Asexual reproduction

Asexual reproduction[1] is a type of reproduction by which offspring arise from a single organism, and inherit the genes of that parent only; it does not involve the fusion of gametes, and almost never changes the number of chromosomes. Asexual reproduction is the primary form of reproduction for single-celled organisms such as archaea and bacteria. Many plants and fungi sometimes 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 in the sense of genetic recombination in meiosis.[2] A complete lack of sexual reproduction is relatively rare among multi-cellular organisms, particularly animals. It is not entirely understood why the ability to reproduce sexually is so common among them. Current hypotheses[3] 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[4] may underlie why few animals have relinquished sexual reproduction completely in their life-cycles. Another constraint on switching from sexual to asexual reproduction would be the concomitant loss of meiosis and the protective recombinational repair of DNA damage afforded as one function of meiosis.[5][6]

Caduco
Asexual reproduction in liverworts: a caducous phylloid germinating

Types

Fission

An important form of fission is binary fission, where the parent organism is replaced by two daughter organisms, because it literally divides in two. Only prokaryotes (the archaea and the bacteria) reproduce asexually through binary fission. 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.[7][8][9]

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,[10][11] sporogony results in sporozoites, and gametogony results in microgametes.

Budding

S cerevisiae under DIC microscopy
The yeast Saccharomyces cerevisiae reproducing by 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 multi-cellular level; an animal example is the hydra, which reproduces by budding. The buds grow into fully matured individuals which eventually break away from the parent organism.

Internal budding is a process of asexual reproduction, favoured by parasites such as Toxoplasma gondii. It involves an unusual process in which two (endodyogeny) or more (endopolygeny) daughter cells are produced inside a mother cell, which is then consumed by the offspring prior to their separation.[12]

Also, budding (external or internal) is present in some worm like Taenia or Echinococci; these worm produce cyst and then produce (invaginated or evaginated) protoscolex with budding.

Vegetative propagation

Bryophyllum daigremontianum nahaufnahme1
Vegetative plantlets of mother-of-thousands, Bryophyllum (Kalanchoe) daigremontianum

Vegetative propagation is a type of asexual reproduction found in plants where new individuals are formed without the production of seeds or spores by meiosis or syngamy.[13] 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 shoots and may form a clonal colony, where all the individuals are clones, and the clones may cover a large area.[14]

Spore formation

Many multi-cellular organisms form spores during their biological life cycle in a process called sporogenesis. Exceptions are animals and some protists, who 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 multi-cellular individuals (called gametophytes in the case of plants) without a fertilization event. These haploid individuals give rise to gametes through mitosis. Meiosis and gamete formation therefore occur in separate generations or "phases" of the life cycle, referred to as alternation of generations. Since sexual reproduction is often more narrowly defined as the fusion of gametes (fertilization), spore formation in plant sporophytes and algae might be considered a form of asexual reproduction (agamogenesis) despite being the result of meiosis and undergoing a reduction in ploidy. However, both events (spore formation and fertilization) are necessary to complete sexual reproduction in the plant life cycle.

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, most Basidiomycota, and many algae, are produced by meiosis.

Fragmentation

Tu - Linckia guildingi
Linckia guildingi "comet", a starfish regrowing from a single arm

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 planarians, many annelid worms including polychaetes[15] and some oligochaetes,[16] turbellarians and sea stars. Many fungi and plants reproduce asexually. Some plants have specialized structures for reproduction via fragmentation, such as gemma in liverworts. Most lichens, which are a symbiotic union of a fungus and photosynthetic algae 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 multi-cellular 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.[17] Researchers claim today that due to many environmental and epigenetic differences, that clones originated in the same ancestor might actually be genetically and epigenetically different.[18]

Agamogenesis

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

Parthenogenesis

Aphid-giving-birth
Aphid giving birth to live young from an unfertilized egg

Parthenogenesis is a form of agamogenesis in which an unfertilized egg develops into a new individual. Parthenogenesis occurs naturally in many plants, invertebrates (e.g. water fleas, rotifers, aphids, stick insects, some ants, bees and parasitic wasps), and vertebrates (e.g. some reptiles, amphibians, 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. 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 the Saharan Cypress Cupressus dupreziana, where the genetic material of the embryo are derived entirely from pollen.

Alternation between sexual and asexual reproduction

Soybeanaphidlifecycle
Aphid populations are often entirely female during the summer, with sexual reproduction only to produce eggs for overwintering.

Some 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, such as aphids which will, under certain conditions, produce eggs that have not gone through meiosis, 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. For example, 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. Another example are monogonont rotifers of the genus Brachionus, which 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.

For example, 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.[19]

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.[20] 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.[21]

Inheritance in asexual species

For example, 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.[22][23]
Inheritance of asexual reproduction by a single recessive locus has also been found in the parasitoid wasp Lysiphlebus fabarum.[24]

Examples in animals

Asexual reproduction is found in nearly half of the animal phyla.[25] Parthenogenesis occurs in the hammerhead shark[26] and the blacktip shark.[27] 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.[28] The female boa could have chosen any number of male partners (and had successfully in the past) but on these occasions she reproduced asexually, creating 22 female babies with WW sex-chromosomes.

Polyembryony is a widespread form of asexual reproduction in animals, whereby the fertilized egg or a later stage of embryonic development splits to form genetically identical clones. Within animals, this phenomenon has been best studied in the parasitic Hymenoptera. In the 9-banded armadillos, this process is obligatory and usually gives rise to genetically identical quadruplets. In other mammals, monozygotic twinning has no apparent genetic basis, though its occurrence is common. There are at least 10 million identical human twins and triplets in the world today.

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.[29]

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.[30]

In the grass thrips genus Aptinothrips there have been several transitions to asexuality, likely due to different causes.[31]

See also

References

  1. ^ "Asexual but Not Clonal: Evolutionary Processes in Automictic Populations | Genetics". Retrieved 2018-08-21.
  2. ^ Narra, H. P.; Ochman, H. (2006). "Of what use is sex to bacteria?". Current Biology. 16 (17): R705–710. doi:10.1016/j.cub.2006.08.024. PMID 16950097.CS1 maint: Multiple names: authors list (link)
  3. ^ Dawson, K.J. (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.
  4. ^ Engelstädter, J. (November 2008). "Constraints on the evolution of asexual reproduction". BioEssays. 30 (11–12): 1138–1150. doi:10.1002/bies.20833. PMID 18937362.
  5. ^ Bernstein, H.; Hopf, F.A.; Michod, R.E. (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.CS1 maint: Multiple names: authors list (link)
  6. ^ 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
  7. ^ "Cell reproduction". Encyclopædia Britannica.
  8. ^ Britannica Educational Publishing (2011). Fungi, Algae, and Protists. The Rosen Publishing Group. ISBN 978-1-61530-463-9.
  9. ^ P.Puranik; Asha Bhate (2007). Animal Forms And Functions: Invertebrata. Sarup & Sons. ISBN 978-81-7625-791-6.
  10. ^ Margulis, Lynn; McKhann, Heather I.; Olendzenski, Lorraine (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.
  11. ^ Yoshinori Tanada; Harry K. Kaya (1993). Insect pathology. Gulf Professional Publishing. ISBN 978-0-12-683255-6.
  12. ^ Smyth, James Desmond; Wakelin, Derek (1994). Introduction to animal parasitology (3 ed.). Cambridge University Press. pp. 101–102. ISBN 0-521-42811-4.
  13. ^ "Asexual Reproduction". Ucmp.berkeley.edu. Retrieved 13 August 2010.
  14. ^ "Celebrating Wildflowers - Fading Gold - How Aspens Grow". Fs.fed.us. 11 May 2010. Archived from the original on 23 September 2010. Retrieved 13 August 2010.
  15. ^ Ruppert, E. E.; Fox, R. S.; Barnes, R. D. (2004). "Annelida". Invertebrate Zoology (7 ed.). Brooks / Cole. pp. 434–441. ISBN 0-03-025982-7.
  16. ^ Ruppert, E.E.; Fox, R.S. & Barnes, R.D. (2004). "Annelida". Invertebrate Zoology (7 ed.). Brooks / Cole. pp. 466–469. ISBN 0-03-025982-7.
  17. ^ Sköld, Helen Nilsson; Obst, Matthias; Sköld, Mattias; Åkesson, Bertil (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.
  18. ^ Neuhof, Moran; Levin, Michael; Rechavi, Oded (26 August 2016). "Vertically and horizontally-transmitted memories – the fading boundaries between regeneration and inheritance in planaria". Biology Open. 5: bio.020149. doi:10.1242/bio.020149. PMC 5051648. PMID 27565761. Retrieved 30 August 2016.
  19. ^ Mehrotra, R. S.; Aneja, K. R. (December 1990). An Introduction to Mycology. New Age International. pp. 83 ff. ISBN 978-81-224-0089-2. Retrieved 4 August 2010.
  20. ^ Cole, Kathleen M.; Sheath, Robert G. (1990). Biology of the red algae. Cambridge University Press. pp. 469–. ISBN 978-0-521-34301-5. Retrieved 4 August 2010.
  21. ^ 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.
  22. ^ 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. PMC 2942836. PMID 20862222.
  23. ^ 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. PMC 3113615. PMID 21576287.
  24. ^ Sandrock, C.; Vorburger, C. (2011). "Single-locus recessive inheritance of asexual reproduction in a parasitoid wasp". Curr. Biol. 21 (5): 433–7. doi:10.1016/j.cub.2011.01.070. PMID 21353557.CS1 maint: Multiple names: authors list (link)
  25. ^ Minelli, Alessandro (2009). Asexual reproduction and regeneration. Perspectives in Animal Phylogeny and Evolution. Oxford University Press. pp. 123–127. ISBN 978-0198566205.
  26. ^ Savage, Juliet Eilperin (23 May 2007). "Female Sharks Can Reproduce Alone, Researchers Find". Washington Post. Retrieved 27 April 2008.
  27. ^ Chapman, D. D.; Firchau, B.; Shivji, M. S. (11 October 2008). "'Virgin Birth' By Shark Confirmed: Second Case Ever". Journal of Fish Biology. Sciencedaily.com. 73 (6): 1473–1477. doi:10.1111/j.1095-8649.2008.02018.x. Retrieved 13 August 2010.
  28. ^ "Boa constrictor produces fatherless babies". CBC News - Technology & Science. 3 November 2010. Retrieved 2014-10-20.
  29. ^ 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.
  30. ^ Schwander, T.; et al. (2011). "Molecular evidence for ancient asexuality in Timema stick insects". Current Biology. 21 (13): 1129–34. doi:10.1016/j.cub.2011.05.026.
  31. ^ van der Kooi, C. J.; Schwander, T. (2014). "Evolution of asexuality via different mechanisms in grass thrips (Thysanoptera: Aptinothrips)". Evolution. 86 (7): 1883–1893. doi:10.1111/evo.12402.

Further reading

  • Avise, J. (2008). Clonality: The Genetics, Ecology, and Evolution of Sexual Abstinence in Vertebrate Animals. Oxford University Press. ISBN 978-0-19-536967-0.
  • Graham, L.; Graham, J.; Wilcox, L. (2003). Plant Biology. Upper Saddle River, NJ: Pearson Education. pp. 258–259. ISBN 0-13-030371-2.
  • Raven, P. H.; Evert, R. F.; Eichhorn, S. E. (2005). Biology of Plants (7th ed.). NY: W.H. Freeman and Company. ISBN 0-7167-6284-6.

External links

Acervulus

An acervulus (pl. acervuli) is a small asexual fruiting body that erupts through the epidermis of host plants parasitised by mitosporic fungi of the form order Melanconiales (Deuteromycota, Coelomycetes). It has the form of a small cushion at the bottom of which short crowded conidiophores are formed. The spores escape through an opening at the top.

Apomixis

In botany, apomixis was defined by Hans Winkler as replacement of the normal sexual reproduction by asexual reproduction, without fertilization. Its etymology is Greek for "away from" + "mixing". This definition notably does not mention meiosis. Thus "normal asexual reproduction" of plants, such as propagation from cuttings or leaves, has never been considered to be apomixis, but replacement of the seed by a plantlet or replacement of the flower by bulbils were categorized as types of apomixis. Apomictically produced offspring are genetically identical to the parent plant.

Some authors included all forms of asexual reproduction within apomixis, but that generalization of the term has since died out.In flowering plants, the term "apomixis" is commonly used in a restricted sense to mean agamospermy, i.e. clonal reproduction through seeds. Although agamospermy could theoretically occur in gymnosperms, it appears to be absent in that group.Apogamy is a related term that has had various meanings over time. In plants with independent gametophytes (notably ferns), the term is still used interchangeably with "apomixis", and both refer to the formation of sporophytes by parthenogenesis of gametophyte cells.

Male apomixis (paternal apomixis) involves replacement of the genetic material of egg by the pollen that of egg

Asexual

Asexual or Asexuals may refer to:

Asexual reproduction

Asexual reproduction in starfish

Asexuality, the lack of sexual attraction to anyone or lack of interest in or desire for sexual activity

Asexuals (band), a Canadian punk rock band

Asexual reproduction in starfish

Asexual reproduction in starfish takes place by fission or through autotomy of arms. In fission, the central disc breaks into two pieces and each portion then regenerates the missing parts. In autotomy, an arm is shed which continues to live independently as a "comet", eventually growing a new disc and further arms. Only certain genera of starfish are able to reproduce in these ways.

Blastospore

A blastospore is an asexual fungal spore produced by budding. Produced by fungi within the phylum Glomeromycota and others. It is also known as a blastoconidium (plural = blastoconidia).

An example of a fungus that forms blastospores is Candida albicans.

Budding

Budding 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. The small bulb like projection coming out from the yeast cell is called a bud. The new organism remains attached as it grows, separating from the parent organism only when it is mature, leaving behind scar tissue. Since the reproduction is asexual, the newly created organism is a clone and is genetically identical to the parent organism.

Organisms such as hydra use regenerative cells for reproduction in the process of budding.

In hydra, a bud develops as an outgrowth due to repeated cell division at one specific site. These buds develop into tiny individuals and, when fully mature, detach from the parent body and become new independent individuals.

Internal budding or endodyogeny is a process of asexual 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.Endopolygeny is the division into several organisms at once by internal budding.

Cutting (plant)

A plant cutting is a piece of a plant that is used in horticulture for vegetative (asexual) propagation. A piece of the stem or root of the source plant is placed in a suitable medium such as moist soil. If the conditions are suitable, the plant piece will begin to grow as a new plant independent of the parent, a process known as striking. A stem cutting produces new roots, and a root cutting produces new stems. Some plants can be grown from leaf pieces, called leaf cuttings, which produce both stems and roots. The scions used in grafting are also called cuttings.Propagating plants from cuttings is an ancient form of cloning. There are several advantages of cuttings, mainly that the produced offspring are practically clones of their parent plants. If a plant has favorable traits, it can continue to pass down its advantageous genetic information to its offspring. This is especially economically advantageous as it allows commercial growers to clone a certain plant to ensure consistency throughout their crops.The poet Theodore Roethke wrote about plant cuttings and root growth behavior in his poems "Cuttings" and "Cuttings (Later)" found in his book The Lost Son: And Other Poems.

Division (horticulture)

Division, in horticulture and gardening, is a method of asexual plant propagation, where the plant (usually an herbaceous perennial) is broken up into two or more parts. Both the root and crown of each part is kept intact. The technique is of ancient origin, and has long been used to propagate bulbs such as garlic and saffron. Division is mainly practiced by gardeners and very small nurseries, as most commercial plant propagation is now done through plant tissue culture.

Division is one of the three main methods used by gardeners to increase stocks of plants (the other two are seed-sowing and cuttings). Division is usually applied to mature perennial plants, but may also be used for shrubs with suckering roots, such as gaultheria, kerria and sarcococca. Annual and biennial plants do not lend themselves to this procedure, as their lifespan is too short.

Most perennials are best divided and replanted every few years to keep them healthy. They may also be divided in order to produce new plants. Those with woody crowns or fleshy roots need to be cut apart, while others can be prized apart using garden forks or hand forks. Each separate section must have both shoots and roots. Division can take place at almost any time of the year, but the best seasons are Autumn and Spring.

Fission (biology)

Fission, in biology, is the division of a single entity into two or more parts and the regeneration of those parts into separate entities resembling the original. The object experiencing fission is usually a cell, but the term may also refer to how organisms, bodies, populations, or species split into discrete parts. The fission may be binary fission, in which a single entity produces two parts, or multiple fission, in which a single entity produces multiple parts.

Fragmentation (reproduction)

Fragmentation in multicellular organisms is a form of asexual reproduction in which an organism is split into fragments. Each of these fragments develop into matured, fully grown individuals that are identical to their parents.

The splitting may or may not be intentional – it may or may not occur due to man-made or natural damage by the environment or predators. This kind of organism may develop specific organs or zones that may be shed or easily broken off. If the splitting occurs without the prior preparation of the organism, both fragments must be able to regenerate the complete organism for it to function as reproduction.

Fragmentation, also known as splitting, as a method of reproduction is seen in many organisms such as filamentous cyanobacteria, molds, lichens, many plants, and animals such as sponges, acoel flatworms, some annelid worms and sea stars.

Gemma (botany)

A gemma (plural gemmae) is a single cell, or a mass of cells, or a modified bud of tissue, that detaches from the parent and develops into a new individual. This type of asexual reproduction is referred to as fragmentation. It is a means of asexual propagation in plants. These structures are commonly found in fungi, algae, liverworts and mosses, but also in some flowering plants such as pygmy sundews and some species of butterworts. Vascular plants have many other methods of asexual reproduction including bulbils and turions.

Gemmule

Gemmules are internal buds found in sponges and are involved in asexual reproduction. It is an asexually reproduced mass of cells, that is capable of developing into a new organism i.e., an adult sponge.

Plant reproduction

Plant reproduction is the production of new individuals or offspring in plants, which can be accomplished by sexual or asexual reproduction. Sexual reproduction produces offspring by the fusion of gametes, resulting in offspring genetically different from the parent or parents. Asexual reproduction produces new individuals without the fusion of gametes, genetically identical to the parent plants and each other, except when mutations occur. In seed plants, the offspring can be packaged in a protective seed, which is used as an agent of dispersal.

Propagule

In biology, a propagule is any material that functions in propagating an organism to the next stage in its life cycle, such as by dispersal. The propagule is usually distinct in form from the parent organism. Propagules are produced by plants (in the form of seeds or spores), fungi (in the form of spores), and bacteria (for example endospores or microbial cysts).In disease biology, pathogens are said to generate infectious propagules, the units that transmit a disease. These can refer to bacteria, viruses, fungi, or protists, and can be contained within host material. For instance, for influenza, the infectious propagules are carried in droplets of host saliva or mucus that are expelled during coughing or sneezing.

In horticulture, a propagule is any plant material used for the purpose of plant propagation. In asexual reproduction, a propagule is often a stem cutting. In some plants, a leaf section or a portion of root can be used. In sexual reproduction, a propagule is a seed or spore. In micropropagation, a type of asexual reproduction, any part of the plant may be used, though it is usually a highly meristematic part such as root and stem ends or buds.

Reproduction

Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parents". Reproduction is a fundamental feature of all known life; each individual organism exists as the result of reproduction. There are two forms of reproduction: asexual and sexual.

In asexual reproduction, an organism can reproduce without the involvement of another organism. Asexual reproduction is not limited to single-celled organisms. The cloning of an organism is a form of asexual reproduction. By asexual reproduction, an organism creates a genetically similar or identical copy of itself. The evolution of sexual reproduction is a major puzzle for biologists. The two-fold cost of sexual reproduction is that only 50% of organisms reproduce and organisms only pass on 50% of their genes.Sexual reproduction typically requires the sexual interaction of two specialized organisms, called gametes, which contain half the number of chromosomes of normal cells and are created by meiosis, with typically a male fertilizing a female of the same species to create a fertilized zygote. This produces offspring organisms whose genetic characteristics are derived from those of the two parental organisms.

Reproductive biology

Reproductive biology includes both sexual and asexual reproduction.Reproductive biology includes a wide number of fields:

Reproductive systems

Endocrinology

Sexual development (Puberty)

Sexual maturity

Reproduction

Fertility

Thelytoky

Thelytoky (from the 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 and reported in about 1,500 species, about 1 in 1000 of described animal species, according to a 1984 study. It is more common in invertebrates, like arthropods, but it can occur in vertebrates, including salamanders, fish, and reptiles such as some whiptail lizards.

Thelytoky can occur by different mechanisms, each of which has a different impact on the level of homozygosity. It is found in several groups of Hymenoptera, including Apidae, Aphelinidae, Cynipidae, Formicidae, Ichneumonidae, and Tenthredinidae. It can be induced in Hymenoptera by the bacteria Wolbachia and Cardinium.

Trematoda

Trematoda is a class within the phylum Platyhelminthes. It includes two groups of parasitic flatworms, known as flukes.

They are internal parasites of molluscs and vertebrates. Most trematodes have a complex life cycle with at least two hosts. The primary host, where the flukes sexually reproduce, is a vertebrate. The intermediate host, in which asexual reproduction occurs, is usually a snail.

Vegetative reproduction

Vegetative reproduction (also known as vegetative propagation, vegetative multiplication or vegetative cloning) is any form of asexual reproduction occurring in plants in which a new plant grows from a fragment of the parent plant or a specialized reproductive structure.Many plants naturally reproduce this way, but it can also be induced artificially. Horticulturalists have developed asexual propagation techniques that use vegetative plant parts to replicate plants. Success rates and difficulty of propagation vary greatly. Monocotyledons typically lack a vascular cambium and therefore are harder to propagate.

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