ASEXUAL REPRODUCTION: CHARACTERISTICS AND TYPES - BIOLOGY - 2024

The asexual reproduction is defined as the multiplication of an individual capable of causing seed without fertilization. Therefore, daughter organisms consist of clones of the parent.

Children produced by asexual reproductive events are assumed to be identical copies of their parents. However, it must be remembered that the copy of the genetic material is subject to changes called "mutations".

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Asexual reproduction is predominant in single-celled organisms, such as bacteria and protists. In most cases, one stem cell gives rise to two daughter cells, in an event called binary fission.

Although animals are usually associated with sexual reproduction and plants with asexual reproduction, it is a wrong relationship and in both lineages we find the two basic models of reproduction.

There are different mechanisms by which an organism can reproduce asexually. In animals, the main types are fragmentation, budding, and parthenogenesis.

In the case of plants, asexual reproduction is characterized by being extremely varied, since these organisms enjoy great plasticity. They can reproduce, by cuttings, rhizomes, cuttings and even by portions of leaves and roots.

Asexual reproduction has a number of advantages. It is fast and efficient, allowing the colonization of environments in a relatively short time. Plus, you don't need to spend time and energy fighting for sexual partners or complex and elaborate courtship dances.

However, its main disadvantage is the lack of genetic variability, which is a sine qua non condition for the mechanisms responsible for biological evolution to act.

The lack of variability in a species can lead to its extinction if they have to face unfavorable conditions, be it pests or extreme climates. Therefore, asexual reproduction is understood as an alternative adaptation in response to conditions that demand uniform populations.

General characteristics

Sexual reproduction occurs when an individual produces new organisms from somatic structures. The offspring are genetically identical to the parent in all aspects of the genome, except in regions that have undergone somatic mutations.

Different terms are used to refer to the production of new individuals starting from somatic tissue or cells. In the literature, sexual reproduction is synonymous with clonal reproduction.

For animals, the term agamic reproduction (from the English agametic reproduction) is often used, while in plants it is common to use the expression vegetative reproduction.

A vast number of organisms reproduce throughout their lives through sexual reproduction. Depending on the group and the environmental conditions, the organism can reproduce exclusively asexually or alternate it with sexual reproduction events.

Asexual reproduction in animals (types)

In animals, offspring can come from a single parent through mitotic divisions (asexual reproduction) or it can occur through the fertilization of two gametes from two different individuals (sexual reproduction).

Various groups of animals can reproduce asexually, predominantly groups of invertebrates. The most important types of asexual reproduction in animals are the following:

Gemmation

Budding consists of the formation of a bulge or discharge from the parental individual. This structure is called the yolk and will give rise to a new organism.

This process occurs in certain cnidarians (jellyfish and related) and tunicates where offspring can be produced by protrusions on the body of the parents. The individual can grow up and become independent or become attached to its parent to form a colony.

There are colonies of cnidarians, the famous rocky corals, which can extend for more than a meter. These structures are made up of individuals formed by budding events, whose gemmules remained connected. Hydras are known for their ability to reproduce asexually by budding.

In the case of porifers (sponges) budding is a fairly common way to reproduce. Sponges can form gemmules to withstand times of unfavorable environmental conditions. However, sponges also exhibit sexual reproduction.

Fragmentation

Animals can divide their bodies in a process of fragmentation, where a piece can give rise to a new individual. This process is accompanied by regeneration, where the cells of the original parental portion divide to generate a complete body.

This phenomenon occurs in different lineages of invertebrates, such as sponges, cnidarians, annelids, polychaetes and tunicates.

Regeneration processes per se should not be confused with asexual reproduction events. For example, sponges when they lose one of the arms, can regenerate a new one. However, it does not imply reproduction since it does not lead to an increase in the number of individuals.

In starfish of the genus Linckia it is possible that a new individual originated from an arm. Thus, an organism with five arms can give rise to five new individuals.

Planarians (Turbellarians) are vermiform organisms with the ability to reproduce both sexually and asexually. A common experience in biology laboratories is to fragment a planarian to observe how a new organism regenerates from each piece.

Parthenogenesis in invertebrates

In some groups of invertebrates, such as insects and crustaceans, an ovum is capable of developing a complete individual, without the need to be fertilized by a sperm. This phenomenon is called parthenogenesis and is widespread in animals.

The clearest example is that of the Hymenopterans, specifically bees. These insects can give rise to males, called drones, through parthenogenesis. As individuals come from an unfertilized egg, they are haploid (they only have half the genetic load).

Aphids - another group of insects - can give rise to new individuals by processes of parthenogenesis or by sexual reproduction.

In the crustacean Daphnia the female produces different kinds of eggs depending on the environmental conditions. The eggs can be fertilized and give rise to a diploid individual or develop by parthenogenesis. The first case is associated with unfavorable environmental conditions, while parthenogenesis occurs in prosperous environments

In the laboratory, parthenogenesis can be induced by the application of chemicals or physical stimuli. In certain echinoderms and amphibians, this process has been carried out successfully and is called experimental parthenogenesis. In the same way, there is a bacterium of the genus Wolbachia capable of inducing the process.

Parthenogenesis in vertebrates

The phenomenon of parthenogenesis extends to the vertebrate lineage. In various genera of fish, amphibians, and reptiles, a more complex form of this process occurs, involving duplication of the chromosome set, leading to diploid zygotes without the participation of a male gamete.

Approximately 15 species of lizards are known for their unique ability to reproduce through parthenogenesis.

Although these reptiles do not directly need a partner to conceive (in fact, these species lack males), they require sexual stimuli from false copulation and courtship sessions with other individuals.

Androgenesis and Gynogenesis

In the androgenesis process, the nucleus from the oocyte degenerates and is replaced by the nucleus from the father through nuclear fusion from two sperm. Although it occurs in some animal species, such as stick insects for example, it is not considered a common process in that kingdom.

On the other hand, gynogenesis consists of the production of new organisms by diploid oocytes (female sex cells) that did not undergo division of their genetic material by meiosis.

Remember that our sex cells only have half of the chromosomes and when fertilization occurs the number of chromosomes is restored.

For gynogenesis to occur, stimulation from the male's sperm is necessary. The offspring product of gynogenesis are females identical to their mother. This pathway is also known as pseudogamy.

Asexual reproduction in plants (types)

In plants there is a wide spectrum of modes of reproduction. They are highly plastic organisms and it is not unusual to find plants that can reproduce sexually and asexually.

However, many species have been found to prefer the asexual route of reproduction, even though their ancestors did so sexually.

In the case of asexual reproduction, plants can generate offspring in different ways, from the development of an unfertilized egg cell to the obtaining of a complete organism by a fragment of the parent.

As in the case of animals, sexual reproduction takes place by events of cell division by mitosis, which result in identical cells. Below we will discuss the most relevant types of vegetative reproduction:

Stolons

Some plants are capable of reproducing on slender, elongated stems that originate along the surface of the soil. These structures are known as stolons and generate roots at spaced intervals. The roots can generate erect stems that eventually develop into independent individuals.

An outstanding example is the strawberry or strawberry species (Fragaria ananassa) that is capable of generating various structures, including leaves, roots and stems of each node of a stolon.

Rhizomes

Both in the case of stolons and rhizomes, the axillary buds of plants can generate a specialized shoot for asexual reproduction. The mother plant represents a reserve source for the shoots.

Rhizomes are indefinitely growing stems that grow under the ground - or above - horizontally. Like stolons, they produce adventitious roots, which will generate a new plant identical to the mother one.

This type of vegetative reproduction is important in the group of grasses (where rhizomes lead to the formation of buds that give rise to stems with leaves and flowers), ornamental perennials, pastures, reeds and bamboos.

Cuttings

Cuttings are bits or pieces of a stem from which a new plant originates. For this event to occur, the stem must be buried in the ground to avoid desiccation and can be treated with hormones that stimulate the growth of adventitious roots.

In other cases, the piece of stem is placed in water to stimulate root formation. After it is transferred to a suitable environment, a new individual can develop.

Grafts

Plants can reproduce by inserting a bud into a previously made cleft in a stem of a woody plant that has roots.

When the procedure is successful, the wound is closed, and the stem is viable. Colloquially it is said that the plant "caught".

Leaves and roots

There are some species where the leaves can be used as structures for vegetative reproduction. The species popularly known as "maternity plant" (Kalanchoe daigremontiana) can generate plants separated from the meristematic tissue located on the edge of their leaves.

These small plants grow attached to the leaves, until they are mature enough to separate from their mother. When the daughter plant falls to the ground it takes root.

In cherry, apple and raspberry, reproduction can occur through the roots. These subterranean structures produce shoots capable of originating new individuals.

There are extreme cases like the dandelion. If someone tries to pull the plant out of the ground and fragment its roots, each of the pieces can give rise to a new plant.

Sporulation

Sporulation occurs in a wide range of plant organisms, including mosses and ferns. The process consists of the formation of a significant number of spores that are capable of withstanding adverse environmental conditions.

Spores are small elements that are easily dispersed, either by animals or by the wind. When they reach a favorable zone, the spore develops in an individual equal to the one that originated it.

Propagules

Propagules are accumulations of cells, typical of bryophytes and ferns but are also found in certain higher plants such as tubers and grasses. These structures come from the thallus, and are small buds with the ability to spread.

Parthenogenesis and apomixis

In botany, it is also often applied in the term parthenogenesis. Although it is used in a narrower sense to describe an event of "gametophytic apomixis". In this case, a sporophyte (the seed) is produced by a cell of an ovule that does not undergo reduction.

Apoxymysis is present in approximately 400 species of angiosperms, while other plants can do so facultatively. Thus, parthenogenesis describes only a part of asexual reproduction in plants. Therefore, it is suggested to avoid the use of the term to plants.

Some authors (see De Meeûs et al. 2007) tend to divide apomixis from vegetative reproduction. Furthermore, they classify apomixis as the gametophytic one already described, and it comes from the sporophyte, where the embryo develops from a nuclear cell or other somatic tissue of the ovary that does not undergo the gametophytic phase.

Advantages of asexual reproduction in plants

In general, asexual reproduction allows the plant to reproduce in identical copies that are well adapted to that particular environment.

Furthermore, asexual reproduction in silvers is a fast and efficient mechanism. For this reason, it is used as a strategy when the organism is in areas where the environments are not very suitable for reproduction by seed.

For example, plants located in arid environments in Patagonia, such as chorions, reproduce in this way, occupying large areas of soil.

On the other hand, farmers have made the most of this type of propagation. They can select a variety and reproduce it asexually to obtain clones. Thus, they will obtain genetic uniformity and allow them to retain some desired characteristic.

Asexual reproduction in microorganisms (types)

Asexual reproduction is very common in single-celled organisms. In prokaryotic lineages, for example bacteria, the most prominent are binary fission, budding, fragmentation and multiple fission. On the other hand, in unicellular eukaryotic organisms there is binary division and sporulation.

Binary fission in bacteria

Binary fission is a process of division of the genetic material, followed by the equitable division of the interior of the cell to obtain two organisms identical to the parent and identical to each other.

Binary fission begins when the bacteria are in an environment where there are enough nutrients and the environment is conducive to reproduction. The cell then experiences a slight elongation event.

Later, the replication of the genetic material begins. In bacteria, DNA is organized on a circular chromosome and is not bounded by a membrane, like the conspicuous and distinctive nucleus in eukaryotes.

In the period of division genetic material is distributed to opposite sides of the dividing cell. At this point, the synthesis of the polysaccharides that form the bacterial wall begins, then the formation of a septum in the middle occurs, and the cell finally separates completely.

In some cases, bacteria can begin to divide and duplicate their genetic material. However, the cells never separate. Examples of this are the clusters of cocci, such as diplococci.

Binary fission in eukaryotes

In unicellular eukaryotes, like Trypanosoma for example, a similar type of reproduction occurs: one cell gives rise to two daughter cells of similar sizes.

Due to the presence of a true cell nucleus, this process becomes more complex and elaborate. A process of mitosis must occur for the nucleus to divide, followed by cytokinesis, which comprises the division of the cytoplasm.

Multiple fission

Although binary fission is the most common reproductive modality, some species, such as Bdellovibrio ¸, are capable of experiencing multiple fissions. The result of this process is multiple daughter cells, and no longer two, as mentioned in binary fission.

Gemmation

It is a process similar to that mentioned for animals, but extrapolated to a single cell. Bacterial budding begins with a small bud that differs from the parent cell. This bulge undergoes a growth process until it gradually separates from the bacteria that originated it.

Budding results in uneven distribution of the material contained in the cell.

Fragmentation

Generally, bacteria of the filamentous type (for example Nicardia sp.) Can reproduce through this route. The cells of the filament separate and begin to grow as new cells.

Sporulation

Sporulation consists of the production of structures called spores. These are highly resistant structures made up of a cell.

This process is linked to the environmental conditions that surround the organism, generally when these become unfavorable due to the lack of nutrients or extreme climates, sporulation is triggered.

Differences between sexual and asexual reproduction

In individuals that reproduce asexually, the offspring consist of virtually identical copies of their parents, that is, clones. The genome of the only parent is copied by mitotic cell divisions, where the DNA is copied and transmitted in equal parts to the two daughter cells.

In contrast, for sexual reproduction to occur, two individuals of opposite sex must participate, with the exception of hermaphrodites.

Each of the parents will carry a gamete or sex cells generated by meiotic events. The offspring consist of unique combinations between both parents. In other words, there is remarkable genetic variation.

To understand the high levels of variation in sexual reproduction, we must focus on the chromosomes during division. These structures are capable of exchanging fragments with each other, leading to unique combinations. Therefore, when we observe siblings from the same parents, they are not identical to each other.

Advantages of asexual versus sexual reproduction

Asexual reproduction has several advantages over sexual reproduction. First, no time and energy is wasted in the complex courtship dances or fights for the female typical of some species, since only one parent is needed.

Second, many individuals who reproduce sexually expend a lot of energy in the production of gametes that are never fertilized. This allows you to colonize new environments quickly and efficiently without the need to find a mate.

Theoretically, the asexual reproduction models mentioned above give more advantages - compared to sexual - to individuals living in stable environments, since they can perpetuate their genotypes in a precise way.

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