Reproduction is the process of producing offspring. It is of two types; asexual and sexual reproduction. In sexual reproduction, two egg cells (dual parents) are fused to form a zygote, whereas in asexual reproduction single parent is only required to produce a new offspring.
Asexual reproduction is the mode of reproduction where a single parent gives rise to their offspring without fusing gametes. Thus, produced offspring in an asexual way are genetically alike and formed rapidly within a short period.
This type of asexual reproduction mostly takes place in unicellular organisms. The organisms that undergo asexual reproduction include; bacteria, Archea, plants, fungi, and certain animals. There are different types of asexual reproduction. These include; binary fission, budding, fragmentation, vegetative propagation, sporogenesis, parthenogenesis, and apomixis.
Types of Asexual Reproduction with Examples
Another type of asexual reproduction is binary fission, where an organism splits or divides in such a plane, producing two identical microorganisms. It mainly occurs in protozoans like Amoeba, Paramecium, Vorticella, and Euglena and lowers metazoans like coelenterates, turbellarians, and annelids.
Types of Binary Fission
- Simple or orthodox binary fission; occurs in irregularly shaped organisms like Amoeba, where the division plane is difficult to observe.
- Transverse binary fission; occurs in Paramecium, coelenterates, turbellarians, and annelids. Here, the plane of the division is always transverse to the longitudinal axis of the body of the organisms.
- Longitudinal binary fission; occurs in certain ciliates and flagellates like Vorticella and Euglena. Here, the nucleus and the cytoplasm divide in the longitudinal plane.
- Oblique binary fission; occurs in most dinoflagellates where the division occurs obliquely.
- Strobilation; occurs in metazoan animals like scyphozoan (Aurelia), certain polychaetes, and ascidians. It is a type of transverse fission that coincides, giving rise to several offsprings that do not immediately separate from each other.
Some multicellular animals like Hydra and certain tunicates reproduce asexually by forming a bud called budding. A bud is a small outgrowth in the parent body. The bud is supplied with food from the parent’s body and gets detached from the parent when fully developed. The process of bud development into a fully grown adult organism is also called blastogenesis. It also occurs in unicellular organisms like bacteria, yeast, protozoans, and metazoans. Endodyogeny or internal budding occurs in parasites like Toxoplasma gondii where a mother cell produces two daughter cells.
Fragmentation and Regeneration
Fragmentation is splitting an organism into smaller fragments, and each fragment develops into an individual organism. Fragmentation occurs in cyanobacteria, fungi, plants, and animals like flatworms, sponges, annelid worms, and sea stars.
Regeneration is regenerating or repairing any single part of an organism. Many organisms carry this kind of ability. It is common in lizards (resurgence of tail) and octopuses (regeneration of blood vessels and tails).
Vegetative propagation is a type of asexual reproduction common in plants where a plant’s vegetative or non-reproductive part gives rise to new offspring. There are commonly two types of vegetative propagation; natural and artificial.
Natural vegetative propagation occurs when no human intervention occurs. Some examples of natural vegetative propagation are; bulbs (in onion, garlic, tulips), rhizomes (in turmeric and ginger), stolons (in strawberries and mints), corms (in gladiolus and taro), and plantlets (in kalanchoe).
Artificial vegetative propagation is performed in laboratories to produce new plants from a parent plant. There is various method of performing artificial vegetative propagation. Cutting, grafting, layering, suckering, and tissue culture are commonly used methods.
Sporogenesis is the type of asexual reproduction by the production of spores. Sporogenesis occurs in some eukaryotic and prokaryotic organisms like plants, bacteria, fungi, and algae. The reproductive spores forms in the reproductive structure called sporangia which has a sporogenous cell that undergoes cell division to give rise to spores.
In meiotic sporogenesis, the diploid mother cell within the sporangium undergoes meiosis, producing a tetrad of haploid spores. In the case of the formation of mitospores, mitotic cell division occurs inside a sporangium called conidia in some fungi like Aspergillus and Penicillium.
An unfertilized ovum develops into a new individual only after fertilization. An exception occurs when the development of the egg takes place without any fertilization, known as pathogenesis. It is a mode of asexual reproduction in an egg do not require fertilization for development. Parthenogenesis occurs in different groups of animals, as in certain insects like Hymenoptera, Homoptera, Coleoptera, crustaceans, rotifers, and some vertebrates like desert lizards, turkeys, and some mammals.
Types of Parthenogenesis
Natural parthenogenesis is of two types; complete and incomplete.
- Complete parthenogenesis is found in some species of earthworms, bdelloid rotifers, grasshoppers, roaches, phasmids, moths, gall flies, fishes, salamanders, and lizards.
- Some insects have generations, sexual and parthenogenetic generation, and occur alternatively. In this case, diploid eggs produce females, and unfertilized eggs produce males. This kind of reproduction is called incomplete parthenogenesis.
Both complete and incomplete parthenogenesis is of two types haploid or arrhenotokous and diploid parthenogenesis.
In arrhenotokous or haploid parthenogenesis, haploid eggs are not fertilized by the sperms and develop into haploid individuals. Here the haploid eggs become male; those fertilized diploid eggs become female. It occurs in bees, wasps, ticks, mites, and spiders.
In diploid or thelytokous parthenogenesis, young individuals develop from diploid eggs and can be either male or female. It is commonly seen in aphids.
The eggs that always develop into young individuals by fertilization can sometimes develop parthenogenetically under artificial conditions known as artificial parthenogenesis. The cause of artificial parthenogenesis can be either physical or chemical.
Physical means include temperature, electrical shocks, ultraviolet light, or any physical damage like pricking by fine glass needles.
Chemical means include the presence of chemicals like chloroform, strychnine, hypertonic and hypotonic sea waters, Chlorides of K+, Ca++, Na++, Mg++, etc., acids like butyric acid, lactic acid, oleic acid, and other fatty acids, fat solvents like toluene, ether, alcohol, benzene and acetone, urea, and sucrose.
Apomixis is the process of asexual reproduction in plants where seed formation occurs without meiosis and fertilization. The thus-formed seed then develops into the embryo. It is a substitution for sexual reproduction without involving nucleus and cell fusion. It is also termed agamospermy. It is classified into the following types; nonrecurrent, recurrent, vegetative, and adventive apomixis.
- Non Recurrent apomixis: The mother cell follows the meiotic cell divisions to form the haploid embryo. The new embryo can develop from the egg or another cell of the gametophyte. The latest offspring is partially identical to the mother, and the process does not repeat in the next generation.
- Recurrent apomixis: Here, the mother and daughter cell has the same number of chromosomes and can also repeat in other generations.
- Vegetative apomixis: A bulb or vegetative propagules develop in the flowers and germinate while still attached to a plant. It is common in Allium, Fargaria, Agave, and grasses.
- Adventive apomixis: The embryos arise from the nucleus cells or the integument. It is a type of sporophytic apomixis. It occurs in different species of Citrus like Garcinia, Euphoria, Magnifier indica, etc.
Verma, P. S., & Agarwal, V. K. (n.d.). Cell Biology, genetics, Molecular Biology, evolution and ecology: Evoloution and ecology.