- What is asexual reproduction?
- Types of asexual reproduction
- Advantages of asexual reproduction
- Disadvantages of asexual reproduction
- Clones and cloning
We explain what asexual reproduction is, the types that exist, their advantages and disadvantages. Also, what is cloning.
Some plants like the dandelion reproduce asexually through their seeds.
What is asexual reproduction?
Asexual reproduction is one that requires a single organism, which does not need to mate to form new individuals. Since there is no intervention of sex cells, in the sexual reproduction there is no exchange or combination of Genetic information.
When an organism reproduces asexually, it does so through methods which consist of the replication or duplication of its genetic content, to give rise to new individuals genetically identical to itself.
Reproduction consists of the production of new individuals of the same species as the progenitor, which allows multiplying and perpetuating the species. Reproduction constitutes one of the main stages in the Lifecycle of everything living being and, although it is not essential for an individual to survive, it is essential for a species to remain in the Earth.
Organisms can reproduce in various ways, which can be grouped into two types of reproduction: sexual or asexual, depending on the number of individuals involved and whether the offspring are genetically identical to the parental organism or organisms.
Sexual reproduction like that of Humans, involves sexual contact between two individuals, a female and a male, each of which contributes a gamete or sex cell. The union between the female and male gametes (ovum and sperm respectively) gives rise to the embryo, which when developing will form a new individual of the same species, whose genetic material will result from the combination of those of its parents. Thus, in sexual reproduction, each parent provides half of the genetic information, and offspring are genetically different from their parents.
Asexual reproduction is typical of single-celled organisms, Like the prokaryotes Y protists, and is common in mushrooms, the invertebrates Y plants. While in the most complex forms of life, sexual reproduction is usually more frequent, there are also some specific cases of animals that reproduce asexually.
Types of asexual reproduction
Many plants can create a new individual from a fragment.
Asexual reproduction can occur through different mechanisms, among which are the following:
- Gemmation. It consists of the production of bumps or bud formations in the body of the parent itself, from which an independent individual emerges, capable of detaching and living independently, or of remaining attached and starting a colony. Budding is a frequent process in porifers, cnidarians, and bryozoans. In addition, some single-celled organisms, such as yeast and some bacteria, reproduce by this method.
- Fragmentation. It consists of the production of new individuals from fragments of the parent's body, thus rebuilding the entire body from a significant piece of it. These fragmentations can be intentional or accidental. Fragmentation is an asexual reproduction mechanism present in many invertebrates, such as starfish, starfish, and planarians. In addition to animals, there are plants that can reproduce by the fragmentation mechanism, directed by human intervention, and which is better known as “artificial vegetative multiplication”.
It is important not to confuse the processes of regeneration by fragmentation with those of asexual reproduction. For example, some lizards are capable of regenerating their tail if they accidentally lose it, but this phenomenon does not imply reproduction since it does not lead to the appearance of new individuals.
- Binary fission. It is the simplest asexual reproduction mechanism and consists of the duplication of genetic material (molecules of DNA) of the parent, followed by the division of its organelles and finally the excision of the cytoplasm, thus obtaining two cells identical where before there was only one. Binary fission is carried out by prokaryotic organisms, which include archaea bacteria. There are also some unicellular eukaryotic organisms that reproduce by a similar mechanism: one cell gives rise to two identical daughter cells of similar sizes. However, in these organisms the presence of a cell nucleus true makes the process a bit more complex and elaborate.
- Sporulation. It consists of reproduction by means of resistant, unicellular structures, capable of resisting extreme conditions, called spores or endospores.Sporulation can be part of the normal life cycle of the organism or, in some cases, be favored or triggered by unfavorable environmental circumstances. The sporulation mechanism is a form of cell division common in fungi, plants and a certain kind of bacteria.
- Apomixis. This mechanism is unique to plants and consists of a form of asexual reproduction by means of seeds, which does not imply fertilization or meiosis. In plants that reproduce by this method, the individual produces seeds that are genetically identical to itself, which allow the species to be extended, but have little adaptability to the environment. There are different types of apomixis in the plant kingdom and it is a fairly frequent type of asexual reproduction in this group of living beings.
- Parthenogenesis. This mode of asexual reproduction involves the development of unfertilized female sex cells, that is to say, possessing the same genetic material as their progenitor, through a segmentation of the unfertilized ovum. This asexual reproduction mechanism is present in groups of invertebrates as well as in vertebrates: it is a usual procedure in certain fish, reptiles, insects, crustaceans Y amphibians, especially in times of risk for the species.
- Polyembryony. It consists of a mode of reproduction in which two or more embryos develop from a single zygote. In reality, it can be said that it constitutes a combination of sexual and asexual reproduction: the first is necessary for fertilization and formation of the zygote, and the second takes place when the embryo divides into several genetically identical, and gives rise to two or more individuals genetically identical to each other, but different from their parents. Depending on the number of embryos generated, polyembryony can be single or multiple. This mode of reproduction is frequent in certain insects, plants and, curiously, in armadillos, whose litter is always monozygotic (it comes from the same embryo). It can also occur in humans, as it happens in univitelline or identical twins, which come from the same zygote (and should not be confused with dizygotic twins).
Advantages of asexual reproduction
Asexual reproduction like binary fission requires very few resources.
Asexual reproduction is quick and simple, since it does not need the production of specialized cells (gametes), nor does it require spending Energy to achieve fertilization, or other similar efforts. Thus, this type of reproduction allows an isolated individual to perfectly give rise to new descendants, sometimes many of them, although always genetically identical to himself and to each other.
This is particularly useful in situations of biological risk or in need of rapid expansion, for example, during the colonization of a territory or the massification of specimens in the face of imminent danger.
Disadvantages of asexual reproduction
The great disadvantage of asexual reproduction is its absence of genetic variability, that is, the fact that the descendants are identical to the parent, except in the case of mutations unforeseen.
Thus, the species evolves at a much slower and less effective rate since the natural selection it cannot favor the fittest individuals. This could kill a colony or even a species very quickly, since its lower genetic variability can prevent a rapid adaptation to a changing environment.
Clones and cloning
Human cloning was banned by UNESCO in 1997.
In genetics, a clone is defined as a set of genetically identical individuals, coming from another individual through asexual reproduction mechanisms. Although these processes are very frequent in the nature (in fact, asexual reproduction long predates sexual), the term clone was created in 1903 by H. J. Weber, with the intention of contributing to the development of the lexicon of the genetics, science that was beginning to develop at that time. At present, asexual reproduction can be called clonal reproduction, although it is not widely used.
The cloning, which derives from the term clone, is the action of producing a biological entity that is genetically identical to another, from an existing one. Although this process can be carried out without greater technical knowledge (for example, when doing vegetative multiplication of plants), when talking about cloning it is usually done more in reference to the artificial techniques used in a laboratory to produce genetically identical individuals.
In the case of vertebrates, artificial cloning is based on removing the nucleus of an ovule and replacing it with that of an adult cell belonging to the individual to be cloned. Then, this modified ovum (which is now equivalent to a viable zygote) is transferred to the body of a female where it will continue its development until its birth. This technique began to be applied in frogs in 1952, but it was only successful in mammals in 1996 with the famous Dolly the sheep.
From a practical point of view, cloning in humans should not have insurmountable technical obstacles in the long term. However, the possibility of using the technique in our species, which is called "reproductive cloning" has given rise to an intense ethical, religious, social and political debate in which multiple actors participate and which is still far from being resolved.