Asexual reproduction

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Asexual reproduction in liverworts: a caducous phylloid germinating

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 definition is agamogenesis which is reproduction without the fusion 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 reproduce asexually as well.

While all prokaryotes reproduce asexually (without the formation and fusion 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)).[1] 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 [2] 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[3] may underlie why few animals have relinquished 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 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))[4][5] (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)))

Types of asexual reproduction[edit]


Main article: Fission (biology)

An important form of fission is binary(二進の) fission. In binary(二進の) fission, 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), reproduce asexually through binary(二進の) fission; most of these are also capable of sexual reproduction.

Another type of fission is multiple fission that is advantageous to the plant life cycle. Multiple fission 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 of the parent cell divides several times by mitosis((No gloss)), producing several nuclei. The cytoplasm((No gloss)) then separates, creating multiple daughter cells.[6][7][8]

In apicomplexans, multiple fission, 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)[9][10] sporogony results in sporozoites, and gametogony results in micro((No gloss))gametes.

In multiple fission, the nucleus of the parent cell divides by mitosis((No gloss)) several times, producing several nuclei. The cytoplasm((No gloss)) then separates, creating multiple daughter cells.[6][7][8]


Main article: Budding

Some cells split via budding (for example baker's yeast), 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 to their separation.[11]

Endopolygeny is the division into several organisms at once by internal budding.[11] 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[edit]

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.[12] 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.[13]

Spore formation[edit]

Main article: Sporogenesis

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 cells called mitospores that develop into a new organism after dispersal.((No gloss)) This method of reproduction is found for example in conidial fungi 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, 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 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[Which ones?], turbellarians and sea stars), fungi, 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 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.[14]


Agamogenesis is any form of reproduction that does((No gloss)) not involve a male gamete. Examples are parthenogenesis and apomixis.


Main article: Parthenogenesis

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, rarely birds). In plants, apomixis may or may not involve parthenogenesis.

Apomixis and nucellar embryony[edit]

Main articles: 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 arises from an unfertilized egg within a diploid embryo 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 is formed from the diploid nucellus tissue surrounding the embryo 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 are derived entirely from pollen. 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[edit]

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 (cyclical parthenogenesis e.g. in Brachionus 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, reptiles, and birds have a similar ability (see parthenogenesis for examples). For example, the freshwater 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 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 (mitosis) as single-celled amoebae(アメーバ) 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 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 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, it releases hundreds of amoebic cells that are the product of meiotic recombination(再結合) between the original two cells.[15]

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.[16] 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.[17]

Inheritance of asexual reproduction in sexual species[edit]

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.[18] [19]
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.[20]

Examples in animals[edit]

There are examples of parthenogenesis in the hammerhead((No gloss)) shark[21] and the blacktip shark.[22] 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 WW offspring, but a (ZW) female boa constrictor((No gloss)) was discovered to have produced viable female offspring with WW chromosomes.(染色体)[23] 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 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 through the Meselson effect that have allowed them to survive better in periods of dehydration.((No gloss))[24]

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.[25]

See also[edit]


  1. ^ Narra HP, Ochman H; Ochman (2006). "Of what use is sex to bacteria?". Current Biology 16 (17): R705–710. doi:10.1016/j.cub.2006.08.024. PMID 16950097. 
  2. ^ Dawson KJ (October 1995). "The Advantage of Asexual Reproduction: When is it Two-fold?". Journal of Theoretical Biology 176 (3): 341–347. doi:10.1006/jtbi.1995.0203. 
  3. ^ Engelstädter J (November 2008). "Constraints on the evolution of asexual reproduction". BioEssays 30 (11–12): 1138–1150. doi:10.1002/bies.20833. PMID 18937362. 
  4. ^ Bernstein H, Hopf FA, Michod RE; Hopf; Michod (1987). "The molecular basis of the evolution of sex". Adv. Genet. Advances in Genetics 24: 323–70. doi:10.1016/s0065-2660(08)60012-7. ISBN 9780120176243. PMID 3324702. 
  5. ^ 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
  6. ^ a b "Cell reproduction". Encyclopædia Britannica. 
  7. ^ a b Britannica Educational Publishing (2011). Fungi, Algae, and Protists. The Rosen Publishing Group. ISBN 978-1-61530-463-9. 
  8. ^ a b P.Puranik, Asha Bhate (2007). Animal Forms And Functions: Invertebrata. Sarup & Sons. ISBN 978-81-7625-791-6. 
  9. ^ Lynn Margulis, Heather I. McKhann, Lorraine Olendzenski (2001). Illustrated glossary of protoctista: vocabulary of the algae, apicomplexa, ciliates, foraminifera, microspora, water molds, slime molds, and the other protoctists. Jones & Bartlett learn. ISBN 978-0-86720-081-2. 
  10. ^ Yoshinori Tanada, Harry K. Kaya (1993). Insect pathology. Gulf Professional Publishing. ISBN 978-0-12-683255-6. 
  11. ^ a b James Desmond Smyth, Derek Wakelin (1994). Introduction to animal parasitology (3 ed.). Cambridge University Press. pp. 101–102. ISBN 0-521-42811-4. 
  12. ^ "Asexual Reproduction". Retrieved 13 August 2010. 
  13. ^ Fun Fact (11 May 2010). "Celebrating Wildflowers - Fading Gold - How Aspens Grow". Archived from the original on 23 September 2010. Retrieved 13 August 2010. 
  14. ^ Helen Nilsson Sköld, Matthias Obst, Mattias Sköld, & Bertil Åkesson (2009). "Stem Cells in Asexual Reproduction of Marine Invertebrates". In Baruch Rinkevich, Valeria Matranga. Stem Cells in Marine Organisms. Springer. p. 125. ISBN 978-90-481-2766-5. 
  15. ^ R. S. Mehrotra; K. R. Aneja (December 1990). An Introduction to Mycology. New Age International. pp. 83 ff. ISBN 978-81-224-0089-2. Retrieved 4 August 2010. 
  16. ^ Kathleen M. Cole; Robert G. Sheath (1990). Biology of the red algae. Cambridge University Press. pp. 469–. ISBN 978-0-521-34301-5. Retrieved 4 August 2010. 
  17. ^ Edward G. Reekie; Fakhri A. Bazzaz (28 October 2005). Reproductive allocation in plants. Academic Press. pp. 99–. ISBN 978-0-12-088386-8. Retrieved 4 August 2010. 
  18. ^ 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. doi:10.1371/journal.pone.0012854. PMID 20862222. 
  19. ^ 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. doi:10.1093/jhered/esr036. 
  20. ^ Sandrock, C.; Vorburger, C. (2011). Curr Biol. 21 (5): 433–7.  Missing or empty |title= (help)
  21. ^ Savage, Juliet Eilperin (23 May 2007). "Female Sharks Can Reproduce Alone, Researchers Find". Washington Post. Retrieved 27 April 2008. 
  22. ^ Chapman, D. D.; Firchau, B.; Shivji, M. S. (11 October 2008). "'Virgin Birth' By Shark Confirmed: Second Case Ever". Journal of Fish Biology ( 73 (6): 1473. doi:10.1111/j.1095-8649.2008.02018.x. Retrieved 13 August 2010. 
  23. ^ "'Boa constrictor produces fatherless babies'". CBC News - Technology&Science. 3 November 2010. Retrieved 2014-10-20. 
  24. ^ Pouchkina-Stantcheva, N. N.; McGee, B. M.; Boschetti, C.; Tolleter, D.; Chakrabortee, S.; Popova, A. V.; Meersman, F.; MacHerel, D.; Hincha, D. K. (2007). "Functional Divergence of Former Alleles in an Ancient Asexual Invertebrate". Science 318 (5848): 268–71. doi:10.1126/science.1144363. PMID 17932297. 
  25. ^ Davies, Ella. "Sticks insects survive one million years without sex". BBC. Archived from the original on 19 July 2011. Retrieved 19 July 2011. 

Further reading[edit]

  • Graham, L., J. Graham, & L. Wilcox. 2003. Plant Biology. Pearson Education, Inc., Upper Saddle River, N.J.: pp. 258–259.
  • Raven, P.H., Evert, R.F., Eichhorn, S.E. 2005. Biology of Plants, 7th Edition. W.H. Freeman and Company Publishers, NY.
  • Avise, J. 2008. Clonality: The Genetics, Ecology, and Evolution of Sexual Abstinence in Vertebrate Animals. Oxford University Press

External links[edit]