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{{Short description|Reproduction without a sexual process}}{{Distinguish|Asexuality}}{{pp-pc|small=yes}}{{Use dmy dates|date=July 2020}}File:caduco.jpg|thumb|Asexual reproduction in liverworts: a caducous phylloid germinating]]Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the full set of genes of their single parent and thus the newly created individual is genetically and physically similar to the parent or an exact clone of the parent. Asexual reproduction is the primary form of reproduction for single-celled organisms such as archaea and bacteria. Many eukaryotic organisms including plants, animals, and fungi can also reproduce asexually.JOURNAL,www.genetics.org/content/206/2/993, Asexual but Not Clonal: Evolutionary Processes in Automictic Populations | Genetics, Genetics, June 2017, 206, 2, 993–1009, 10.1534/genetics.116.196873, 2018-08-21, Engelstädter, Jan, 28381586, 5499200, 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. Komodo dragons and some monitor lizards can reproduce asexually.JOURNAL, Dudgeon, Christine L., Coulton, Laura, Bone, Ren, Ovenden, Jennifer R., Thomas, Severine, 2017-01-16, Switch from sexual to parthenogenetic reproduction in a zebra shark, Scientific Reports, en, 7, 1, 40537, 10.1038/srep40537, 28091617, 5238396, 2017NatSR...740537D, 2045-2322, 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.JOURNAL, Narra, H. P., Ochman, H., Of what use is sex to bacteria?, Current Biology, 2006, 16, R705–710, 16950097, 10.1016/j.cub.2006.08.024, 17, free, WEB,www.livescience.com/9460-female-komodo-dragon-virgin-births.html, Female Komodo Dragon Has Virgin Births, Jeanna Bryner, 20 December 2006, livescience.com, 18 December 2022,

Types of asexual reproduction

Fission

Prokaryotes (Archaea and Bacteria) reproduce asexually through binary fission, in which the parent organism divides in two to produce two genetically identical daughter organisms. Eukaryotes (such as protists and unicellular fungi) may reproduce in a functionally similar manner by mitosis; most of these are also capable of sexual reproduction.Multiple fission at the cellular level occurs in many protists, e.g. sporozoans and algae. The nucleus of the parent cell divides several times by mitosis, producing several nuclei. The cytoplasm then separates, creating multiple daughter cells.ENCYCLOPEDIA,www.britannica.com/EBchecked/topic/498542/reproduction/76146/Multiple-fission, Cell reproduction, Encyclopædia Britannica, BOOK, Britannica Educational Publishing, Fungi, Algae, and Protists, The Rosen Publishing Group, 2011, 978-1-61530-463-9,books.google.com/books?id=_U9MB4iUpDIC&q=multiple%20fission&pg=PA101, BOOK, P.Puranik, Asha Bhate, Animal Forms And Functions: Invertebrata, Sarup & Sons, 2007, 978-81-7625-791-6,books.google.com/books?id=-kdq6RyyVE0C&q=multiple%20fission&pg=PA87, In apicomplexans, multiple fission, or schizogony appears either as merogony, sporogony or gametogony. Merogony results in merozoites, which are multiple daughter cells, that originate within the same cell membrane,BOOK, Margulis, Lynn, Lynn Margulis, McKhann, Heather I., Olendzenski, Lorraine, Illustrated glossary of protoctista: vocabulary of the algae, apicomplexa, ciliates, foraminifera, microspora, water molds, slime molds, and the other protoctists, Jones & Bartlett learn, 2001,books.google.com/books?id=y55Efu3baksC&q=Merogony%20multiple%20division&pg=PA72, 978-0-86720-081-2, BOOK, Yoshinori Tanada, Harry K. Kaya, Insect pathology, Gulf Professional Publishing, 1993, 978-0-12-683255-6,books.google.com/books?id=99YwOQnsgGUC&q=difference%20between%20merogony%20and%20schizogony&pg=PA415, sporogony results in sporozoites, and gametogony results in microgametes.

Budding

File:S cerevisiae under DIC microscopy.jpg|thumb|The yeast Saccharomyces cerevisiae reproducing by buddingbuddingSome cells divide by budding (for example baker’s yeast), resulting in a “mother” and a “daughter” cell that is initially smaller than the parent. Budding is also known on a multicellular level; an animal example is the hydra,JOURNAL, 1703-12-31, IV. Part of a letter from Mr Antony van Leeuwenhoek, F. R. S. concerning green weeds growing in water, and some animalcula found about them., Philosophical Transactions of the Royal Society of London, en, 23, 283, 1304–1311, 10.1098/rstl.1702.0042, 186209549, 0261-0523, Leeuwenhoek, Antoni Van, 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.BOOK, Smyth, James Desmond, Wakelin, Derek, Introduction to animal parasitology, 1994, Cambridge University Press, 101–102, 3, 978-0-521-42811-8, Also, budding (external or internal) occurs in some worms like Taenia or Echinococcus; these worms produce cysts and then produce (invaginated or evaginated) protoscolex with budding.

Vegetative propagation

File:Bryophyllum daigremontianum nahaufnahme1.jpg|thumb|Vegetative plantlets of mother-of-thousands, Bryophyllum daigremontianum (Kalanchoe daigremontiana)]]Vegetative propagation is a type of asexual reproduction found in plants where new individuals are formed without the production of seeds or spores and thus without syngamy or meiosis.WEB,www.ucmp.berkeley.edu/glossary/gloss6/asexual.html, Asexual Reproduction, Ucmp.berkeley.edu, 13 August 2010, 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). Some plants reproduce by forming bulbs or tubers, for example tulip bulbs and Dahlia tubers. In these examples, all the individuals are clones, and the clonal population may cover a large area.WEB,www.fs.fed.us/wildflowers/communities/aspen/grow.shtml, Celebrating Wildflowers – Fading Gold – How Aspens Grow, Fs.fed.us, 11 May 2010,www.fs.fed.us/wildflowers/communities/aspen/grow.shtml," title="web.archive.org/web/20100923073834www.fs.fed.us/wildflowers/communities/aspen/grow.shtml,">web.archive.org/web/20100923073834www.fs.fed.us/wildflowers/communities/aspen/grow.shtml, 23 September 2010, live,

Spore formation

Many multicellular organisms produce spores during their biological life cycle in a process called 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, without a fertilization event. These haploid individuals produce gametes through mitosis. Meiosis and gamete formation therefore occur in separate multicellular 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.Fungi and some algae can also utilize true asexual spore formation, which involves mitosis giving rise to reproductive cells called mitospores that develop into a new organism after dispersal. This method of reproduction is found for example in conidial fungi and the red algae Polysiphonia, and involves sporogenesis without meiosis. Thus the chromosome number of the spore cell is the same as that of the parent producing the spores. However, mitotic sporogenesis is an exception and most spores, such as those of plants and many algae, are produced by meiosis.“Plant.” Britannica Academic, Encyclopædia Britannica, 15 Jun. 2021. Accessed 20 Jan. 2022.Card, V. (2016). Algae. In M. S. Hill (Ed.), Biology (2nd ed., Vol. 1, pp. 21–23). Macmillan Reference USA.“Fungus.” Britannica Academic, Encyclopædia Britannica, 4 Oct. 2018. Accessed 20 Jan. 2022.

Fragmentation

File:Tu - Linckia guildingi cropped.jpg|thumb |right |alt=Regeneration from an arm |Linckia guildingi “comet”, a starfishstarfishFragmentation 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. Animals that reproduce asexually include planarians, many annelid worms including polychaetesBOOK, Ruppert, E. E., Fox, R. S., Barnes, R. D., Invertebrate Zoology, Annelida, Brooks / Cole, 7, 978-0-03-025982-1, 2004, 434–441,archive.org/details/isbn_9780030259821/page/434, and some oligochaetes,BOOK, Ruppert, E.E., Fox, R.S., Barnes, R.D., amp, Invertebrate Zoology, Annelida, Brooks / Cole, 7, 978-0-03-025982-1, 2004, 466–469,archive.org/details/isbn_9780030259821/page/466, turbellarians and sea stars. Many fungi and plants reproduce asexually. Some plants have specialized structures for reproduction via fragmentation, such as gemmae in mosses and liverworts. Most lichens, which are a symbiotic union of a fungus and photosynthetic algae or cyanobacteria, reproduce through fragmentation to ensure that new individuals contain both symbionts. These fragments can take the form of soredia, dust-like particles consisting of fungal hyphae 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.BOOK, Sköld, Helen Nilsson, Obst, Matthias, Sköld, Mattias, Ã…kesson, Bertil, Baruch Rinkevich, Valeria Matranga, Stem Cells in Marine Organisms, Stem Cells in Asexual Reproduction of Marine Invertebrates, Springer, 2009, 125, 978-90-481-2766-5,books.google.com/books?id=-eP2p0VF6VcC&q=Fissiparity&pg=PA125, Due to many environmental and epigenetic differences, clones originating from the same ancestor might actually be genetically and epigenetically different.JOURNAL, Neuhof, Moran, Levin, Michael, Rechavi, Oded, Vertically and horizontally-transmitted memories – the fading boundaries between regeneration and inheritance in planaria, Biology Open, 26 August 2016, 1177–1188, 10.1242/bio.020149, 27565761, 5051648, 5, 9,

Agamogenesis

Agamogenesis is any form of reproduction that does not involve a male gamete. Examples are parthenogenesis and apomixis.

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.WEB, parthenogenesis {{!, Definition, Types, & Facts|url=https://www.britannica.com/science/parthenogenesis|access-date=2020-12-03|website=Encyclopedia Britannica|language=en}} 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.JOURNAL, Kono, Tomohiro, Obata, Yayoi, Wu, Quiong, Niwa, Katsutoshi, Ono, Yukiko, Yamamoto, Yuji, Park, Eun Sung, Seo, Jeong-Sun, Ogawa, Hidehiko, 2004-04-22, Birth of parthenogenetic mice that can develop to adulthood,pubmed.ncbi.nlm.nih.gov/15103378/, Nature, 428, 6985, 860–864, 10.1038/nature02402, 1476-4687, 15103378, 2004Natur.428..860K, 4353479, JOURNAL, Ramachandran, R., McDaniel, C. D., 2018, Parthenogenesis in birds: a review, Reproduction, 155, 6, R245–R257, 10.1530/REP-17-0728, 1741-7899, 29559496, free, 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.{{Citation|last1=Ozias-Akins|first1=Peggy|title=16 – Regulation of apomixis|date=2012-01-01|url=http://www.sciencedirect.com/science/article/pii/B978012381466100016X|work=Plant Biotechnology and Agriculture|pages=243–254|editor-last=Altman|editor-first=Arie|place=San Diego|publisher=Academic Press|language=en|isbn=978-0-12-381466-1|access-date=2020-12-12|last2=Conner|first2=Joann A.|editor2-last=Hasegawa|editor2-first=Paul Michael}} Parthenogenetic organisms can be split into two main categories: facultative and obligate.

Facultative parthenogenesis

(File:Zebra_Shark.jpg|thumb|Zebra shark)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.WEB, Yam, Philip, Strange but True: Komodo Dragons Show that “Virgin Births” Are Possible,www.scientificamerican.com/article/strange-but-true-komodo-d/, 2020-12-13, Scientific American, en, WEB, 2010-09-10, Komodo dragon,www.nationalgeographic.com/animals/reptiles/k/komodo-dragon/,www.nationalgeographic.com/animals/reptiles/k/komodo-dragon/," title="web.archive.org/web/20161017001707www.nationalgeographic.com/animals/reptiles/k/komodo-dragon/,">web.archive.org/web/20161017001707www.nationalgeographic.com/animals/reptiles/k/komodo-dragon/, dead, 17 October 2016, 2020-12-13, Animals, en, File:Aphid-giving-birth.jpg|thumb|AphidAphidHeterogony 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.JOURNAL, Stern, David L., 2008-06-24, Aphids, Current Biology, 18, 12, R504–R505, 10.1016/j.cub.2008.03.034, 0960-9822, 2974440, 18579086, However, some aphid species are obligate parthenotes.JOURNAL, Dedryver, C-A, Le Gallic, J-F, Mahéo, F, Simon, J-C, Dedryver, F, 2013, The genetics of obligate parthenogenesis in an aphid species and its consequences for the maintenance of alternative reproductive modes, Heredity, 110, 1, 39–45, 10.1038/hdy.2012.57, 0018-067X, 3522239, 22990313,

Obligate parthenogenesis

(File:DesertGrasslandWhiptailLizard_AspidoscelisUniparens56.jpg|thumb|Desert grassland whiptail lizard)In obligate parthenogenesis, females only reproduce asexually. One example of this is the desert grassland whiptail lizard, a hybrid of two other species. Typically hybrids are infertile but through parthenogenesis this species has been able to develop stable populations.JOURNAL, Crews, D., Fitzgerald, K. T., 1980, “Sexual” behavior in parthenogenetic lizards (Cnemidophorus), Proceedings of the National Academy of Sciences of the United States of America, 77, 1, 499–502, 10.1073/pnas.77.1.499, 0027-8424, 16592761, 348299, 1980PNAS...77..499C, free, 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.JOURNAL, Tobler, Michael, Schlupp, Ingo, 2005-06-22, Parasites in sexual and asexual mollies (Poecilia, Poeciliidae, Teleostei): a case for the Red Queen?, Biology Letters, 1, 2, 166–168, 10.1098/rsbl.2005.0305, 1744-9561, 1626213, 17148156,

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. In flowering plants, the term “apomixis” is now most often used for agamospermy, the formation of seeds without fertilization, but was once used to include vegetative reproduction. An example of an apomictic plant would be the triploid European dandelion. 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. In nucellar embryony, the embryo is formed from the diploid nucellus tissue surrounding the embryo sac. Nucellar embryony occurs in some citrus seeds. Male apomixis can occur in rare cases, such as in the Saharan Cypress Cupressus dupreziana, where the genetic material of the embryo is derived entirely from pollen.WEB, Apomixis {{!, reproduction {{!}} Britannica |url=https://www.britannica.com/science/apomixis |access-date=2023-03-23 |website=www.britannica.com |language=en}}JOURNAL, Zhang, Siqi, Liang, Mei, Wang, Nan, Xu, Qiang, Deng, Xiuxin, Chai, Lijun, 2018-03-01, Reproduction in woody perennial Citrus: an update on nucellar embryony and self-incompatibility,doi.org/10.1007/s00497-018-0327-4, Plant Reproduction, en, 31, 1, 43–57, 10.1007/s00497-018-0327-4, 29457194, 254022638, 2194-7961, BOOK, Lotsy, Johannes Paulus,www.biodiversitylibrary.org/item/46760, Progressus rei botanicae = Fortschritte der Botanik = Progrès de la botanique = Progress of botany, botanistes, Association internationale des, 1907, G. Fischer, 2, Jena,

Alternation between sexual and asexual reproduction

{{See also |Plant reproduction#Sexual reproduction}}File:Soybeanaphidlifecycle.gif|thumb|upright=1.7|AphidAphidSome species can alternate between sexual and asexual strategies, an ability known as heterogamy, depending on many conditions. Alternation is observed in several rotifer species (cyclical parthenogenesis e.g. in Brachionus species) and a few types of insects.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 is triggered by environmental changes in the fall and causes females to develop eggs instead of embryos. This dynamic reproductive cycle allows them to produce specialized offspring with polyphenism, a type of polymorphism where different phenotypes have evolved to carry out specific tasks.The cape bee Apis mellifera subsp. capensis can reproduce asexually through a process called thelytoky. The freshwater crustacean Daphnia reproduces by parthenogenesis in the spring to rapidly populate ponds, then switches to sexual reproduction as the intensity of competition and predation increases. 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.{{which|date=November 2021}}{{which|date=February 2020}}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.BOOK, Mehrotra, R. S., Aneja, K. R., An Introduction to Mycology,books.google.com/books?id=UUorj_O2dcsC&pg=PA83, December 1990, New Age International, 978-81-224-0089-2, 83 ff, 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.BOOK, Cole, Kathleen M., Sheath, Robert G., Biology of the red algae,books.google.com/books?id=F7CWXuYZFq8C&pg=PA469, 1990, Cambridge University Press, 978-0-521-34301-5, 469, 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.BOOK, Edward G. Reekie, Fakhri A. Bazzaz, Fakhri A. Bazzaz, Reproductive allocation in plants,books.google.com/books?id=_KJdaiLKAogC&pg=PA99, 2005, Academic Press, 978-0-12-088386-8, 99,

Inheritance in asexual species

In the rotifer Brachionus calyciflorus asexual reproduction (obligate parthenogenesis) can be inherited by a recessive allele, which leads to loss of sexual reproduction in homozygous offspring.JOURNAL, Stelzer, C.-P., Schmidt, J., Wiedlroither, A., Riss, S., 2010, Loss of Sexual Reproduction and Dwarfing in a Small Metazoan, PLOS ONE, 5, 9, e12854, 10.1371/journal.pone.0012854, 20862222, 2942836, 2010PLoSO...512854S, free, JOURNAL, Scheuerl, T., Riss, S., Stelzer, C.P., 2011, Phenotypic effects of an allele causing obligate parthenogenesis in a rotifer, Journal of Heredity, 102, 4, 409–415, 10.1093/jhered/esr036, 21576287, 3113615, Inheritance of asexual reproduction by a single recessive locus has also been found in the parasitoid wasp Lysiphlebus fabarum.JOURNAL, Sandrock, C., Vorburger, C., Single-locus recessive inheritance of asexual reproduction in a parasitoid wasp, Curr. Biol., 21, 5, 433–7, 2011, 21353557, 10.1016/j.cub.2011.01.070, 1438839,www.dora.lib4ri.ch/eawag/islandora/object/eawag%3A6665, free,

Examples in animals

Asexual reproduction is found in nearly half of the animal phyla.BOOK, Minelli, Alessandro, 2009, Asexual reproduction and regeneration, Perspectives in Animal Phylogeny and Evolution, Oxford University Press, 123–127,books.google.com/books?id=jIASDAAAQBAJ, 978-0198566205, Parthenogenesis occurs in the hammerhead sharkNEWS, Savage, Juliet Eilperin, Female Sharks Can Reproduce Alone, Researchers Find, The Washington Post, 23 May 2007,www.washingtonpost.com/wp-dyn/content/article/2007/05/22/AR2007052201405.html, and the blacktip shark.JOURNAL,www.sciencedaily.com/releases/2008/10/081010173054.htm, ‘Virgin Birth’ By Shark Confirmed: Second Case Ever, 10.1111/j.1095-8649.2008.02018.x, Sciencedaily.com, 11 October 2008, Chapman, D. D., Firchau, B., Shivji, M. S., Journal of Fish Biology, 73, 6, 1473–1477, 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.WEB,www.cbc.ca/news/technology/boa-constrictor-produces-fatherless-babies-1.947269, Boa constrictor produces fatherless babies, CBC News – Technology & Science, 3 November 2010, 2014-10-20, The female boa could have chosen any number of male partners (and had successfully in the past) but on this occasion 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 nine-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.Bdelloid rotifers reproduce exclusively asexually, and all individuals in the class Bdelloidea are females. 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.JOURNAL, Pouchkina-Stantcheva, N. N., McGee, B. M., Boschetti, C., Tolleter, D., Chakrabortee, S., Popova, A. V., Meersman, F., MacHerel, D., Hincha, D. K., Functional Divergence of Former Alleles in an Ancient Asexual Invertebrate, Science, 318, 5848, 268–71, 2007, 17932297, 10.1126/science.1144363, 2007Sci...318..268P, free, Bdelloid rotifers are extraordinarily resistant to damage from ionizing radiation due to the same DNA-preserving adaptations used to survive dormancy.JOURNAL, Gladyshev, Eugene, Meselson, Matthew, amp, Extreme resistance of bdelloid rotifers to ionizing radiation, Proceedings of the National Academy of Sciences, 105, 1 April 2008, 5139–5144, 10.1073/pnas.0800966105, 13, 18362355, 2278216, 2008PNAS..105.5139G, free, These adaptations include an extremely efficient mechanism for repairing DNA double-strand breaks.JOURNAL, Hespeels B, Knapen M, Hanot-Mambres D, Heuskin AC, Pineux F, LUCAS S, Koszul R, Van Doninck K, Gateway to genetic exchange? DNA double-strand breaks in the bdelloid rotifer Adineta vaga submitted to desiccation, J. Evol. Biol., 27, 7, 1334–45, July 2014, 25105197, 10.1111/jeb.12326,hal-pasteur.archives-ouvertes.fr/pasteur-01420001/file/hespeels%20et%20al%202014.pdf,ghostarchive.org/archive/20221009/https://hal-pasteur.archives-ouvertes.fr/pasteur-01420001/file/hespeels%20et%20al%202014.pdf, 2022-10-09, live, free, This repair mechanism was studied in two Bdelloidea species, Adineta vaga, and Philodina roseola.JOURNAL, Welch, David B. Mark, Welch, Jessica L. Mark, Meselson, Matthew, amp, Evidence for degenerate tetraploidy in bdelloid rotifers, Proceedings of the National Academy of Sciences, 105, 13, 5145–9, 1 April 2008, 18362354, 2278229, 10.1073/pnas.0800972105, 2008PNAS..105.5145M, free, and appears to involve mitotic recombination between homologous DNA regions within each species.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.JOURNAL, T., Schwander, Lee, Henry, Bernard J., Crespi, 1, 2011, Molecular evidence for ancient asexuality in Timema stick insects, Current Biology, 21, 13, 1129–34, 10.1016/j.cub.2011.05.026, 21683598, 2053974,pure.rug.nl/ws/files/147541304/Molecular_Evidence_for_Ancient_Asexuality_in_Timema_Stick_Insects.pdf,ghostarchive.org/archive/20221009/https://pure.rug.nl/ws/files/147541304/Molecular_Evidence_for_Ancient_Asexuality_in_Timema_Stick_Insects.pdf, 2022-10-09, live, 11370/8c189a5e-f36b-4199-934c-53347c0e2131, free, Similar findings suggest that the mite species Oppiella nova may have reproduced entirely asexually for millions of years.BOOK, Jens, Brandt, Alexander Van, Patrick Tran Bluhm, Christian Anselmetti, Yoann Dumas, Zoe Figuet, Emeric Francois, Clementine M. Galtier, Nicolas Heimburger, Bastian Jaron, Kamil S. Labedan, Marjorie Maraun, Mark Parker, Darren J. Robinson-Rechavi, Marc Schaefer, Ina Simion, Paul Scheu, Stefan Schwander, Tanja Bast,worldcat.org/oclc/1312207506, Haplotype divergence supports long-term asexuality in the oribatid mite Oppiella nova, 2021, NATL ACAD SCIENCES, 1312207506, In the grass thrips genus Aptinothrips there have been several transitions to asexuality, likely due to different causes.JOURNAL, C. J., van der Kooi, T., Schwander, 2014, Evolution of asexuality via different mechanisms in grass thrips (Thysanoptera: Aptinothrips), Evolution (journal), Evolution, 86, 7, 1883–1893, 10.1111/evo.12402, 24627993, 14853526,

Adaptive significance of asexual reproduction

A complete lack of sexual reproduction is relatively rare among multicellular organisms, particularly animals. It is not entirely understood why the ability to reproduce sexually is so common among them. Current hypothesesJOURNAL, Dawson, K.J., The Advantage of Asexual Reproduction: When is it Two-fold?, Journal of Theoretical Biology, October 1995, 176, 341–347, 3, 10.1006/jtbi.1995.0203, 1995JThBi.176..341D, 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 constraintsJOURNAL, Engelstädter, J., Constraints on the evolution of asexual reproduction, BioEssays, November 2008, 30, 1138–1150, 11–12, 18937362, 10.1002/bies.20833, 5357709, may underlie why few animals have relinquished sexual reproduction completely in their life-cycles. Almost all asexual modes of reproduction maintain meiosis either in a modified form or as an alternative pathway.JOURNAL, 10.1007/s00497-013-0234-7, free, 3825497, 23995700, The oxidative damage initiation hypothesis for meiosis, 2013, Hörandl, Elvira, Hadacek, Franz, Plant Reproduction, 26, 4, 351–367, Facultatively apomictic plants increase frequencies of sexuality relative to apomixis after abiotic stress. 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.JOURNAL, Bernstein, H., Hopf, F.A., Michod, R.E., The molecular basis of the evolution of sex, Adv. Genet., 24, 323–70, 1987, 3324702, 10.1016/s0065-2660(08)60012-7, Advances in Genetics, 9780120176243, Avise, J. (2008) Clonality: The Genetics, Ecology and Evolution of Sexual Abstinence in Vertebrate Animals. See pp. 22-25. Oxford University Press. {{ISBN |019536967X}} {{ISBN |978-0195369670}}

See also

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• Alternation of generations
• Self-fertilization
• Bacterial conjugation
• Biological life cycle
• Biological reproduction, also simply reproduction
• Cloning
• Hermaphrodite
• Plant reproduction
• Sex

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References

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Further reading

• BOOK, Avise, J., 2008, Clonality: The Genetics, Ecology, and Evolution of Sexual Abstinence in Vertebrate Animals, Oxford University Press, 978-0-19-536967-0,
• BOOK, Graham, L., Graham, J., Wilcox, L., 2003, Plant Biology, Pearson Education, Upper Saddle River, NJ, 258–259, 978-0-13-030371-4,
• BOOK, Raven, P. H., Evert, R. F., Eichhorn, S. E., 2005, Biology of Plants,archive.org/details/biologyofplants00rave_0, registration, 7th, W.H. Freeman and Company, NY, 978-0-7167-6284-3,

External links

{{Commons category |Asexual reproduction}}

• www.tiscali.co.uk/reference/encyclopaedia/hutchinson/m0030820.html" title="web.archive.org/web/20050708073801www.tiscali.co.uk/reference/encyclopaedia/hutchinson/m0030820.html">Asexual reproduction
• Intestinal Protozoa

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