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Parthenogenesis Definition
By definition, reproduction entails the generation of new children. The process is known as reproduction, and it is one of the characteristics of a living organism. It’s a way to keep a species alive.
Sexual and asexual reproduction are the two types of reproduction. Sexual reproduction is described in biology as a way of reproduction in which a gamete with just one pair of chromosomes (haploid) unites with the other gamete to produce a diploid progeny.
Animals, for example, generate a diploid zygote by combining a haploid egg cell (also known as an ovum) and a haploid sperm cell. In multicellular eukaryotes, including plants, fungi, and mammals, sexual reproduction is the most frequent mode of reproduction.
Asexually reproducing creatures, on the other hand, may not divide through meiosis to create gametes. Instead, they use a mechanism akin to mitosis to pass on their whole genome to their progeny. This can be observed in species that reproduce exclusively through asexual methods.
What is Parthenogenesis?
Parthenogenesis is a type of asexual reproduction in which a zygote is produced from an unfertilized egg by self-impregnation. As a result, many people refer to it as “virgin birth.” Both asexual animals and plants go through parthenogenesis. The embryo develops in animals from an unfertilized egg.
Apomixis is a component of the process in plants. Parthenogenesis is a process that occurs in many invertebrates, particularly scorpions, worms, mites, water fleas, wasps, some bees, and other insects.
Parthenogenesis has been seen in a variety of vertebrate creatures, including amphibians, fish, reptiles, and a few bird species. Sharks such as the blacktip shark, bonnethead shark, and zebra shark have been reported to undergo parthenogenesis. Parthenogenesis allows the zebra shark (Stegostoma fasciatum) to switch between sexual and asexual reproduction.
In the absence of a possible mate, they choose asexuality. Honeybees and ants are among the insects that may reproduce asexually through parthenogenesis. Water fleas are another example of a parthenogenetic invertebrate (daphnia of order Cladocera). Water fleas are tiny crustaceans, not insects.
They reproduce mostly by parthenogenesis with some sexual reproduction, depending on the environment. They reproduce sexually by laying resting eggs that can withstand severe environmental conditions. However, in ideal circumstances, they replicate asexually, creating female clones. When circumstances become unfavourable, they asexually generate male progeny to mate with females, allowing sexual reproduction to take place.
Parthenogenesis is a type of asexual reproduction in which the child grows from the egg or female gamete without the need for male gamete fertilisation. Because a zygote develops without the combination of female and male gametes, parthenogenesis is considered an asexual reproduction method.
Plants, invertebrates (such as water fleas, aphids, stick insects, certain ants, bees, and parasitic wasps), and some vertebrates use it to reproduce (such as some reptiles, amphibians, fish, and few birds).
Types of Parthenogenesis
Automixis or apomixis parthenogenesis are the two most frequent types of parthenogenesis. The egg cell unites with the polar body to form an embryo in automixis parthenogenesis. This union can cause the mother’s genes to move, resulting in novel alleles in the child that are related to but not identical to the mother.
The mother provides two X chromosomes to the offspring. As a result, only female progeny are produced through automixis parthenogenesis. Fertile men with only one X chromosome are extremely rare. Because their sperm cells solely have the X chromosome, they are fertile.
The egg cells do not undergo meiotic division in apomixis parthenogenesis, thus when an egg cell splits, it is a duplicate of the original cell in terms of the genes present. As a result, the child will be a complete clone of the mother.
Plants are the most common hosts of this type. This kind of parthenogenesis gives birth to identical kids who are genetically identical to their parents. Apomictic or automictic parthenogenesis are also possible. Apomictic parthenogenesis is a kind of parthenogenesis in which mature egg cells (which are diploid) generated by cell division grow straight into embryos. The children are exact clones of their mother.
The gametes in automictic parthenogenesis undergo meiosis and are hence haploid. Parthenogenesis can be voluntary or mandatory. The female reproduces either sexually or asexually in facultative parthenogenesis. Mayflies have the ability to go through facultative parthenogenesis. When there are no viable males in the environment, they go through parthenogenesis.
Obligate parthenogenesis occurs when an organism reproduces exclusively through asexual methods. Only a few reptile species (mostly lizards) are capable of obligatory parthenogenesis.
How Parthenogenesis Happens?
Egg cells are haploid cells generated by meiosis from ovaries. After meiosis, a precursor cell generates an ovum and three polar bodies (small byproducts of meiotic division). The polar bodies gradually degenerate during sexual reproduction. That’s what happens after meiosis in human egg cells.
The ovum may merge with the polar body in other species, as in the instance of automixis parthenogenesis (also known as meiotic parthenogenesis) mentioned in the preceding section. The second polar body unites with the egg.
Despite the fact that no sperm cell is involved in increasing gene variety, this type of parthenogenesis allows for some genetic variability among children. After meiosis, the egg cell is usually haploid, which means it has half the amount of chromosomes as the parent cell.
Parthenogenesis produces diploid or haploid offspring, depending on the process of parthenogenesis. The condition can be defined as follows based on the outcome of the offspring:
1. When an unfertilized egg grows parthenogenetically into a male, it is called arrhenotokous.
2. Thelytokous refers to the development of a female from an unfertilized egg.
3. Deuterotokous, a condition in which an unfertilized egg can grow into either a man or a female.
Some insects are capable of both sexual and asexual reproduction. When the conditions are good, they tend to reproduce asexually (a instance of facultative parthenogenesis or cyclic parthenogenesis), which allows for a fast rise in the number of progeny.
They reproduce sexually in adverse settings, which is necessary to generate a latent egg that can survive for a long period and hatch when the environmental conditions improve. In terms of increasing genetic variety, sexual reproduction can be helpful. As a result, having the capacity to reproduce sexually is beneficial, even if they already have the ability to reproduce asexually.
Some social insects use gametes to generate workforce and soldiers, whereas queens use parthenogenesis to reproduce asexually. Queens are created when sperm gates in eggs are closed, preventing sperm cells from entering. Other communities lack males, thus they never reproduce sexually and instead rely on obligatory parthenogenesis to reproduce asexually.
Advantages of Parthenogenesis
In the absence of a spouse, parthenogenesis is a form of self-reproduction in which the egg cells grow into children. As a result, parthenogenetic animals and plants may continue to reproduce their own species without having to spend time and effort finding a suitable spouse.
The woman can then utilise this time to look for food and shelter in areas where these are available. Aphids, for example, reproduce asexually through parthenogenesis in the summer because green leaves are plentiful and the days are longer.
Asexual parthenogenesis allows a population to expand twice as rapidly as a sexually reproducing population. Without the requirement for fertilisation, parthenogenesis allows for fast reproduction and population growth. Because parthenogenesis does not require the presence of a male, it is quicker and easier than sexual reproduction. In reality, parthenogenetic females may produce the same number of children as sexually reproduced females.
One person of a parthenogenetic species can create a colony without mating, generating female progeny that can develop and reproduce to generate a high number of offspring in a short amount of time. Furthermore, certain parthenogens can keep their capacity to integrate additional genes throughout the sexual reproduction cycle.
As a result, they may continue to evolve while also generating a large number of progeny. Parthenogenesis has less genetic variety than sexual reproduction. Parthenogenesis, on the other hand, may be advantageous in that it creates clones with the same genes for desirable characteristics as their parents. As a result, if the mother lives in a habitat to which she has adapted, her kids will inherit the same genes, ensuring their survival in that environment.
Human eggs are currently being investigated for parthenogenesis, which stimulates the egg to develop without fertilisation. This technology might aid researchers in developing a novel way to produce human stem cells from an unfertilized egg or perhaps cloning people.
Disadvantages of Parthenogenesis
Asexual reproduction is known as parthenogenesis. Since a result, it is marked by a lack of genetic variety, as the offspring receives all genetic material from a single parent. Due to a lack of genetic variety, the kids are predisposed to the same illnesses and disorders as their parents. Negative mutations, as well as undesirable characteristics, will be passed on across generations.
Likewise, because parthenogenesis generates clones of the parent, the offspring are unable to adapt or survive when the environment changes. As a result, parthenogenesis may result in a huge number of creatures that are unable to adapt to even minor changes in their environment.
Parthenogenesis Examples
Rotifers, daphnia, nematodes, aphids, and other invertebrates and plants all go through parthenogenesis on their own. Birds, snakes, sharks, and lizards are the only vertebrate animals that can breed strictly via parthenogenesis. Other amphibians, fish, and reptiles, on the other hand, can go through several stages of hybridogenesis, which is a type of incomplete parthenogenesis.
Other species, such as blacktip and hammerhead sharks, as well as the Komodo dragon, typically reproduce sexually; however, parthenogenesis may allow these species to breed asexually. In animals, parthenogenesis may be intentionally produced to generate animal clones, which are genetically identical to their mothers. The egg cell is artificially encouraged to enter mitosis in order to generate a new creature in this procedure.
Parthenogenesis in Reptiles
Many snake species breed sexually, while some have the ability to reproduce asexually. Aside from snakes, lizards are another type of reptile that reproduces asexually. Asexual reproduction in snakes and lizards can take the form of either facultative or obligatory parthenogenesis.
When a sexually reproducing reptile’s population is depleted of males, it undergoes facultative parthenogenesis. In this instance, their population is made up of individuals who may or may not be clones of the mother’s DNA, as they reproduce by mating when the chance arises.
Obligate parthenogenetic species, such as rock lizards and some geckos, reproduce only by parthenogenesis. The female egg in these animals develops into a new child without the help of the male in fertilisation. Only one snake species and around 50 lizard species reproduce through obligatory parthenogenesis. In reptiles, parthenogenesis can result in either complete clones with all of the mother’s genes or a half-clone with the reptile egg’s haploid genome.
Full clones are created when the genome of a female’s germ cell is doubled in a process known as pre-meiotic genome doubling, which occurs during the regular haploid egg production process. Two-division cycles of meiosis create a diploid egg rather than a haploid egg in this process, which occurs when two comparable sister chromosomes formed during premeiotic genome doubling are split during meiosis I.
Instead of the two homologous chromosomes that pair in sexual reproduction, identical chromosomes pair. During meiosis II, two identical sister chromatids are split. Full clones could also be found in both obligatory and facultative parthenotes, like Lacerta and therefore the Burmese python.
Half-clones are produced by other species. The majority of these species reproduce by facultative parthenogenesis. Between an egg cell and a haploid polar body, terminal fusion occurs. Meiosis produces the polar body as a byproduct.
The egg and polar body fuse to form a diploid nucleus, which grows into a diploid child. The formation of a diploid nucleus from the merger of two distinct cells is comparable to the fusing of an egg and sperm in sexual reproduction. The kid is a homozygote, meaning it inherits roughly half of its mother’s genetic variety.
This kind of parthenogenesis might result in either a female or a male offspring. Because the meiosis process occurs regularly in individuals generated by this kind of parthenogenesis, these animals, like certain snakes and Komodo dragons, can reproduce sexually or asexually.
Several Lizard species, such as the Caucasian rock lizards of the genus Lacerta, are parthenogenetic. Lacerta are genuine parthenotes, which means their egg is fertilised without the need of sperm. Another genus of whiptail lizards is the Teiid, which is the most well-known evolutionary form of parthenogenesis-reproducing reptiles.
Whiptails have a unique kind of parthenogenesis in which two females share reproduction; one is ovulating and fulfils the conventional function of an ovulating female, while the other participates in courting behaviour that is often seen in males.
There are six gecko species that breed via parthenogenesis, which are divided into five genera. This species reproduces by female-to-female mating. They can, however, generate male progeny, which is considered to be caused by hormonal inversion that is unrelated to genes. The men generated are anatomically normal, yet they are infertile because they produce defective sperm.
The brahminy blind snake is the only snake species that goes through obligatory parthenogenesis. Parthenogenesis-producing species can evolve because a triploid species created by parthenogenesis was intentionally impregnated by a sperm in a laboratory.The tetraploid creature that resulted gained additional genes and, as a result, new traits.
Parthenogenesis in Humans
Parthenogenesis occurs in certain animals, such as the whiptail lizard, when an egg develops without being fertilized by a sperm. As a result, there is no paternal inheritance, and the offspring’s genome is entirely inherited from the mother. Human parthenogenesis, as well as androgenetic processes, may occur spontaneously in humans.
They can, however, only develop malignancies like ovarian teratoma. The stimulation of the oocyte, meiosis, and genetic imprinting are all required for the creation of a parthenogenesis human. In mammals, parthenogenesis is not a frequent mode of reproduction.
The stimulation of the mammalian egg, which was subsequently activated and developed into an embryo through parthenogenesis, was used to artificially induce parthenogenesis in animals. Due to a lack of paternal genes, which are important for the development of a proper placenta, the resultant embryo will not grow further.
Creating a completely viable animal through parthenogenesis, on the other hand, is feasible thanks to genetic modification technology, which manipulates the genome of the animal generated from an activated oocyte.
In humans, asexual reproduction is uncommon. As a result, there are legal and ethical concerns with parthenogenetic activation of a human oocyte. However, there are medical, scientific, and economic grounds for using this technique of reproduction. Parthenogenesis, for example, can be used to a variety of professions.
In many studies employing the oocyte, human stem cells have been examined for their regulatory systems that are responsible for the formation of an embryo or cloning attempts. As a result, clinical human parthenogenesis in people might create cells like embryonic stem cells, which could be beneficial to a variety of patients.
It has a low risk of organ rejection and can be utilised in organ transplantation. The activation of human oocytes is a complicated process that necessitates numerous stimuli at various stages of the cell division cycle. Without a stimulation, the unfertilized oocyte will not develop in metaphase II. The stimulus could be caused by spermatozoon fertilisation or the use of an artificial agent.
The artificial agent allows for the shift to anaphase, the separation of sister chromatids, and the expulsion of the second polar body. To initiate the formation of an embryo, the artificial agent must replicate the activity of sperm. Different sorts of stimuli can be used to activate parthenogenetic activity in oocytes.
Chemical and physical stimuli are two types of stimuli. Even though many techniques stimulate oocytes, they have varying degrees of embryonic development and activation rates, which impact the degree of success.
The resulting embryo is a pseudodiploid, with the two maternal sister chromatid chromosomes present in the egg. Because the resultant embryo includes one copy of sister chromatids in order to grow into a homozygous embryo, spontaneous diploidization occurs.
The activating substance then caused the second polar body to extrude and the cell to exit metaphase II without diploidization. As a result, the embryo produced at this stage is haploid. The use of parthenogenesis to activate the human oocyte to treat infertility is gaining popularity because it can be used to produce embryos in areas like assisted reproduction, somatic cell, and nuclear transfer experiments, and the development of clinical-grade pluripotent embryonic stem cells for regenerative medicine. To trigger human and nonhuman female gametes at various stages of embryonic development, several techniques have been used.
Parthenogenesis in Birds
In certain birds, parthenogenesis may occur spontaneously. However, because the unfertilized egg’s growth is influenced by its surroundings, it is generally aborted. Birds can reproduce by facultative parthenogenesis, resulting in only diploid males.
Although the process of parthenogenesis in birds is unknown, some experts believe that it may influence both normal fertilisation and the natural development of an embryo. Parthenogenesis-reproducing turkeys and virgin quail, for example, have exhibited lower reproductive success following sexual mating.
Parthenogenesis in birds can be triggered by environmental influences as well as genetic selection such as vaccination exposure. Parthenogenesis in birds can be researched in order to learn more about parthenogenesis in vertebrates. This knowledge aids in the study of the aetiology of ovarian malignancies resulting from parthenogenesis of mammalian oocytes.
Because of their tiny size, short generation intervals, and early sexual maturity than hens and turkeys, avian species are excellent models for such investigations. Parthenogenesis, on the other hand, is a failure in most birds, including pigeons, quail, turkeys, and zebra finches.
Researchers were able to hatch a tiny fraction of the turkeys’ unfertilized eggs when they were exposed to genetic selection. The meiotic early stage of parthenogenesis is comparable to a fertilised egg in that the number of chromosomes is decreased.
As a result, the haploid egg nucleus and the haploid second polar body are merged or endomitosis is utilised to restore the diploid number of chromosomes. As a result, the parthenogen created has the same diploid chromosomal number as the parent.
Parthenogenesis in Bees
Honey bees’ diploid fertilised eggs generally grow into females, but haploid unfertilized eggs develop into males through haploid parthenogenesis. Meiosis reduces the amount of chromosomes in the honeybee’s somatic parthenogenesis. Unlike apomictic parthenogenesis, which does not include meiosis.
Male honeybees are generated from unfertilized haploid eggs that grow into haploid males during parthenogenesis. Unfertilized eggs can be generated by the queen or unmated workers who use parthenogenesis to make males. Workers seldom lay eggs, although this can happen in unusual situations, such as when the queen dies.
The presence of pheromones generated by the queen prevents workers from reproducing. As a result, they exclusively spawn drones (males) through parthenogenesis. When the queen dies, however, certain honeybee species can use parthenogenesis to generate a diploid queen to replace her and prevent the colony from collapsing.
Diploid fertilised eggs create honeybee females; nevertheless, a haploid egg that can recover the diploid amount of chromosomes during meiosis II can also produce a diploid female. A worker or a queen female can be generated in cape workers by fusing two haploid pronuclei to form a diploid female during second division meiosis.
This characteristic is used by certain colonies to parasitize neighbouring colonies by generating a large number of female progeny. To put two allele pairs on the four chromatids, many recombination processes occur between the centrosome and a locus.
As a result, parthenogenesis that uses this form of recombination has a one in three probability of producing homozygous offspring, because any chromatid will be picked at random to carry its alternative allele.
To increase the number of drones and workers, a honeybee may create haploid males through parthenogenesis and diploid females through fertilisation.
Parthenogenesis in Plants
Animals aren’t the only ones who go through parthenogenesis. It may also be found in nature in plants, particularly lower plants like algae, mosses, and approximately 10% of ferns, as well as about 1% of higher blooming plants.
In plants, parthenogenesis generally takes the form of a mixture of autonomous endospore creation and apomeiosis, which refers to meiosis omission, and is referred to as apomixis, which implies clonal seed generation. A haploid megaspore is produced by female meiosis in plants, which develops into a gametophyte.
Following fertilisation with male sperm, the haploid female gametophyte alternates with a diploid sporophytic generation (haploid). The archegonia is a unique area that arises in lower primordial plants such as ferns and mosses. Furthermore, the female gametophyte is bigger and more free-living than the male. Because haploid embryos are not viable, parthenogenesis is a process that recovers or maintains the diploid number of chromosomes.
Plant parthenogenesis involves a variety of processes that result in a whole or partial clone of the mother DNA. Apospory is a process in which the ovule sporophytic cell grows straight into a gametophyte.
Another approach is diplospory, in which the parent cell megaspore undergoes endoreplication before meiosis. Both methods are capable of producing complete clones of the mother. When a haploid egg cell grows without the diploid number of chromosomes being restored, the child is haploid and sterile.
Other variables, such as nutrition, influence the growth of parthenogenetically generated plant embryos. Angiosperms are involved in the feeding of an embryo through endosperm, which is produced during fertilisation.
In animal parthenogenesis, this step is unimportant since embryo nourishment is generally provided by the mother. Instead of the egg cell, the embryo in lower plants develops by parthenogenesis from gametophytic cells. Somatic embryogenesis is a kind of parthenogenesis in plants in which the embryo develops spontaneously from a sporophytic cell.
The lack of embryonic sac development, seed coat, and endosperm are all drawbacks of this method. The formation of an embryo from cells other than egg cells is another unusual type of parthenogenesis in plants. Instead, the embryo is formed from the plant’s leaves, protoplasts, pollen, gametophytic cells, or other tissues.
External cues, such as heat stress or hormones, cause these components to grow. Researchers believe that parthenogenesis artificial induction in various sections of the plant might be beneficial in plant breeding.
Parthenogenesis Citations
- Patterns and mechanisms in instances of endosymbiont-induced parthenogenesis. J Evol Biol . 2017 May;30(5):868-888.
- Parthenogenesis and Human Assisted Reproduction. Stem Cells Int . 2016;2016:1970843.
- Parthenogenesis in Insects: The Centriole Renaissance. Results Probl Cell Differ . 2017;63:435-479.
- Identifying and Engineering Genes for Parthenogenesis in Plants. Front Plant Sci . 2019 Feb 19;10:128.
- Parthenogenesis in birds: a review. Reproduction . 2018 Jun;155(6):R245-R257.
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