Asexual reproduction is a mode of reproduction by which offspring arise from a single organism, and inherit the genes of that parent only; it is reproduction which almost never involves ploidy or reduction. The offspring will be exact genetic copies of the parent, except in the specific case of automixis. A more stringent(説得力のある,切迫した,厳しい) definition is agamogenesis which is reproduction without the fusion(核融合,融解,融合,溶融,合併,連合,統合,総合) of gametes. Asexual reproduction is the primary form of reproduction for single-celled organisms such as the archaebacteria, eubacteria, and protists. Many plants and fungi((カビ・キノコなどの)菌(類),キノコ,茸,黴,急に発生するもの,菌類) reproduce asexually as well.
While all prokaryotes reproduce asexually (without the formation and fusion(核融合,融解,融合,溶融,合併,連合,統合,総合) of gametes), mechanisms for lateral(1.側面の,横の,横からの,2.<地位などが>横滑りの,横移動の) gene transfer such as conjugation((動詞の)活用,共役), transformation and transduction are sometimes likened to sexual reproduction (or at least with sex, in the sense of genetic recombination(再結合)). 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 hypotheses  suggest that asexual(無性(生殖)の,(交際が)セックス抜きの) 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(所有物を放棄する,をやめる,手放す,手を引く,断念する L.liquere(leave),断念する,やめる) sexual reproduction completely in their life-cycles. Another constraint on switching from sexual to asexual(無性(生殖)の,(交際が)セックス抜きの) reproduction would be the concomitant(付随物,付随する,随伴する,相伴う,同時に生ずる,両立する) loss of meiosis(緩叙法(控えめに言ってかえって印象を強める,誇張hyperboleの反対)) and the protective recombinational repair of DNA damage afforded as one function of meiosis.(緩叙法(控えめに言ってかえって印象を強める,誇張hyperboleの反対)) (Also see Meiosis section: Origin and function of meiosis(緩叙法(控えめに言ってかえって印象を強める,誇張hyperboleの反対)))
- 1 Types of asexual reproduction
- 2 Alternation between sexual and asexual reproduction
- 3 Inheritance of asexual reproduction in sexual species
- 4 Examples in animals
- 5 See also
- 6 References
- 7 Further reading
- 8 External links
Types of asexual reproduction
An important form of fission(分裂,核分裂) is binary(二進の) fission.(分裂,核分裂) In binary(二進の) fission,(分裂,核分裂) the parent organism is replaced by two daughter organisms, because it literally divides in two. Organisms, both prokaryotes (the archaea and the bacteria), and eukaryotes (such as protists and unicellular fungi((カビ・キノコなどの)菌(類),キノコ,茸,黴,急に発生するもの,菌類)), reproduce asexually through binary(二進の) fission;(分裂,核分裂) most of these are also capable of sexual reproduction.
Another type of fission(分裂,核分裂) is multiple fission(分裂,核分裂) that is advantageous to the plant life cycle. Multiple fission(分裂,核分裂) at the cellular(細胞の,細胞質の,多孔性の,独房使用の,細胞状の) level occurs in many protists, e.g. sporozoans and algae(藻類). The nucleus(細胞核,核,原子核,心,中軸) of the parent cell divides several times by mitosis((No gloss)), producing several nuclei.(細胞核,核,原子核,心,中軸) The cytoplasm((No gloss)) then separates, creating multiple daughter cells.
In apicomplexans, multiple fission,(分裂,核分裂) or schizogony, is manifested either as merogony, sporogony or gametogony. Merogony results in merozoites, which are multiple daughter cells, that originate within the same cell membrane,(薄膜,膜,皮膜,膜組織) sporogony results in sporozoites, and gametogony results in micro((No gloss))gametes.
In multiple fission,(分裂,核分裂) the nucleus(細胞核,核,原子核,心,中軸) of the parent cell divides by mitosis((No gloss)) several times, producing several nuclei.(細胞核,核,原子核,心,中軸) The cytoplasm((No gloss)) then separates, creating multiple daughter cells.
Some cells split via budding (for example baker's yeast(酵母,イースト菌)), resulting in a 'mother' and 'daughter' cell. The offspring organism is smaller than the parent. Budding is also known on a multicellular level; an animal example is the hydra(ヒュードラ,根絶しにくい害悪,ひとすじなわではいかない難問), which reproduces by budding. The buds grow into fully matured individuals which eventually break away from the parent organism.
Internal budding or Endodyogeny is a process of asexual(無性(生殖)の,(交際が)セックス抜きの) reproduction, favoured by parasites such as Toxoplasma gondii. It involves an unusual process in which two daughter cells are produced inside a mother cell, which is then consumed by the offspring prior(前の,先の,より重要な,優先する,小修道院長,大修道院副院長) to their separation.
Endopolygeny is the division into several organisms at once by internal budding. also budding (external or internal) is present in some worm like Taenia or Echinococci; these worm produce cyst(膿胞,胞嚢,嚢胞(のうほう)) and then produce (invaginated or evaginated) protoscolex with budding.
Vegetative reproduction is a type of asexual(無性(生殖)の,(交際が)セックス抜きの) reproduction found in plants where new individuals are formed without the production of seeds or spores(因子,胞子,胚種) by meiosis(緩叙法(控えめに言ってかえって印象を強める,誇張hyperboleの反対)) or syngamy. Examples of vegetative reproduction include the formation of miniaturized plants called plantlets on specialized leaves (for example in kalanchoe) and some produce new plants out of rhizomes or stolon (for example in strawberry). Other plants reproduce by forming bulbs or tubers (for example tulip bulbs and dahlia tubers). Some plants produce adventitious((要素などが)外来の,本質的でない,付加的な,偶発的な,付随的な,外来的な,偶然に得た) shoots and suckers(吸う人(物),だまされやすい人,お人好し,世間知らず,ばか,考えが甘い人,カモ,乳児,吸着盤,吸盤,もの(thing)) that form along their lateral(1.側面の,横の,横からの,2.<地位などが>横滑りの,横移動の) roots. Plants that reproduce vegetatively may form a clonal colony, where all the individuals are clones, and the clones may cover a large area.
Many multicellular organisms form spores(因子,胞子,胚種) during their biological life cycle in a process called sporogenesis. Exceptions are animals and some protists, who undergo meiosis(緩叙法(控えめに言ってかえって印象を強める,誇張hyperboleの反対)) immediately followed by fertilization.(授精,豊かにすること) Plants and many algae(藻類) on the other hand undergo sporic meiosis(緩叙法(控えめに言ってかえって印象を強める,誇張hyperboleの反対)) where meiosis(緩叙法(控えめに言ってかえって印象を強める,誇張hyperboleの反対)) leads to the formation of haploid spores(因子,胞子,胚種) rather than gametes. These spores(因子,胞子,胚種) grow into multicellular individuals (called gametophytes in the case of plants) without a fertilization(授精,豊かにすること) event. Then in the plant life cycle.
Fungi and some algae(藻類) can also utilize true asexual(無性(生殖)の,(交際が)セックス抜きの) spore(因子,胞子,胚種) formation, which involves mitosis((No gloss)) giving rise to reproductive(再生(再現)の,生殖の,多産の) cells called mitospores that develop into a new organism after dispersal.((No gloss)) This method of reproduction is found for example in conidial fungi((カビ・キノコなどの)菌(類),キノコ,茸,黴,急に発生するもの,菌類) and the red alga Polysiphonia, and involves sporogenesis without meiosis.(緩叙法(控えめに言ってかえって印象を強める,誇張hyperboleの反対)) Thus the chromosome(染色体) number of the spore(因子,胞子,胚種) cell is the same as that of the parent producing the spores.(因子,胞子,胚種) However, mitotic sporogenesis is an exception and most spores,(因子,胞子,胚種) such as those of plants, most Basidiomycota, and many algae,(藻類) are produced by meiosis(緩叙法(控えめに言ってかえって印象を強める,誇張hyperboleの反対)).
A mode of reproduction resembling multiple fission,(分裂,核分裂) common among Protozoa, in which the organism breaks up into a number of pieces, or spores,(因子,胞子,胚種) each of which eventually develops into an organism like the parent form. The formation of reproductive(再生(再現)の,生殖の,多産の) cells or spores,(因子,胞子,胚種) as in the growth of bacilli. Even if the plants are healthy the outcome of the seed will always be unexplained.
Fragmentation is a form of asexual(無性(生殖)の,(交際が)セックス抜きの) 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 such as animals (some annelid((No gloss)) worms, turbellarians and sea stars), fungi,((カビ・キノコなどの)菌(類),キノコ,茸,黴,急に発生するもの,菌類) and plants. Some plants have specialized structures for reproduction via fragmentation, such as gemma in liverworts. Most lichens(地衣類(岩や木につくコケ)), which are a symbiotic((No gloss)) union of a fungus((カビ・キノコなどの)菌(類),キノコ,茸,黴,急に発生するもの,菌類) and photosynthetic algae(藻類) or bacteria, reproduce through fragmentation to ensure that new individuals contain both symbiont. 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(無性(生殖)の,(交際が)セックス抜きの) 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.
Parthenogenesis is a form of agamogenesis in which an unfertilized egg develops into a new individual. Parthenogenesis occurs naturally in many plants, invertebrates(脊椎のない,背骨のない,無脊椎動物(の),気骨のないひと) (e.g. water fleas, rotifers, aphids,((No gloss)) stick insects, some ants, bees and parasitic wasps), and vertebrates(脊椎骨のある) (e.g. some reptiles, amphibians,(両生類の,水陸両用の,(名)両生類,水陸両用機,両生動物,水陸両用,両性動物) rarely birds). In plants, apomixis may or may not involve parthenogenesis.
Apomixis and nucellar embryony
Apomixis in plants is the formation of a new sporophyte without fertilization.(授精,豊かにすること) 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,(授精,豊かにすること) 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(胎児,(発達の)初期,幼虫) arises from an unfertilized egg within a diploid embryo(胎児,(発達の)初期,幼虫) sac(【生物】袋,嚢) that was formed without completing meiosis.(緩叙法(控えめに言ってかえって印象を強める,誇張hyperboleの反対)) In nucellar embryony, the embryo(胎児,(発達の)初期,幼虫) is formed from the diploid nucellus tissue surrounding the embryo(胎児,(発達の)初期,幼虫) sac.(【生物】袋,嚢) Nucellar embryony occurs in some citrus(柑橘類) seeds. Male apomixis can occur in rare cases, such as the Saharan Cypress Cupressus dupreziana, where the genetic material of the embryo(胎児,(発達の)初期,幼虫) are derived entirely from pollen(【植物】花粉). The term "apomixis" is also used for asexual(無性(生殖)の,(交際が)セックス抜きの) reproduction in some animals, notably water-fleas, Daphnia.
Alternation between sexual and asexual reproduction
Some species alternate between the sexual and asexual(無性(生殖)の,(交際が)セックス抜きの) strategies, an ability known as heterogamy, depending on conditions. Alternation is observed in several rotifer species and a few types of insects, such as aphids((No gloss)) which will, under certain conditions, produce eggs that have not gone through meiosis,(緩叙法(控えめに言ってかえって印象を強める,誇張hyperboleの反対)) thus cloning themselves. The cape bee Apis mellifera subsp. capensis can reproduce asexually through a process called thelytoky. A few species of amphibians,(両生類の,水陸両用の,(名)両生類,水陸両用機,両生動物,水陸両用,両性動物) reptiles, and birds have a similar ability (see parthenogenesis for examples). For example, the freshwater(淡水) crustacean(甲殻類の) Daphnia reproduces by parthenogenesis in the spring to rapidly populate ponds, then switches to sexual reproduction as the intensity of competition and predation(補食,補食関係) increases. Another example are monogonont rotifers of the genus((動植物分類上の)属,種類) Brachionus, which reproduce via cyclical((No gloss)) 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((カビ・キノコなどの)菌(類),キノコ,茸,黴,急に発生するもの,菌類) alternate between sexual and asexual(無性(生殖)の,(交際が)セックス抜きの) reproduction.
For example, the slime(どろどろした物,ねば土,ヘドロ,粘液,を泥で覆う) mold Dictyostelium undergoes binary(二進の) fission(分裂,核分裂) (mitosis) as single-celled amoebae(アメーバ) under favorable conditions. However, when conditions turn unfavorable, the cells aggregate(1.集まった,集合した,総計の,2.骨材,2.集合,総量,3.集める,統合する,総計〜となる) and follow one of two different developmental pathways, depending on conditions. In the social pathway, they form a multicellular slug which then forms a fruiting body with asexually generated spores.(因子,胞子,胚種) In the sexual pathway, two cells fuse(ヒューズ,信管,導火線,にヒューズを付ける,溶かす,溶ける,融合・連合する,ヒューズが飛ぶ,ヒューズを飛ばす) to form a giant cell that develops into a large cyst.(膿胞,胞嚢,嚢胞(のうほう)) When this macrocyst germinates,((種子など)が芽を出す,成長する,発生する,発生させる,生長し始める,芽を出す) it releases hundreds of amoebic cells that are the product of meiotic recombination(再結合) between the original two cells.
The hyphae of the common mold (Rhizopus) are capable of producing both mitotic as well as meiotic spores.(因子,胞子,胚種) Many algae(藻類) similarly switch between sexual and asexual(無性(生殖)の,(交際が)セックス抜きの) reproduction. A number of plants use both sexual and asexual(無性(生殖)の,(交際が)セックス抜きの) means to produce new plants, some species alter their primary modes of reproduction from sexual to asexual(無性(生殖)の,(交際が)セックス抜きの) under varying environmental conditions.
Inheritance of asexual reproduction in sexual species
For example, in the rotifer Brachionus calyciflorus asexual(無性(生殖)の,(交際が)セックス抜きの) reproduction (obligate parthenogenesis) can be inherited by a recessive((No gloss)) allele,((No gloss)) which leads to loss of sexual reproduction in homozygous offspring. Inheritance of asexual(無性(生殖)の,(交際が)セックス抜きの) reproduction by a single recessive((No gloss)) locus((No gloss)) has also been found in the parasitoid wasp Lysiphlebus fabarum.
Examples in animals
There are examples of parthenogenesis in the hammerhead((No gloss)) shark and the blacktip shark. 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.
Reptiles use the ZW sex-determination system, which produces either males (with ZZ sex chromosomes)(染色体) or females (with ZW or WW sex chromosomes).(染色体) Until 2010, it was thought that the ZW chromosome(染色体) system used by reptiles was incapable of producing viable(1.<計画・考えが>実行可能な,実用的な,2.生存できる,生存能力のある) WW offspring, but a (ZW) female boa constrictor((No gloss)) was discovered to have produced viable(1.<計画・考えが>実行可能な,実用的な,2.生存できる,生存能力のある) female offspring with WW chromosomes.(染色体) 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(無性(生殖)の,(交際が)セックス抜きの) reproduction in animals, whereby(なにによって,そのために,何について,どのようにして,どういう手段で,それによって) the fertilized egg or a later stage of embryonic(胚に関する,胎児の) 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.(四つ子) 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(三つ揃い,三連音符) 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(無性(生殖)の,(交際が)セックス抜きの) reproduction has allowed the animals to evolve new proteins(タンパク質,蛋白質,たん白質) through the Meselson effect that have allowed them to survive better in periods of dehydration.((No gloss))
Molecular evidence strongly suggest that at least two species of the stick insect genus((動植物分類上の)属,種類) Timema have used only asexual(無性(生殖)の,(交際が)セックス抜きの) (parthenogenetic) reproduction for one million years, the longest period known for any insect.
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