Not to be confused with Asexuality.
The offspring that arise by asexual reproduction, either from from a single cell or from a multicellular organism, inherit the genes of their single parent.
In vertebrates, the most common form of asexual reproduction is parthenogenesis which is typically used as an alternative to sexual reproduction in times when reproductive opportunities are limited.
While all prokaryotes reproduce without the formation and fusion of gametes, mechanisms for lateral gene transfer such as conjugation, transformation and transduction can be likened to sexual reproduction in the sense of genetic recombination in meiosis.
Main article: Fission (biology)
The cytoplasm then separates, creating multiple daughter cells.
Main article: Budding
Budding is also known on a multicellular level; an animal example is the hydra, which reproduces by budding.
The buds grow into fully matured individuals which eventually break away from the parent organism.
Internal budding is a process of asexual reproduction, favoured by parasites such as Toxoplasma gondii.
It involves an unusual process in which two (endodyogeny) or more (endopolygeny) daughter cells are produced inside a mother cell, which is then consumed by the offspring prior to their separation.
Main article: Vegetative propagation
Examples of vegetative reproduction include the formation of miniaturized plants called plantlets on specialized leaves, for example in kalanchoe (Bryophyllum daigremontianum) and many produce new plants from rhizomes or stolon (for example in strawberry).
In these examples, all the individuals are clones, and the clonal population may cover a large area.
Main article: Sporogenesis
Exceptions are animals and some protists, which undergo meiosis immediately followed by fertilization.
Plants and many algae on the other hand undergo sporic meiosis where meiosis leads to the formation of haploid spores rather than gametes.
These spores grow into multicellular individuals (called gametophytes in the case of plants) without a fertilization event.
Meiosis and gamete formation therefore occur in separate generations or "phases" of the life cycle, referred to as alternation of generations.
Since sexual reproduction is often more narrowly defined as the fusion of gametes (fertilization), spore formation in plant sporophytes and algae might be considered a form of asexual reproduction (agamogenesis) despite being the result of meiosis and undergoing a reduction in ploidy.
However, both events (spore formation and fertilization) are necessary to complete sexual reproduction in the plant life cycle.
Thus the chromosome number of the spore cell is the same as that of the parent producing the spores.
Main article: Fragmentation (reproduction)
Fragmentation is a form of asexual reproduction where a new organism grows from a fragment of the parent.
Each fragment develops into a mature, fully grown individual.
Fragmentation is seen in many organisms.
Many fungi and plants reproduce asexually.
These fragments can take the form of soredia, dust-like particles consisting of fungal hyphen wrapped around photobiont cells.
Clonal Fragmentation in multicellular or colonial organisms is a form of asexual reproduction or cloning where an organism is split into fragments.
Each of these fragments develop into mature, fully grown individuals that are clones of the original organism.
In echinoderms, this method of reproduction is usually known as fissiparity.
Due to many environmental and epigenetic differences, clones originating from the same ancestor might actually be genetically and epigenetically different.
Agamogenesis is any form of reproduction that does not involve a male gamete.
Main article: Parthenogenesis
Parthenogenesis is a form of agamogenesis in which an unfertilized egg develops into a new individual.
It has been documented in over 2,000 species.
Parthenogenesis occurs in the wild in many invertebrates (e.g. water fleas, rotifers, aphids, stick insects, some ants, bees and parasitic wasps) and vertebrates (mostly reptiles, amphibians, and fish).
It has also been documented in domestic birds and in genetically altered lab mice.
Plants can engage in parthenogenesis as well through a process called apomixis.
However this process is considered by many to not be an independent reproduction method, but instead a breakdown of the mechanisms behind sexual reproduction .
Parthenogenetic organisms can be split into two main categories: facultative and obligate.
In facultative parthenogenesis, females can reproduce both sexually and asexually.
Because of the many advantages of sexual reproduction, most facultative parthenotes only reproduce asexually when forced to.
This typically occurs in instances when finding a mate becomes difficult.
For example, female Zebra Sharks will reproduce asexually if they are unable to find a mate in their ocean habitats.
Parthenogenesis was previously believed to rarely occur in vertebrates, and only be possible in very small animals.
However, it has been discovered in many more species in recent years.
Today, the largest species that has been documented reproducing parthenogenically is the Komodo Dragon at 10 feet long and over 300 pounds.
Heterogony is a form of facultative parthenogenesis where females alternate between sexual and asexual reproduction at regular intervals (see Alternation between sexual and asexual reproduction).
Aphids are one group of organism that engages in this type of reproduction.
They use asexual reproduction to reproduce quickly and create winged offspring that can colonize new plants and reproduce sexually in the fall to lay eggs for the next season.
However, some aphid species are obligate parthenotes.
In obligate parthenogenesis, females only reproduce asexually.
Typically hybrids are infertile but through parthenogenesis this species has been able to develop stable populations.
Gynogenesis is a form of obligate parthenogenesis where a sperm cell is used to initiate reproduction.
However, the sperm's genes never get incorporated into the egg cell.
The best known example of this is the Amazon Molly.
Because they are obligate parthenotes, there are no males in their species so they depend on males from a closely related species (the Sailfin Molly) for sperm.
Apomixis and nucellar embryony
Apomixis in plants is the formation of a new sporophyte without fertilization.
It is important in ferns and in flowering plants, but is very rare in other seed plants.
Apomixis mainly occurs in two forms: In gametophytic apomixis, the embryo arises from an unfertilized egg within a diploid embryo sac that was formed without completing meiosis.
Nucellar embryony occurs in some citrus seeds.
Alternation between sexual and asexual reproduction
See also: Plant reproduction § Sexual reproduction
Some species can alternate between sexual and asexual strategies, an ability known as heterogamy, depending on many conditions.
One example of this is aphids which can engage in heterogony.
In this system, females are born pregnant and produce only female offspring.
This cycle allows them to reproduce very quickly.
However, most species reproduce sexually once a year.
This switch it triggered by environmental changes in the fall and causes females to develop eggs instead of embryos.
Monogonont rotifers of the genus Brachionus reproduce via cyclical parthenogenesis: at low population densities females produce asexually and at higher densities a chemical cue accumulates and induces the transition to sexual reproduction.
Many protists and fungi alternate between sexual and asexual reproduction.
A few species of amphibians, reptiles, and birds have a similar ability.
The slime mold Dictyostelium undergoes binary fission (mitosis) as single-celled amoebae under favorable conditions.
However, when conditions turn unfavorable, the cells aggregate and follow one of two different developmental pathways, depending on conditions.
In the social pathway, they form a multi-cellular slug which then forms a fruiting body with asexually generated spores.
In the sexual pathway, two cells fuse to form a giant cell that develops into a large cyst.
When this macrocyst germinates, it releases hundreds of amoebic cells that are the product of meiotic recombination between the original two cells.
The hyphae of the common mold (Rhizopus) are capable of producing both mitotic as well as meiotic spores.
Many algae similarly switch between sexual and asexual reproduction.
A number of plants use both sexual and asexual means to produce new plants, some species alter their primary modes of reproduction from sexual to asexual under varying environmental conditions.
Inheritance in asexual species
Inheritance of asexual reproduction by a single recessive locus has also been found in the parasitoid wasp Lysiphlebus fabarum.
Examples in animals
Asexual reproduction is found in nearly half of the animal phyla.
In both cases, the sharks had reached sexual maturity in captivity in the absence of males, and in both cases the offspring were shown to be genetically identical to the mothers.
The New Mexico whiptail is another example.
Some reptiles use the ZW sex-determination system, which produces either males (with ZZ sex chromosomes) or females (with ZW or WW sex chromosomes).
Until 2010, it was thought that the ZW chromosome system used by reptiles was incapable of producing viable WW offspring, but a (ZW) female boa constrictor was discovered to have produced viable female offspring with WW chromosomes.
The female boa could have chosen any number of male partners (and had successfully in the past) but on these occasions she reproduced asexually, creating 22 female babies with WW sex-chromosomes.
Polyembryony is a widespread form of asexual reproduction in animals, whereby the fertilized egg or a later stage of embryonic development splits to form genetically identical clones.
Within animals, this phenomenon has been best studied in the parasitic Hymenoptera.
In the 9-banded armadillos, this process is obligatory and usually gives rise to genetically identical quadruplets.
In other mammals, monozygotic twinning has no apparent genetic basis, though its occurrence is common.
There are at least 10 million identical human twins and triplets in the world today.
Asexuality evolved in these animals millions of years ago and has persisted since.
There is evidence to suggest that asexual reproduction has allowed the animals to evolve new proteins through the Meselson effect that have allowed them to survive better in periods of dehydration.
Molecular evidence strongly suggests that several species of the stick insect genus Timema have used only asexual (parthenogenetic) reproduction for millions of years, the longest period known for any insect.
Adaptive significance of asexual reproduction
It is not entirely understood why the ability to reproduce sexually is so common among them.
Current hypotheses suggest that asexual reproduction may have short term benefits when rapid population growth is important or in stable environments, while sexual reproduction offers a net advantage by allowing more rapid generation of genetic diversity, allowing adaptation to changing environments.
Developmental constraints may underlie why few animals have relinquished sexual reproduction completely in their life-cycles.
Another constraint on switching from sexual to asexual reproduction would be the concomitant loss of meiosis and the protective recombinational repair of DNA damage afforded as one function of meiosis.
Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Asexual reproduction.