longitudinal binary fission: The plane of division passes along the longitudinal axis of the animal. It occurs in Euglena, Vorticella.
(iii) Transverse binary fission: The plane of division runs along the transverse axis of the individual, e.g., Paramoecium, Planaria, diatoms, bacteria. In Paramoecium, the meganucleus divides by amitosis, while the micronucleus divides by mitosis. It produces two dissimilar daughters, one anterior and the other posterior. Both develop deficient components and become similar.
(iv) Oblique binary fission: The plane of division is oblique. It occurs in Ceratium, Gonyaulax.
The organisms that undergo binary fission are said to be immortal. Because in binary fission, the parent body as a whole form the reproductive unit and disappears after the formation of two daughter individuals. So the parent cannot be said as died.
B. Multiple Fission
Multiple fission is also an asexual method of reproduction in organisms. In multiple fission, the parent Organism splits (or divides) to form many new organisms at the same time. This happens as follows: Sometimes (particularly during unfavourable conditions), a cyst or protective wall is formed around the cell of a single-celled organism (like that of Plasmodium) [see Figure].
Reproduction by Multiple Fission
Inside the cyst, the nucleus of the cell splits (or divides) several times to form many smaller nuclei called daughter nuclei. Little bits of cytoplasm collect around each daughter nuclei and thin membranes are formed around them. In this way, many new daughter cells are formed from a single parent cell within the cyst [see Figure], In fact, as many daughter cells are formed as the number of daughter nuclei produced by the divisions of the parent nucleus.
When favourable conditions arrive, the cyst breaks open and the many daughter cells present in it are released, each forming a new organism [see Figure], In this way, a single-celled parent undergoes multiple fission to reproduce many daughter cells at the same time. Plasmodium is a protozoan (a microscopic, single-celled animal) that reproduces by the asexual method of multiple fission. About 1000 daughter cells are produced by the multiple fission of one Plasmodium cell. Plasmodium is the malarial parasite that produces malaria disease in human beings. The malarial parasite Plasmodium is carried by female Anopheles mosquitoes from one person to another thereby spreading the malaria disease.
This is a highly enlarged photograph of the red blood cell of a person suffering from malaria disease. This red blood cell contains two Plasmodium cells which have been coloured blue.
The female Anopheles mosquito carries the parasite of malaria disease called Plasmodium. So, it transmits malaria.
A health department worker is fumigating homes to remove mosquitoes so as to prevent malaria disease.
Before we discuss the next asexual method of reproduction called budding, we should know the meaning of the term ‘bud’. The ‘bud’ here means a ‘small outgrowth’ from the body of a living organism. Let us discuss the method of ‘budding’ now.
This type of reproduction occurs in Amoeba, Plasmodium, etc., where repeated nuclear and cytoplasmic division produces numerous daughter cells, each of which ultimately gives rise to a new individual. Under unfavourable conditions, Amoeba withdraws; its pseudopodia and secretes a three-layered wall, i.e., cyst wall. This phenomenon is referred to as encystment. This cyst wall is extremely resistant to unfavourable environmental conditions and even to antibiotics. Amoebas can survive inside the cyst for a very long time. On return of favourable conditions, multiple fission occurs and a good number of spores, i.e., pseudopodiospores are formed inside, which get liberated and each grows into an adult Amoeba.
In Plasmodium, multiple fission occurs in the schizont, the process is called schizogony and the daughters are called merozoites. The process of multiple fission in oocyst is called sporogony and the daughter individuals are known as sporozoites.
Differences between binary fission and multiple fission:
| Binary Fission | Multiple Fission |
| 1. In this type of asexual reproduction, the unicellular organisms split into two equal daughter cells by cell division. | 1. In this type of asexual reproduction, unicellular organisms divide into many daughter cells. |
| 2. Here, the nucleus and cytoplasm divide simultaneously. | 2. Here, firstly the nucleus divides into many nuclei, and then each nuclei is surrounded by the cytoplasm, and the daughter nuclei are released by the rupture of the parent cell. |
| 3. In binary fission, many different patterns of division are found like, transverse and longitudinal. Example: Amoeba. | 3. In multiple fission, no definite pattern of division is found. Example: Malaria parasite. |
| 4. Binary fission occurs during favourable conditions. | 4. Multiple fission can take place under favourable conditions, as well as under unfavourable conditions. |
C. Plasmotomy
It is modified by multiple fission. It occurs in syncytial organisms. Here the multinucleate parent divides into many multi-nucleate individuals without the division of nuclei. Nuclear division occurs later on to maintain a normal number of nuclei, e.g., Opalina.
2. Budding – An Overview of Budding in Hydra and Yeast Cells
Budding is an asexual method of reproduction. In budding, a small part of the body of the parent organism grows out as a ‘bud’ which then detaches and becomes a new organism. The asexual reproduction by budding is observed in Hydra and yeast. This is described below.
Hydra is a simple multicellular animal [see Figure(a)]. Hydra reproduces by the process of budding (by using its regenerative cells). This happens as follows: In Hydra, first, a small outgrowth called ‘bud’ is formed on the side of its body by the repeated mitotic divisions of its cells [see Figure(b)]. This bud then grows gradually to form a small Hydra by developing a mouth and tentacles [see Figure(c)], And finally the tiny new Hydra detaches itself from the body of the parent Hydra and lives as a separate organism [see Figure(d)], In this way, the parent Hydra has produced (or created) a new Hydra. Thus, Hydra reproduces asexually by growing buds from its body. This is called budding. Please note that the bud formed in a Hydra is not a single cell. It is a group of cells.
Hydra Reproducing by the method of Budding
We will now describe the reproduction in yeast plants by the process of budding. Please note that every single cell of yeast is a complete plant in itself. Yeast is a tiny, unicellular, non-green plant (which is a fungus). Yeast reproduces by budding. The figure shows a parent yeast cell (which is a complete plant). In yeast, first, a bud appears on the outside of the cell wall [Figure (b)]. The nucleus of the parent yeast cell then divides into two parts and one part of the nucleus moves into the bud [Figure(b)]. Ultimately, the bud separates from the parent yeast cell and forms a new yeast cell (or new yeast plant) [Figure(c)]. The budding in yeast, however, often takes place so fast that the first buds start forming their own buds and all of them remain attached to the parent yeast cell forming a chain of yeast cells [Figure(d)], After some time, all the yeast cells of the chain separate from one another and form individual yeast plants.
Yeast reproduces by the method of budding.
We can study the process of asexual reproduction in yeast by budding in the laboratory as follows: Take 100 mL of water in a conical flask and dissolve 10 grams of sugar in it. Then add 5 grams of yeast powder (or yeast granules) to this sugar solution and stir it well with a glass rod. Put a cotton plug in the neck of the conical flask. This conical flask containing sugar solution and yeast mixture is kept aside in a warm place for 3 to 5 days.
When froth is observed in the flask, the yeast culture is ready for examination. Take out a small quantity of the yeast culture solution from near the bottom of the conical flask with the help of a dropper and place a drop of this culture solution on a clean slide. Add a very little iodine solution over the culture solution drop to stain it. Place a coverslip over the slide. Keep the slide under the microscope and observe it first under low power and then under high power of the microscope. Note the formation of buds on the yeast cells and how they separate from the parent cell (see Figure).
This is yeast powder. It is used for making yeast culture solutions in the laboratory.
Yeast cells reproduce by budding (as seen in yeast culture solution under the high power of microscope).
In some organisms like sponges and corals, the buds remain attached to the parent organism permanently. These buds then grow and produce buds of their own. In this way, a colony of sponges or corals is formed. Before we discuss the next asexual method of reproduction called ‘spore formation’, we should know something about ‘spores’. Spores are the microscopic ‘asexual reproductive bodies’ which are covered by a hard protective coat. This coat enables them to survive under unfavourable conditions like lack of food, lack of water, and extreme temperatures.
But when the conditions are favorable (food and water are available, and the temperature is suitable), then the spores grow to produce new plants. Thus, spores are a kind of seeds of plants. These spores are very light and keep floating in the air all around us. They are so small that we cannot see them with the naked eye. Keeping these points in mind, it will now be easier for us to understand asexual reproduction by spore formation.
In this process, the daughter individuals are formed from the parent as small outgrowth or bud which grows gradually and acquires the form of a parent.
A. Budding in yeast: A small outgrowth is formed from the cell wall. Which is initially attached to the parent body and gradually increases in size. Later on, it gets separated and matures into a new organism. In Mucor, Rhizopus, the apical portion of special branches of hypha form oidia somewhat similar to budding, the process of which is called Torulation. The yeast bud resembles the fungal genus Torula and the stage represents the torula stage.
B. Budding in Animals: It is of two types.
(i) Exogenous or External budding: A bud develops from the body surface of the parent individual. It grows in size and forms a young individual. The bud separates from the parent to lead an independent life as in Hydra or remains attached to the parent to form a colony, e.g., Scypha (= Sycon). Exogenous budding also occurs in urochordates or tunicates (Salpa).
(ii) Endogenous or Internal budding: In freshwater sponges (e.g., Spongilla) and in marine sponges (e.g., Sycori), the parental individual releases a specialized mass of cells enclosed in a common opaque envelope, called the gemmules. Each gemmule consists of a small group of totipotent cells called archaeocytes. During favourable conditions, the mass of archaeocytes comes out through micropyle and later on forms a new colony.
Differences between binary fission and budding:
| Binary Fission | Budding |
| 1. The body of the parent divides into two equal halves and from each half a new individual is formed. | 1. A small bud is formed from the parent body which grows in size and separates from the parent body. |
| 2. The division is equal. | 2. Division is unequal. |
| 3. Parent’s body disappears. Examples: Bacteria, Amoeba, Paramoecium, Euglena, etc. | 3. Parent’s body remains as it is. Examples: Yeast, Hydra, Sycon, etc. |
| 4. Binary fission occurs during favourable conditions. | 4. Multiple fission can take place under favourable conditions. |
C. Strobilation: Strobilation or transverse fission is a form of asexual reproduction consisting of the spontaneous transverse segmentation of the body. The segmented body is called the strobila larva and each of the segments is called the ephyra larva. Strobilation occurs in the Aurelia and the neck of the Taenia. This mode of reproduction is characterized by high offspring output.
3. Sporulation (Spore Formation) – Method, Diagram, and Examples
Spore formation is the asexual method of reproduction. Reproduction by spore formation takes place in plants. In spore formation, the parent plant produces hundreds of microscopic reproductive units called ‘spores’. When the spore case of the plant bursts, then the spores spread into the air. When these air-borne spores land on food (or soil) under favourable conditions (like damp and warm conditions), they germinate and produce new plants. Most of the fungi (like Rhizopus, Mucor, etc.), bacteria, and non-flowering plants such as ferns and mosses reproduce by the method of spore formation. The common bread mould is a fungus plant whose scientific name is Rhizopus. The common bread mould (or Rhizopus fungus) reproduces by the method of spore formation. This is described below.
The tiny spores of ‘bread mould’ (a fungus plant) are almost always present in the air. If we keep a moist slice of bread aside for a few days, then the spores of bread mould plant present in the air settle on the moist bread and germinate to form new fungus plants. The bread mould plants first look like a white cottony mass covering the bread slice which later on turns black. If we observe the surface of this slice of bread through a magnifying glass, then the bread mould plant growing on it will appear to be like that shown in Figure.
This is a common bread mould plant (or Rhizopus fungus). It reproduces by forming spores.
The common bread mould plant consists of fine, threadlike projections called hyphae and thin stems having knoblike structures called sporangia (see Figure). Each knob-like structure (or sporangium) contains hundreds of minute spores enclosed in a spore case. When the spore case bursts, the tiny spores are dispersed in the air (see Figure). These spores are the asexual reproductive units that can produce more bread mould plants under suitable conditions. Actually, it was one such air-borne spore that grew on the moist slice of bread kept aside by us for a few days. If we remove one sporangium from the bread mould, keep it on a slide, put a coverslip over it, and observe this slide through a microscope, we can see the spores.
Bread mould (Rhizopus fungus) grows on a slice of bread.
Bread mould fungus as seen through a magnifying glass. The white threads are ‘hyphae’. Each black dot is a ‘sporangium’ which contains thousands of tiny spores.
This is Penicillium fungus. It also reproduces by forming spores.
The antibiotic drug called penicillin is made from Penicillium fungus.
The spore formation method of asexual reproduction is used by unicellular organisms as well as by multicellular organisms. For example, bacteria are unicellular organisms that reproduce by spore formation whereas fungi such as Rhizopus (bread mould) and Macor, and non-flowering plants such as ferns and mosses are multicellular organisms that reproduce by spore formation method.
Spore formation is a common form of asexual reproduction, which is widely distributed among plants and in certain protozoans (Sporozoa). Spores are minute, single-celled reproductive units formed by cell division of the parent body either exogenously or endogenously. Each spore after its release from the parent body germinates to form a new individual. Sporulation is common in members of monera, protista, fungi, and algae.
Asexual reproduction takes place by means of spore formation in the life history of most cryptogamic plants like algae, fungi, bryophytes, and pteridophytes. Spores are formed inside the specialized organ called sporangium. After liberation from the sporangium, each spore independently germinates into a new plant.
Spores are of the following types:
Endospore:
Endospore is a dormant, tough, and non-reproductive structure produced by certain bacteria. It is a seed-like form, but it is not a true spore. It is a stripped-down, dormant form to which the bacterium can reduce itself. Endospore formation is usually triggered by a lack of nutrients. During adverse environmental conditions, some bacteria produce resting spores by forming a hard impermeable coat around them called an endospore. With the return of favorable conditions, they rupture to give vegetative cells.
Zoospore:
Zoospores are unicellular flagellate motile spores that are produced endogenously within the sac-like structure called zoosporangium, e.g., Ulothrix (algae); Synchitrium (Fungi).
In the case of zoospores, two types of distinct flagella are found in various combinations. Those are
(a) Tinsel flagella: This flagellum has lateral filaments i.e., mastigonemes which are perpendicular to the main axis, allowing for more surface area and disturbance of the medium, giving it the property of a rudder, i.e., the purpose of being used for steering.
(b) Whiplash flagella: They are straight, and used to power the zoospore through its medium. This is also the ‘default’ zoospore, which only has the propelling, ‘whiplash’ flagella. Both tinsel and whiplash flagella beat in a sinusoidal wave pattern, but when both are present, the tinsel will beat in the opposite direction of the whiplash, to give 2-axes of control of motility.
Aplanospore:
These are unicellular, non-motile, non-flagellate, and endogenously produced spores formed within the sporangium, e.g., Mucor. Different types of aplanospores are discussed in the following:
(a) Sporangiospore: The more primitive fungi (Rhizopus, Mucor) produce spores in the sporangia, which are sac-like sporophores, whose entire cytoplasmic contents cleave into several spores. These are called sporangiospores. Thus, they differ from modern advanced fungi in that their asexual spores are endogenous in nature. Sporangiospores are either naked and flagellated (as zoospores) or walled and non-motile (as aplanospores).
(b) Conidia: These are non-motile spores cut off externally either singly or in chains from the tip of the conidiophore. It is a type of asexual reproductive spore of fungi (Penicillium). It is usually produced at the tip or side of the hyphae or on special spore-producing structures i.e., conidiophores. The spores attach when mature. They vary widely in shape, size, and colour. According to the size, the larger one is called macroconidia and the smaller one is called microconidia.
(c) Oidia: It is also an asexually produced fungal spore, that is presumed not to constitute the main reproductive preoccupation of the fungi at that time. When the hyphae break up into small pieces of component cells then those are developed into the spores. Oidia cannot survive in unfavourable conditions because they do not store or reserve food. In the case of Rhizopus such spores are produced.
(d) Chlamydospores: These are multicellular, non-motile, thick-walled resting spores formed endogenously within the cell. Chlamyodospores are usually dark-colored, spherical, and have a smooth surface. The cells are connected by pores in septae between cells. These types of spores are found in Ascomycota and Basidiomycota.
(e) Akinete: During unfavorable conditions certain cells of the filamentous algae enlarge due to the storage of reserve food matter. They produce thick walls around them and behave as reproductive units called akinetes. It is a thick-walled dormant cell derived from the enlargement of a vegetative cell. It serves as a survival structure. It is a resting cell of cyanobacteria and unicellular, filamentous green algae. The akinetes are filled with food reserves and have a normal cell wall surrounded with the 3-layer coat.
(f) Hypnospore: They are thick-walled, rigid aplanospores. The hypnospores are filled with food reserves. In the case of Chlamydomonas, hypnospores are found.
(g) Hormospore: These spores are terminally bored in the hormogonium of some blue-green algae with cells modified in shape and having exceptionally thick walls.
(h) Homospore: These spores are equal-sized and formed in the sporangium of moss (Riccia) and fern (Lycopodium).
(i) Heterospore: There are two different types of sporangium where smaller and larger shaped spores are formed. Smaller spores are called microspore which germinates into the male gametophyte and the larger one is the megaspore which germinates into the female gametophyte. These types of spores are found in Selaginella.
(j) Ascopore: These spores are contained in an ascus. These kinds of spores are specific to fungi classified as ascomycetes. Typically, a single ascus will contain eight ascospores which are produced by meiosis followed by a mitotic division. Two meiotic divisions turn the original diploid zygote nucleus into four haploid ones. That is, the single original diploid cell from which the whole process begins contains two complete sets of chromosomes. In preparation for meiosis, all the DNA of both sets is duplicated, to make a total of four sets. The nucleus that contains the four sets divides twice, separating into four new nuclei. each of which has one complete set of chromosomes. Following this process, each of the four new nuclei duplicates its DNA and undergoes a division by mitosis. As a result, the ascus will contain four pairs of spores.
(k) Basidiospores: A basidiospore is a reproductive spore produced by Basidiomycete fungi. Each basidiospore typically contains one haploid nucleus that is the product of meiosis, and they are produced by specialized fungal cells called basidia. In gills under a cap of one common species in the phylum of Basidiomycota, there exist millions of basidia. The mature state of basidia has the base usually topped with four basidiospores which contain one of the two haploid nuclei obtained from the process of meiosis. Because of this, a single mushroom has the ability to release a billion spores. Most basidiospores are forcibly discharged and are thus considered ballistospores.
Name and Occurence of Different Asexual Spores:
| Name of Asexual Spores | Occurrence |
| 1. Zoospore | Chlamydomonas, Volvox (Algae), Synchitrium (Fungi) |
| 2. Endospore | Dermocarpa (Algae), Bacillus (Bacteria) |
| 3. Oidia | Mucor (Fungi) |
| 4. Sporangiospore | Rhizopus, Mucor (Fungi) |
| 5. Conidia | Penicillium (Fungi) |
| 6. Homospore | Lycopodium (Pteridophytes) |
| 7. Synzoospore | Vaucheria (Algae) |
| 8. Carpospore | Polysiphonia (Red Algae) |
| 9. Tetraspore | Polysiphonia (Red Algae) |
| 10. Neutralspore | Porphyra (Red Algae) |
| 11. Hypnospore | Chlamydomonas (Algae) |
| 12. Blastospore | Cladosporium (Fungi) |
| 13. Chlamydospore | Mucor (Fungi) |
| 14. Arthrospore | Geofrichum (Fungi) |
| 15. Ascospore | Ascobolus (Fungi) |
| 16. Basidiospore | Agaricus (Fungi) |
Differences between Zoospore and Zygote:
| Zoospore | Zygote |
| 1. Due to asexual reproduction zoospore is formed. | 1. Due to sexual reproduction zygote is formed. |
| 2. The chromosome number of zoospores is usually haploid. | 2. The chromosome number of the zygote is diploid. |
| 3. It occurs inside the zoosporangium and is motile in nature. | 3. By the union of two gametes, the zygote is formed and is non-motile in nature. |
| 4. After germination, if gives birth to a new offspring. | 4. Developing zygote-formed embryo which gives birth to a new offspring. |
Differences between Aplanospore and Zoospore:
| Aplanospore | Zoospore |
| 1. These are non-motile, unicellular endogenously produced spores. | 1. These are unicellular, flagellate motile spores, which are endogenously produced. |
| 2. Locomotary structure absent so dispersed by the help of agents. | 2. Locomotary organ flagella are present, so, they are able to move. |
| 3. Inside the sporangium, spores are produced. | 3. Inside the zoosporangium, spores are produced. |
| 4. In the case of Mucor, aplanospores are produced. | 4. In the case of Synchitrium, zoospores are produced. |
| 5. Capable of enduring harsh environmental conditions. | 5. Incapable of enduring harsh environmental conditions. |
Importance or Advantages of Asexual Reproduction
- Asexual reproduction is capable of producing new individuals in a short time.
- There is no opportunity for variation among the offspring.
- It is a simple and easy method of reproduction.
- Only one individual is required for this process.
- Regarding characters, offsprings resemble the parents.
- It is a safe reproductive process in adverse situations.
Disadvantages of Asexual Reproduction
- It never produces new varieties.
- There is no possibility of variation among the offspring. As a result, all the fixed characters are transmitted to the organism generation after generation.
- Due to the presence of the same characters in the offspring, they are incapable of adapting themselves to the changed environment. So, there is a chance of extinction of the species in the future.
Differences between Vegetative and Asexual Reproduction:
| Vegetative Reproduction | Asexual Reproduction |
| 1. It is mainly found in plants. | 1. It is found in both plants and animals. |
| 2. Any part of the body can produce a new individual. | 2. Specific parts of the body can produce new individuals. |
| 3. Vegetative reproduction takes place simply by cell division. | 3. Asexual reproduction takes place through the formation of different spores. |
| 4. Generally, the meiotic division does not take place in vegetative reproduction. | 4. In a few cases, the meiotic cell division is found in plants before spore formation. |
| 5. Vegetative reproduction also takes place by artificial methods. | 5. Asexual reproduction does not take place by artificial methods. |
4. Explore How Regeneration in Planaria Occurs
In some organisms (plants as well as animals) small cut parts of their body can grow (or regenerate) to form whole new organisms complete in all respects. The process of getting back a full organism from its body parts is called regeneration. Simple animals like Hydra and Planaria show regeneration. This means that in these organisms, whole new organisms can be reproduced from their cut body parts. In other words, if Hydra or Planaria somehow get cut into a number of pieces, then each body piece can grow into a complete organism. This point will become clear from the following example.
Planaria is a flatworm that is found in freshwater ponds and slow-moving streams. Planaria possesses great power of regeneration. If the body of Planaria somehow gets cut into a number of pieces, then each body piece can regenerate into a complete Planaria by growing all the missing parts. This is shown in Figure.
Regeneration in Planaria
Figure(a) shows one Planaria worm. This Planaria worm somehow gets cut into three pieces [see Figure(b)], After a certain time, each cut piece of the body of the Planaria worm grows into a complete Planaria worm [see Figure(c)]. In this way, three Planaria worms are produced from just one Planaria worm. Similarly, if the body of a Hydra gets cut into a number of pieces, then each body piece of the Hydra can grow into a complete Hydra. Please note that regeneration is not exactly the same as reproduction because most simple animals would not depend on being cut into pieces to be able to reproduce.
The regeneration of an organism from its cut body part occurs by the process of growth and development. This happens as follows: The cells of the cut body part of the organism divide rapidly to make a ‘ball of cells. The cells present in the ‘ball of cells’ move to their proper places within the ball where they have to form various organs and body parts of the organism. The cells then change their shapes (or become specialized) to form different types of tissues. These different tissues form various organs and body parts of the organism. In this way, a complete organism is regenerated.
This type of reproduction is found in Planaria, Hydra, and flatworms. If the body of the animal is fragmented by any means into pieces, each of the fragmented parts develops into a new animal through cell division.
Regeneration is divided into two following types:
- Repairative Regeneration: In this type of regeneration, only damaged tissue can be regenerated.
- Restorative Regeneration: In this regeneration some body parts can be reformed or a complete body develops from a body part.
5. Fragmentation in Plants
In multi-cellular organisms with relatively simple body organization, simple reproductive methods can still work. Spirogyra, for example, simply breaks up into smaller pieces upon maturation. These pieces or fragments grow into new individuals. This is not true for all multi-cellular organisms. They cannot simply divide cell by cell. The reason is that many multi-cellular organisms, as we have seen, are not simply a random collection of cells. Specialized cells are organized as tissues and tissues are organized into organs, which then have to be placed at definite positions in the body. In such a carefully organized situation, the cell-by-cell division would be impractical. Multi-cellular organisms, therefore, need to use more complex ways of reproduction.
A basic strategy used in multi-cellular organisms is that different cell types perform different specialized functions. Following this general pattern, reproduction in such organisms is also the function of a specific cell type. How is reproduction to be achieved from a single cell type, if the organism itself consists of many cell types? The answer is that there must be a single cell type in the organism that is capable of growing, proliferating and making other cell types under the right circumstances.
Spirogyra (a filament-type green alga plant) reproduces by the method of fragmentation.
Spirogyra as seen under the microscope.
We can study Spirogyra in the laboratory as follows: Collect some water from a pond (or lake) that appears dark green and contains long filament-type (thread-type) structures. Take out the green-colored mass from the pond water sample and separate its threads or filaments by using two needles. Place one filament on a clean slide, put a drop of glycerine over it, and cover it with a coverslip. Keep this slide under the microscope and see it first under the low power and then under the high power microscope. Observe the detailed structure of the green filament of Spirogyra and draw a diagram accordingly.
Please note that the main difference between fission and fragmentation is that in fission, a unicellular organism breaks up to form two (or more) daughter organisms, whereas in fragmentation, a multicellular organism breaks up to form two (or more) daughter organisms.
In this method, the body of the adult organism breaks apart into two or more pieces, each of which then grows and reforms the deficient parts to reconstitute a complete animal. It is found in sponges, coelenterates, and echinoderms such as starfish.
Fragmentation is also found in algae (Spirogyra), fungi (Rhizopus), bryophytes (Riccia, Marchantia), pteridophytes (Selaginella rupestris), etc.
6. Vegetative Propagation in Plants and its Types
Vegetative propagation is an asexual method of reproduction. The reproduction by vegetative propagation occurs only in plants. In vegetative propagation, new plants are obtained from the parts of old plants (like stems, roots, and leaves), without the help of any reproductive organs. Vegetative propagation usually involves the growth and development of one (or more) buds present on the old part of the plant to form a new plant. These buds are in the dormant state (inactive state) in the old part of the plant. When provided with suitable conditions (like moisture, warmth, etc.), these buds grow to form new plants. Please note that vegetative propagation is also called vegetative reproduction. Here is an example of vegetative propagation (or vegetative reproduction) in the grass.
It is a common observation that green grass plants spring up in dry fields after the rains. This happens due to vegetative propagation as follows: The fields have dry stems of the old grass plants all over them. These dry stems have buds that are in an inactive state. By getting rainwater, the buds present on dry grass stems get activated and grow to produce new grass plants. Thus, the green grass grows in the fields after rains from the dry, old stems of grass plants present in the fields, by the method of vegetative propagation.
Buds are present on the stems as well as the leaves of the Bryophyllum plant which can develop into new plants. So, Bryophyllum plants can be reproduced by vegetative propagation by using either a piece of its stem or its leaves. For example, if we plant a broken piece of the stem of a Bryophyllum plant in the ground, we will get a new Bryophyllum plant growing from it in a week’s time. Even the leaves of a Bryophyllum plant can produce new plants. This happens as follows: The leaves of a Bryophyllum plant have a special type of buds in their margins (or edges) [see Figure(a)], These buds may get detached from the leaves, fall to the ground and then grow to produce new Bryophyllum plants.
The buds can also drop to the ground together with the leaf and then grow to produce new plants. Sometimes even before a leaf drops off from a Bryophyllum plant, we can see new plantlets already growing on it [see Figure(b)], When such a mature leaf of the Bryophyllum plant falls on the ground, then each plantlet can grow into a new plant. Thus, the leaves of the Bryophyllum plant can produce new plants. Another plant called Begonia also reproduces by vegetative propagation through its leaves.
(a) A leaf of Bryophyllum plant with buds in its margin (or edges)
(b) Plantlets growing in the margin of Bryophyllum leaf
Money plant can also be grown by vegetative propagation by using a piece of its stem as follows: Cut a piece of the stem of a money plant in such a way that it contains at least one leaf on it (The point on a stem where a leaf is attached is called a node). Dip one end of this stem in water. After a few days, we will find that new roots appear at the point where the leaf was attached. The piece of stem will gradually grow into a new money plant. Please note that if we cut the stem of the money plant in between two leaves, then it will not grow into a new plant. This is because it does not have a growing point (here a node) in it.
We will now describe the vegetative propagation of plants by using tubers which are the modified, underground stems (or roots). A tuber is the thickened, underground stem (or root) of a plant that is swollen with stored food. The tuber has a number of ‘buds’ (called ‘eyes’). Each bud (or eye) of the tuber grows into a new plant when the old tuber is planted in the soil in the next growing season. There are two types of tubers: stem tubers and root tubers. Potato is a stem tuber whereas sweet potato is a root tuber. We will now describe how vegetative reproduction in potatoes takes place by using tubers.
The potato tuber is an underground stem of the potato plant. Potato tuber can be used for the vegetative reproduction of potato plants. Each potato tuber can produce more than one plant. This happens as follows: A potato tuber has many buds (called eyes) on its body (see Figure). These buds act as organs for vegetative reproduction. When a potato tuber is planted in the soil, then the various buds of the potato tuber start growing to form new potato plants. In Figure, we have shown the new potato plants growing from only two buds of the potato tuber. Other buds can also do the same.
Vegetative propagation (reproduction) of potato plant from a potato tuber. A new potato plant grows from each bud on the old potato.
A potato plant with tubers.
Please note that it is not necessary to plant the whole potato tuber in the ground to produce new potato plants. We can even plant ‘cut pieces’ of a potato tuber in the ground to obtain new potato plants. But all these cut pieces of potato tuber should have a bud (or eye) on them. So, if we cut a potato tuber into a number of pieces in such a way that every piece has a bud (or eye) on it and plant them in the ground, then each cut piece of potato tuber will produce a new potato plant in due course of time. Each potato plant produces more than one tuber and each tuber has more than one bud (which produces more than one new plant). Due to this, the vegetative propagation method of producing potato plants by tubers is much faster than the production of potato plants from seeds.
We can study the vegetative propagation of potatoes as follows: Take a potato and cut it into small pieces in such a way that some pieces contain a bud (or eye) in them. Place the potato pieces having buds on wet cotton kept in a tray. Keep the tray aside for a few days (but sprinkle water on the cotton daily to keep it wet). We will see that green shoots and roots appear from the buds of the potato pieces. These are the new potato plants. If, however, we take potato pieces without buds in this experiment, then no new potato plants will grow from them.
The roots of a guava plant have buds that can develop into new guava plants. In fact, a large number of plants can be reproduced by the method of vegetative propagation. Some examples of plants that can be reproduced by vegetative propagation are Bryophyllum, Guava, Potato, Onion, Banana, Garlic, Water hyacinth, Tulip, Mint, Strawberry, and Lily. We will now describe the artificial propagation of plants.
The bud-like organ is called a gemmule which is formed inside the body of some sponge. These gemmules after maturation detach from the mother’s body and develop into new individuals. There are small groups of cells i.e., archaeocytes enclosed by a protective coat. Archaeocytes come out through micropyle during favourable conditions and later form a new colony. In the case of liverworts, gemmae develop in small receptacles known as gemma cups.