asexual reproduction
   HOME

TheInfoList



OR:

Asexual reproduction is a type of
reproduction
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
archaea
and bacteria. Many
eukaryotic
eukaryotic
organisms including
plant
plant
s,
animal
animal
s, and
fungi
fungi
can also reproduce asexually. 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 also reproduce asexually. 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 Sexual reproduction is a type of reproduction that involves a complex Biological life cycle, life cycle in which a gamete (haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to p ...
in the sense of
genetic recombination Genetic recombination (also known as genetic reshuffling) is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. In eukaryot ...
in
meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately ...
.


Types of asexual reproduction


Fission

Prokaryote A prokaryote () is a Unicellular organism, single-celled organism that lacks a cell nucleus, nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek language, Greek wikt:πρό#Ancient Greek, πρό (, 'before') a ...
s (
Archaea Archaea ( ; singular archaeon ) is a Domain (biology), domain of Unicellular organism, single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially Taxonomy (biology), classified as bacter ...
and
Bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometre The micrometre (Amer ...
) reproduce asexually through binary fission, in which the parent organism divides in two to produce two genetically identical daughter organisms.
Eukaryote Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
s (such as
protist A protist () is any eukaryotic organism (that is, an organism whose Cell (biology), cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a Common descent, common ancestor (the last eukary ...
s and unicellular
fungi A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...
) may reproduce in a functionally similar manner by
mitosis In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is maintai ...
; most of these are also capable of sexual reproduction. Multiple fission at the cellular level occurs in many
protist A protist () is any eukaryotic organism (that is, an organism whose Cell (biology), cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a Common descent, common ancestor (the last eukary ...
s, e.g.
sporozoa The Apicomplexa (also called Apicomplexia) are a large phylum of parasitic Alveolata, alveolates. Most of them possess a unique form of organelle that comprises a type of non-photosynthetic plastid called an apicoplast, and an apical complex stru ...
ns and
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthesis, photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from u ...
. The
nucleus Nucleus (plural, : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA ...
of the parent cell divides several times by
mitosis In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is maintai ...
, producing several nuclei. The cytoplasm then separates, creating multiple daughter cells. In apicomplexans, multiple fission, or
schizogony Fission, in biology, is the division of a single entity into two or more parts and the regeneration of those parts to separate entities resembling the original. The object experiencing fission is usually a cell (biology), cell, but the term may al ...
appears either as
merogony Apicomplexans, a group of intracellular parasites, have Biological life cycle, life cycle stages that allow them to survive the wide variety of environments they are exposed to during their complex life cycle. Each stage in the life cycle of an a ...
,
sporogony Apicomplexa The Apicomplexa (also called Apicomplexia) are a large phylum of parasitic Alveolata, alveolates. Most of them possess a unique form of organelle that comprises a type of non-photosynthetic plastid called an apicoplast, and an apic ...
or gametogony. Merogony results in
merozoite Apicomplexans, a group of intracellular parasites, have Biological life cycle, life cycle stages that allow them to survive the wide variety of environments they are exposed to during their complex life cycle. Each stage in the life cycle of an a ...
s, which are multiple daughter cells, that originate within the same cell membrane, sporogony results in sporozoites, and gametogony results in micro gametes.


Budding

Some cells divide by
budding Budding or blastogenesis is a type of asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site. For example, the small bulb-like projection coming out from the yeast cell is know ...
(for example
baker's yeast Baker's yeast is the common name for the strains of yeast commonly used in baking bread and other bakery products, serving as a leavening agent which causes the bread to rise (expand and become lighter and softer) by converting the fermentation ( ...
), 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, 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 ''Toxoplasma gondii'' () is an Obligate intracellular parasite, obligate intracellular parasitic protozoan (specifically an apicomplexan) that causes toxoplasmosis. Found worldwide, ''T. gondii'' is capable of infecting virtually all warm-bloode ...
''. 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. Also, budding (external or internal) occurs in some worms like '' Taenia'' or ''
Echinococcus ''Echinococcus'' is a genus within Cestoda Cestoda is a Class (biology), class of parasitic worms in the flatworm phylum (Platyhelminthes). Most of the species—and the best-known—are those in the subclass Eucestoda; they are ribbon-li ...
''; these worms produce cysts and then produce (invaginated or evaginated) protoscolex with
budding Budding or blastogenesis is a type of asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site. For example, the small bulb-like projection coming out from the yeast cell is know ...
.


Vegetative propagation

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 Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes A gamete (; , ultimately ) is a Ploidy#Haploid and monoploid, haploid cell that fuse ...
or
meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately ...
. 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
rhizome In botany and dendrology, a rhizome (; , ) is a modified subterranean plant stem that sends out roots and shoots from its Node (botany), nodes. Rhizomes are also called creeping rootstalks or just rootstalks. Rhizomes develop from axillary bud ...
s or
stolon In biology, stolons (from Latin ''wikt:stolo, stolō'', genitive ''stolōnis'' – "branch"), also known as runners, are horizontal connections between organisms. They may be part of the organism, or of its skeleton; typically, animal stolons are ...
(for example in
strawberry The garden strawberry (or simply strawberry; ''Fragaria × ananassa'') is a widely grown Hybrid (biology), hybrid species of the genus ''Fragaria'', collectively known as the strawberries, which are cultivated worldwide for their fruit. The f ...
). Other plants reproduce by forming
bulb In botany, a bulb is structurally a short Plant stem, stem with fleshy leaf, leaves or leaf basesBell, A.D. 1997. ''Plant form: an illustrated guide to flowering plant morphology''. Oxford University Press, Oxford, U.K. that function as food st ...
s or
tuber Tubers are a type of enlarged structure used as storage organs for nutrients in some plants. They are used for the plant's perennation (survival of the winter or dry months), to provide energy and nutrients for regrowth during the next growing s ...
s (for example
tulip Tulips (''Tulipa'') are a genus of spring-blooming perennial herbaceous bulbiferous geophytes (having bulbs as storage organs). The flowers are usually large, showy and brightly coloured, generally red, pink, yellow, or white (usually in warm ...
bulbs and ''
Dahlia Dahlia (, ) is a genus of bushy, tuberous root, tuberous, herbaceous perennial plants native to Mexico and Central America. A member of the Asteraceae (former name: Compositae) family of dicotyledonous plants, its garden relatives thus include ...
'' tubers). Some plants produce
adventitious Important structures in plant development are buds, shoots, roots, leaf, leaves, and flowers; plants produce these tissues and structures throughout their life from meristems located at the tips of organs, or between mature tissues. Thus, a living p ...
shoots and may form a
clonal colony A clonal colony or genet is a group of genetics, genetically identical individuals, such as plants, fungi, or bacteria, that have grown in a given location, all originating vegetative reproduction, vegetatively, not Sexual reproduction, sexually ...
. In these examples, all the individuals are clones, and the clonal population may cover a large area.


Spore formation

Many multicellular organisms form
spore In biology, a spore is a unit of sexual reproduction, sexual or asexual reproduction that may be adapted for biological dispersal, dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of ...
s during their
biological life cycle In biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of Cell (biology), cel ...
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 Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Sets of chromosomes refer to the number of mat ...
spores rather than gametes. These spores grow into multicellular individuals (called
gametophyte A gametophyte () is one of the two alternation of generations, alternating multicellular organism, multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has on ...
s in the case of plants) without a fertilization event. These haploid individuals give rise to gametes through
mitosis In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is maintai ...
. Meiosis and gamete formation therefore occur in separate generations or "phases" of the life cycle, referred to as
alternation of generations Alternation of generations (also known as metagenesis or heterogenesis) is the predominant type of Biological life cycle, life cycle in plants and algae. It consists of a Multicellular organism, multicellular haploid sexual phase, the gametophy ...
. Since sexual reproduction is often more narrowly defined as the fusion of gametes (
fertilization Fertilisation or fertilization (see American and British English spelling differences#-ise, -ize (-isation, -ization), spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give ...
), spore formation in plant
sporophyte A sporophyte () is the diploid multicellular stage in the biological life cycle, life cycle of a plant or alga which produces asexual Spore, spores. This stage alternates with a multicellular haploid gametophyte phase. Life cycle The sporophyt ...
s and algae might be considered a form of asexual reproduction (agamogenesis) despite being the result of
meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately ...
and undergoing a reduction in
ploidy Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Sets of chromosomes refer to the number of mat ...
. 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 In biology, a spore is a unit of sexual reproduction, sexual or asexual reproduction that may be adapted for biological dispersal, dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of ...
formation, which involves
mitosis In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is maintai ...
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 Red algae, or Rhodophyta (, ; ), are one of the oldest groups of eukaryotic algae. The Rhodophyta also comprises one of the largest phyla of algae, containing over 7,000 currently recognized species with taxonomic revisions ongoing. The majority ...
''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 Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately ...
.


Fragmentation

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. Animals that reproduce asexually include
planarian A planarian is one of the many flatworms of the traditional class (biology), class Turbellaria. It usually describes free-living flatworms of the order Tricladida (triclads), although this common name is also used for a wide number of free-li ...
s, many
annelid The annelids (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant taxon, extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to v ...
worms including
polychaete Polychaeta () is a paraphyletic class (biology), class of generally marine invertebrate, marine annelid worms, common name, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that ...
s and some
oligochaete Oligochaeta () is a scientific classification, subclass of animals in the phylum Annelida, which is made up of many types of aquatic and terrestrial worms, including all of the various earthworms. Specifically, oligochaetes comprise the terrest ...
s,
turbellarians The Turbellaria are one of the traditional sub-divisions of the phylum Platyhelminthes (flatworms), and include all the sub-groups that are not exclusively parasitic. There are about 4,500 species, which range from to large freshwater forms more ...
and
sea stars Starfish or sea stars are Star polygon, star-shaped echinoderms belonging to the class (biology), class Asteroidea (). Common usage frequently finds these names being also applied to brittle star, ophiuroids, which are correctly referred to ...
. Many fungi and plants reproduce asexually. Some plants have specialized structures for reproduction via fragmentation, such as '' gemmae'' in
liverworts The Marchantiophyta () are a division of non-vascular plant, non-vascular embryophyte, land plants commonly referred to as hepatics or liverworts. Like mosses and hornworts, they have a gametophyte-dominant life cycle, in which cells of the pla ...
. Most
lichen A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among hypha, filaments of multiple fungus, fungi species in a mutualism (biology), mutualistic relationship.symbiotic Symbiosis (from Ancient Greek, Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different Organism, biological organisms, be it Mutualism (biolog ...
union of a fungus and
photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemica ...
algae or
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum (biology), phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's ...
, 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 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
echinoderm An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the Crinoi ...
s, this method of reproduction is usually known as ''fissiparity''. Due to many environmental and
epigenetic In biology, epigenetics is the study of stable phenotypic changes (known as ''marks'') that do not involve alterations in the DNA sequence. The Ancient Greek, Greek prefix ''wikt:epi-, epi-'' ( "over, outside of, around") in ''epigenetics'' imp ...
differences, clones originating from the same ancestor might actually be genetically and epigenetically different.


Agamogenesis

Agamogenesis is any form of reproduction that does not involve a male gamete. Examples are parthenogenesis and
apomixis In botany, apomixis is asexual reproduction without fertilization. Its etymology is Greek for "away from" + "mixing". This definition notably does not mention meiosis. Thus "normal Plant reproduction#Asexual reproduction, asexual reproduction" o ...
.


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
invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordata, ...
s (e.g. water fleas,
rotifer The rotifers (, from the Latin , "wheel", and , "bearing"), commonly called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic Coelom#Pseudocoelomates, pseudocoelomate animals. They were first d ...
s, aphids, stick insects, some ants, bees and parasitic wasps) and
vertebrate Vertebrates () comprise all animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually ...
s (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 In botany, apomixis is asexual reproduction without fertilization. Its etymology is Greek for "away from" + "mixing". This definition notably does not mention meiosis. Thus "normal Plant reproduction#Asexual reproduction, asexual reproduction" o ...
. 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.


= Facultative parthenogenesis

= 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 The Komodo dragon (''Varanus komodoensis''), also known as the Komodo monitor, is a member of the monitor lizard family Varanidae that is endemic to the Indonesian islands of Komodo (island), Komodo, Rinca, Flores, and Gili Motang. It is the ...
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).
Aphid Aphids are small sap-sucking insects and members of the Taxonomic rank, superfamily Aphidoidea. Common names include greenfly and blackfly, although individuals within a species can vary widely in color. The group includes the fluffy white Erio ...
s 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.


= Obligate parthenogenesis

= In obligate parthenogenesis, females only reproduce asexually. One example of this is the desert grassland whiptail lizard, a
hybrid Hybrid may refer to: Science * Hybrid (biology) In biology, a hybrid is the offspring resulting from combining the qualities of two organisms of different breeds, varieties, species or genera through sexual reproduction. Hybrids are not alway ...
of two other species. Typically hybrids are infertile but through parthenogenesis this species has been able to develop stable populations.
Gynogenesis Gynogenesis, a form of parthenogenesis Parthenogenesis (; from the Greek grc, παρθένος, translit=parthénos, lit=virgin, label=none + grc, γένεσις, translit=génesis, lit=creation, label=none) is a natural form of asexual repro ...
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 The sailfin molly (''Poecilia latipinna'') is a species of fish of the genus ''Poecilia''. They inhabit Freshwater, fresh, Brackish water, brackish, Saline water, salt, and coastal waters from North Carolina to Texas and the Yucatán Peninsula of ...
) for sperm.


Apomixis and nucellar embryony

Apomixis in plants is the formation of a new
sporophyte A sporophyte () is the diploid multicellular stage in the biological life cycle, life cycle of a plant or alga which produces asexual Spore, spores. This stage alternates with a multicellular haploid gametophyte phase. Life cycle The sporophyt ...
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 Vegetative reproduction (also known as vegetative propagation, vegetative multiplication or cloning) is any form of asexual reproduction Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or chan ...
. An example of an
apomictic In botany, apomixis is asexual reproduction without fertilization. Its etymology is Greek for "away from" + "mixing". This definition notably does not mention meiosis. Thus "normal Plant reproduction#Asexual reproduction, asexual reproduction" o ...
plant would be the
triploid Polyploidy is a condition in which the biological cell, cells of an organism have more than one pair of (Homologous chromosome, homologous) chromosomes. Most species whose cells have Cell nucleus, nuclei (eukaryotes) are diploid, meaning they ha ...
European
dandelion ''Taraxacum'' () is a large genus of flowering plants in the family Asteraceae, which consists of species commonly known as dandelions. The scientific and hobby study of the genus is known as taraxacology. The genus is native to Eurasia and Nor ...
. 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 Nucellar embryony (notated Nu+) is a form of seed A seed is an Plant embryogenesis, embryonic plant enclosed in a testa (botany), protective outer covering, along with a food reserve. The formation of the seed is a part of the process of re ...
, the embryo is formed from the diploid
nucellus In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: the ''integument'', forming its outer layer, the ''nucellus'' (or remnant of the sporangium, megasporangium), ...
tissue surrounding the embryo sac. Nucellar embryony occurs in some
citrus ''Citrus'' is a genus of flowering plant, flowering trees and shrubs in the rue family, Rutaceae. Plants in the genus produce citrus fruits, including important crops such as Orange (fruit), oranges, Lemon, lemons, grapefruits, pomelos, and lim ...
seeds. Male apomixis can occur in rare cases, such as the Saharan Cypress ''
Cupressus dupreziana ''Cupressus dupreziana'', the Saharan cypress, or tarout, is a very rare coniferous Conifers are a group of conifer cone, cone-bearing Spermatophyte, seed plants, a subset of gymnosperms. Scientifically, they make up the phylum, division Pi ...
'', where the genetic material of the embryo are derived entirely from
pollen Pollen is a powdery substance produced by Spermatophyte, seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects ...
.


Alternation between sexual and asexual reproduction

Some species can alternate between sexual and asexual strategies, an ability known as '' heterogamy'', depending on many conditions. Alternation is observed in several
rotifer The rotifers (, from the Latin , "wheel", and , "bearing"), commonly called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic Coelom#Pseudocoelomates, pseudocoelomate animals. They were first d ...
species (cyclical parthenogenesis e.g. in Brachionus species) and a few types of insects. One example of this is
aphid Aphids are small sap-sucking insects and members of the Taxonomic rank, superfamily Aphidoidea. Common names include greenfly and blackfly, although individuals within a species can vary widely in color. The group includes the fluffy white Erio ...
s 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 Thelytoky (from the Greek language, Greek ''thēlys'' "female" and ''tokos'' "birth") is a type of parthenogenesis in which females are produced from unfertilized eggs, as for example in aphids. Thelytokous parthenogenesis is rare among animals a ...
. The freshwater crustacean ''
Daphnia ''Daphnia'' is a genus of small planktonic crustaceans, in length. ''Daphnia'' are members of the Order (biology), order Anomopoda, and are one of the several small aquatic crustaceans commonly called water fleas because their Saltation (gait), ...
'' reproduces by parthenogenesis in the spring to rapidly populate ponds, then switches to
sexual reproduction Sexual reproduction is a type of reproduction that involves a complex Biological life cycle, life cycle in which a gamete (haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to p ...
as the intensity of competition and predation increases. Monogonont
rotifer The rotifers (, from the Latin , "wheel", and , "bearing"), commonly called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic Coelom#Pseudocoelomates, pseudocoelomate animals. They were first d ...
s 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 ''Dictyostelium'' is a genus of single- and multi-celled eukaryote, eukaryotic, Phagocytosis, phagotrophic bacterivores. Though they are Protista and in no way Fungus, fungal, they traditionally are known as "slime molds". They are present i ...
'' 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 ''Rhizopus'' is a genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biologica ...
'') 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

In the
rotifer The rotifers (, from the Latin , "wheel", and , "bearing"), commonly called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic Coelom#Pseudocoelomates, pseudocoelomate animals. They were first d ...
'' Brachionus calyciflorus'' asexual reproduction (obligate parthenogenesis) can be inherited by a recessive allele, which leads to loss of sexual reproduction in homozygous offspring.
Inheritance of asexual reproduction by a single recessive locus has also been found in the
parasitoid wasp Parasitoid wasps are a large group of hymenopteran Superfamily (zoology), superfamilies, with all but the wood wasps (Orussoidea) being in the wasp-waisted Apocrita. As parasitoids, they lay their eggs on or in the bodies of other arthropods, ...
''Lysiphlebus fabarum''.


Examples in animals

Asexual reproduction is found in nearly half of the animal phyla. Parthenogenesis occurs in the
hammerhead shark The hammerhead sharks are a group of sharks that form the family Sphyrnidae, so named for the unusual and distinctive structure of their heads, which are flattened and laterally extended into a "hammer" shape called a cephalofoil. Most hammerhe ...
and the
blacktip shark The blacktip shark (''Carcharhinus limbatus'') is a species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as ...
. 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 The New Mexico whiptail (''Aspidoscelis neomexicanus'') is a female-only species of lizard found in the southwestern United States in New Mexico and Arizona, and in northern Mexico in Chihuahua (state), Chihuahua. It is the official state reptil ...
is another example. Some reptiles use the
ZW sex-determination system The ZW sex-determination system is a Chromosome, chromosomal system that determines the sex of offspring in birds, some fish and crustaceans such as the Macrobrachium rosenbergii, giant river prawn, some insects (including butterflies and moths) ...
, 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 The boa constrictor (scientific name also ''Boa constrictor''), also called the red-tailed boa, is a species of large, non-venomous, heavy-bodied snake that is frequently kept and bred in captivity. The boa constrictor is a member of the family B ...
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 this occasion she reproduced asexually, creating 22 female babies with WW sex-chromosomes.
Polyembryony Polyembryony is the phenomenon of two or more embryos developing from a single fertilized egg. Due to the embryos resulting from the same egg, the embryos are identical to one another, but are genetically diverse from the parents. The genetic differ ...
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 Hymenoptera is a large order (biology), order of insects, comprising the sawfly, sawflies, wasps, bees, and ants. Over 150,000 living species of Hymenoptera have been described, in addition to over 2,000 extinct ones. Many of the species are Par ...
. 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
rotifer The rotifers (, from the Latin , "wheel", and , "bearing"), commonly called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic Coelom#Pseudocoelomates, pseudocoelomate animals. They were first d ...
s reproduce exclusively asexually, and all individuals in the class
Bdelloidea Bdelloidea ( Greek ''βδέλλα'', ''bdella'', "leech") is a class of rotifers found in freshwater habitats all over the world. There are over 450 described species In biology, a species is the basic unit of Taxonomy (biology), clas ...
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. Bdelloid rotifers are extraordinarily resistant to damage from
ionizing radiation Ionizing radiation (or ionising radiation), including nuclear radiation, consists of subatomic particles or electromagnetic waves that have sufficient energy to ionization, ionize atoms or molecules by detaching electrons from them. Some particles ...
due to the same DNA-preserving adaptations used to survive dormancy. These adaptations include an extremely efficient mechanism for repairing DNA double-strand breaks. This repair mechanism was studied in two Bdelloidea species, ''Adineta vaga'', and ''Philodina roseola''. 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. In the grass
thrips Thrips (Order (biology), order Thysanoptera) are minute (mostly long or less), slender insects with fringed wings and unique asymmetrical mouthparts. Different thrips species feed mostly on plants by puncturing and sucking up the contents, al ...
genus '' Aptinothrips'' there have been several transitions to asexuality, likely due to different causes.


Adaptive significance of asexual reproduction

A complete lack of
sexual reproduction Sexual reproduction is a type of reproduction that involves a complex Biological life cycle, life cycle in which a gamete (haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to p ...
is relatively rare among multicellular organisms, particularly s. 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. Almost all asexual modes of reproduction maintain meiosis either in a modified form or as an alternative pathway. 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.Avise, J. (2008) Clonality: The Genetics, Ecology and Evolution of Sexual Abstinence in Vertebrate Animals. See pp. 22-25. Oxford University Press.


See also

*
Alternation of generations Alternation of generations (also known as metagenesis or heterogenesis) is the predominant type of Biological life cycle, life cycle in plants and algae. It consists of a Multicellular organism, multicellular haploid sexual phase, the gametophy ...
*
Self-fertilization Autogamy, or self-fertilization, refers to the Cell fusion, fusion of two gametes that come from one individual. Autogamy is predominantly observed in the form of self-pollination, a Reproduction, reproductive mechanism employed by many Flowering ...
*
Bacterial conjugation Bacterial conjugation is the transfer of genetic material between Bacteria, bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells. This takes place through a pilus. It is a parasexual mode of reproduction i ...
*
Biological life cycle In biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of Cell (biology), cel ...
*
Biological reproduction Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual org ...
, also simply *
Cloning Cloning is the process of producing individual organisms with identical or virtually identical DNA, either by natural or artificial means. In nature, some organisms produce clones through asexual reproduction. In the field of biotechnology, cl ...
*
Hermaphrodite In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many Taxonomy (biology), taxonomic groups of animals (mostly invertebrate ...
*
Plant reproduction Plant reproduction is the production of new offspring in plants, which can be accomplished by Sexual reproduction, sexual or asexual reproduction, asexual reproduction. Sexual reproduction produces offspring by the fusion of gametes, resulting in o ...
* Sex


References


Further reading

* * *


External links


Asexual reproduction


{{DEFAULTSORT:Asexual Reproduction