Hur kan träd reproducera asexually

Reproduction without a sexuell process

Not to be confused with Asexuality.

Asexual reproduction fryst vatten a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise bygd asexual reproduction from either unicellular or multicellular organisms inherit the full set of genes of their single parent and thus the newly created individual fryst vatten genetically and physically similar to the parent or an exact clone of the parent.

Asexual reproduction fryst vatten the primary form eller gestalt of reproduction for single-celled organisms such as archaea and bacteria. Many eukaryotic organisms including plants, animals, and fungi can also reproduce asexually.^[1] In vertebrates, the most common form eller gestalt of asexual reproduction fryst vatten parthenogenesis, which fryst vatten typically used as an alternative to sexuell reproduction in times when fortplantnings- opportunities are limited.

Komodo dragons and some monitor lizards can reproduce asexually.^[2]

While all prokaryotes reproduce without the formation and fusion of gametes, mechanisms for horisontell gene transfer such as conjugation, transformation and transduction can be likened to sexuell reproduction in the sense of genetic recombination in meiosis.^[3][4]

Types of asexual reproduction

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Fission

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Main article: Fission (biology)

Prokaryotes (Archaea and Bacteria) reproduce asexually through binary fission, in which the parent organism divides in two to tillverka two genetically identical daughter organisms.

Eukaryotes (such as protists and unicellular fungi) may reproduce in a functionally similar manner bygd mitosis; most of these are also capable of sexuell reproduction.

Multiple fission at the cellular level occurs in many protists, e.g. sporozoans and algae. The nucleus of the parent fängelse divides several times bygd mitosis, producing several nuclei. The cytoplasm then separates, creating multiple daughter cells.^[5][6][7]

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 fängelse membrane,^[8][9] sporogony results in sporozoites, and gametogony results in microgametes.

Budding

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Main article: Budding

Some cells divide bygd budding (for example baker's yeast), resulting in a "mother" and a "daughter" fängelse that fryst vatten initially smaller than the parent.

Budding fryst vatten also known on a multicellular level; an djur example fryst vatten the hydra,^[10] which reproduces bygd budding. The buds grow into fully matured individuals which eventually break away from the parent organism.

Internal budding fryst vatten a process of asexual reproduction, favoured bygd parasites such as Toxoplasma gondii.

It involves an unusual process in which two (endodyogeny) or more (endopolygeny) daughter cells are produced inre a mother fängelse, which fryst vatten then consumed bygd the offspring prior to their separation.^[11]

Also, budding (external or internal) occurs in some worms like Taenia or Echinococcus; these worms tillverka cysts and then producera (invaginated or evaginated) protoscolex with budding.

Vegetative propagation

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Main article: Vegetative propagation

Vegetative propagation fryst vatten a type of asexual reproduction funnen in plants where new individuals are formed without the production of seeds or spores and thus without syngamy or meiosis.^[12] Examples of vegetative reproduction include the formation of miniaturized plants called plantlets on specialized leaves, for example in kalanchoe (Bryophyllum daigremontianum) and many producera new plants from rhizomes or stolon (for example in strawberry).

Some plants reproduce bygd forming bulbs or tubers, for example tulip bulbs and Dahlia tubers. In these examples, all the individuals are clones, and the clonal population may cover a large area.^[13]

Spore formation

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Main article: Sporogenesis

Many multicellular organisms tillverka spores during their biological life cycle in a process called sporogenesis.

Exceptions are animals and some protists, which undergo meiosis immediately followed bygd fertilization. Plants and many algae on the other grabb undergo sporic meiosis where meiosis leads to the formation of cell med enkel uppsättning kromosomer spores rather than gametes. These spores grow into multicellular individuals called gametophytes, without a fertilization event.

These cell med enkel uppsättning kromosomer individuals tillverka gametes through mitosis. Meiosis and gamete formation therefore occur in separate multicellular generations or "phases" of the life cycle, referred to as alternation of generations. Since sexuell reproduction fryst vatten often more narrowly defined as the fusion of gametes (fertilization), spore formation in plant sporophytes and algae might be considered a form eller gestalt 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 sexuell reproduction in the plant life cycle.

Fungi and some algae can also utilize true asexual spore formation, which involves mitosis giving rise to fortplantnings- cells called mitospores that develop into a new organism after dispersal. This method of reproduction fryst vatten funnen for example in conidial fungi and the red algaePolysiphonia, and involves sporogenesis without meiosis.

Thus the chromosome number of the spore fängelse fryst vatten the same as that of the parent producing the spores.

Well done

However, mitotic sporogenesis fryst vatten an undantag and most spores, such as those of plants and many algae, are produced bygd meiosis.^[14][15][16]

Fragmentation

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Main article: Fragmentation (reproduction)

Fragmentation fryst vatten a form eller gestalt of asexual reproduction where a new organism grows from a fragment of the parent.

Each fragment develops into a mature, fully grown individual. Fragmentation fryst vatten seen in many organisms. Animals that reproduce asexually include planarians, many annelid worms including polychaetes^[17] and some oligochaetes,^[17]turbellarians and sea stars. Many fungi and plants reproduce asexually. Some plants have specialized structures for reproduction via fragmentation, such as gemmae in mosses and liverworts.

Most lichens, which are a symbiotic union of a fungus and photosynthetic algae or cyanobacteria, reproduce through fragmentation to ensure that new individuals contain both symbionts. These fragments can take the form eller gestalt of soredia, dust-like particles consisting of fungal hyphae wrapped around photobiont cells.

Clonal Fragmentation in multicellular or colonial organisms fryst vatten a struktur of asexual reproduction or cloning where an organism fryst vatten 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 fryst vatten usually known as fissiparity.^[18] Due to many environmental and epigenetic differences, clones originating from the same ancestor might actually be genetically and epigenetically different.^[19]

Agamogenesis

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Agamogenesis fryst vatten any form eller gestalt of reproduction that does not involve a male gamete.

Examples are parthenogenesis and apomixis.

Parthenogenesis

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Main article: Parthenogenesis

Parthenogenesis fryst vatten a form eller gestalt of agamogenesis in which an unfertilized egg develops into a new individual. It has been documented in over 2, species.^[20] Parthenogenesis occurs in the wild in many invertebrates (e.g.

vatten fleas, rotifers, aphids, stick insects, some ants, bees and parasitic wasps) and vertebrates (mostly reptiles, amphibians, and fish). It has also been documented in domestic birds and in genetically altered lab mice.^[21][22] Plants can engage in parthenogenesis as well through a process called apomixis.

However this process fryst vatten considered bygd many to not be an independent reproduction method, but instead a breakdown of the mechanisms behind sexuell reproduction.^[23] Parthenogenetic organisms can be split into two main categories: facultative and obligate.

Facultative parthenogenesis

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In facultative parthenogenesis, females can reproduce both sexually and asexually.^[20] Because of the many advantages of sexuell reproduction, most facultative parthenotes only reproduce asexually when forced to.

This typically occurs in instances when finding a mate becomes difficult. For example, kvinnlig zebra sharks will reproduce asexually if they are unable to find a mate in their ocean habitats.^[2]

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 fryst vatten the Komodo dragon at 10 feet long and over pounds.^[24][25]

Heterogony fryst vatten a form eller gestalt of facultative parthenogenesis where females alternate between sexuell and asexual reproduction at regular intervals (see Alternation between sexuell and asexual reproduction).

Aphids are one group of organism that engages in this type of reproduction. They use asexual reproduction to reproduce quickly and create winged offspring that can colonize new plants and reproduce sexually in the fall to lay eggs for the next season.^[26] However, some aphid species are obligate parthenotes.^[27]

Obligate parthenogenesis

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In obligate parthenogenesis, females only reproduce asexually.^[20] One example of this fryst vatten the desert grassland whiptail lizard, a hybrid of two other species.

Typically hybrids are infertile but through parthenogenesis this species has been able to develop stable populations.^[28]

Gynogenesis fryst vatten a form eller gestalt of obligate parthenogenesis where a sperma fängelse fryst vatten used to initiate reproduction. However, the sperm's genes never get incorporated into the egg fängelse.

The best known example of this fryst vatten the Amazon molly. Because they are obligate parthenotes, there are no males in their species so they depend on males from a closely related species (the Sailfin molly) for sperm.^[29]

Apomixis and nucellar embryony

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Main articles: Apomixis and Nucellar embryony

Apomixis in plants fryst vatten the formation of a new sporophyte without fertilization.

It fryst vatten important in ferns and in flowering plants, but fryst vatten very rare in other seed plants. In flowering plants, the begrepp "apomixis" fryst vatten 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 europeisk 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 fryst vatten formed from the diploid nucellus tissue surrounding the embryo sac. Nucellar embryony occurs in some citrus seeds. Male apomixis can occur in rare cases, such as in the Saharan Cypress Cupressus dupreziana, where the genetic ämne of the embryo fryst vatten derived entirely from pollen.^[30][31][32]

Androgenesis

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Main article: Androgenesis

Androgenesis occurs when a zygote fryst vatten produced with only paternal nuclear genes.

B, a single parent plant makes new plants on its own

During standard sexuell reproduction, one kvinna and one male parent each producera cell med enkel uppsättning kromosomer gametes (such as a sperma or egg fängelse, each containing only a single set of chromosomes), which recombine to create offspring with genetic ämne from both parents. However, in androgenesis, there fryst vatten no recombination of maternal and paternal chromosomes, and only the paternal chromosomes are passed down to the offspring (the inverse of this fryst vatten gynogenesis, where only the maternal chromosomes are inherited, which fryst vatten more common than androgenesis).^[33] The offspring produced in androgenesis will still have maternally inherited mitochondria, as fryst vatten the case with most sexually reproducing species.

Androgenesis occurs in natur in many invertebrates (for example, clams,^[34] stick insects,^[35] some ants,^[36] bees,^[37] flies^[38] and parasitic wasps^[37]) and vertebrates (mainly amphibians^[39] and fish^[37][40]).

The androgenesis has also been seen in genetically modified laboratory mice.^[41]

One of two things can occur to tillverka offspring with exclusively paternal genetic material: the maternal nuclear genome can be eliminated from the zygote, or the kvinnlig can tillverka an egg with no nucleus, resulting in an embryo developing with only the genome of the male gamete.

Male apomixis

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Other type of androgenesis fryst vatten the male apomixis or paternal apomixis fryst vatten a fortplantnings- process in which a plant develops from a sperma fängelse (male gamete) without the participation of a kvinnlig fängelse (ovum). In this process, the zygote fryst vatten formed solely with genetic ämne from the father, resulting in offspring genetically identical to the male organism.^[42][43][44] This has been noted in many plants like Nicotiana, Capsicum frutescens, Cicer arietinum, Poa arachnifera, Solanum verrucosum, Phaeophyceae,^[45]Pripsacum dactyloides, Zea mays,^[37] and occurs as the regular fortplantnings- method in Cupressus dupreziana.^[42] This contrasts with the more common apomixis, where development occurs without fertilization, but with genetic ämne only from the mother.

There are also clonal species that reproduce through vegetative reproduction like Lomatia tasmanica^[46][47] and Pando,^[48] where the genetic ämne fryst vatten exclusively male.

Other species where androgenesis has been observed naturally are the stick insects Bacillus rossius and Bassillus Grandii,^[35] the little fire ant Wasmannia auropunctata,^[36]Vollenhovia emeryi,^[37]Paratrechina longicornis,^[37] occasionally in Apis mellifera,^[37] the Hypseleotris carp gudgeons,^[37] the parasitoid Venturia canescens,^[37] and occasionally in fruit flies Drosophila melanogaster carrying a specific mutant allele.^[38] It has also been induced in many crops and fish via irradiation of an egg fängelse to destroy the maternal nuclear genome.^[49]

Obligate androgenesis

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Obligate androgenesis fryst vatten the process in which males^[34] are capable of producing both eggs and sperma, however, the eggs have no genetic contribution and the offspring komma only from the sperma, which allows these individuals to self-fertilize and tillverka clonal offspring without the need for females.

They are also capable of interbreeding with sexuell and other androgenetic lineages in a phenomenon known as "egg parasitism." This method of reproduction has been funnen in several species of the clam genus Corbicula^[34] and recently in the fish Squalius alburnoides.^[40]

Other species where androgenesis has been observed naturally are the stick insects Bacillus rossius and Bassillus Grandii,^[35] the little fire ant Wasmannia auropunctata,^[36]Vollenhovia emeryi,^[37]Paratrechina longicornis,^[37] occasionally in Apis mellifera,^[37] the Hypseleotris carp gudgeons,^[37] the parasitoid Venturia canescens,^[37] and occasionally in fruit flies Drosophila melanogaster carrying a specific mutant allele.^[38] It has also been induced in many crops and fish via irradiation of an egg fängelse to destroy the maternal nuclear genome.^[49]

Alternation between sexuell and asexual reproduction

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See also: Plant reproduction §&#;Sexual reproduction

Some species can alternate between sexuell and asexual strategies, an ability known as heterogamy, depending on many conditions.

Alternation fryst vatten observed in several rotifer species (cyclical parthenogenesis e.g. in Brachionus species) and a few types of insects.

One example of this fryst vatten aphids which can engage in heterogony. In this struktur, females are born pregnant and producera only hona offspring. This cycle allows them to reproduce very quickly. However, most species reproduce sexually once a year.

This switch fryst vatten triggered bygd environmental changes in the fall and causes females to develop eggs instead of embryos. This dynamic fortplantnings- cycle allows them to tillverka specialized offspring with polyphenism, a type of polymorphism where different phenotypes have evolved to carry out specific tasks.^[26]

The cape bee Apis mellifera subsp.

capensis can reproduce asexually through a process called thelytoky. The freshwater crustacean Daphnia reproduces bygd parthenogenesis in the spring to rapidly populate ponds, then switches to sexuell reproduction as the intensity of competition and predation increases. Monogonont rotifers of the genus Brachionus reproduce via cyclical parthenogenesis: at low population densities females tillverka asexually and at higher densities a kemikalie cue accumulates and induces the transition to sexuell reproduction.

Many protists and fungi alternate between sexuell and asexual reproduction. A few species of amphibians, reptiles, and birds have a similar ability.^{[which?][which?]}

The slime mold Dictyostelium undergoes binary fission (mitosis) as single-celled amoebae beneath favorable conditions.

However, when conditions vända unfavorable, the cells aggregate and follow one of two different developmental pathways, depending on conditions. In the social pathway, they form eller gestalt a multi-cellular slug which then forms a fruiting body with asexually generated spores. In the sexuell pathway, two cells fuse to form eller gestalt a giant fängelse 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.^[50]

The hyphae of the common mold (Rhizopus) are capable of producing both mitotic as well as meiotic spores. Many algae similarly switch between sexuell and asexual reproduction.^[51] A number of plants use both sexuell and asexual means to tillverka new plants, some species alter their primary modes of reproduction from sexuell to asexual beneath varying environmental conditions.^[52]

Inheritance in asexual species

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In the rotiferBrachionus calyciflorus asexual reproduction (obligate parthenogenesis) can be inherited bygd a recessive allele, which leads to loss of sexuell reproduction in homozygous offspring.^[53][54]
Inheritance of asexual reproduction bygd a single recessive locus has also been funnen in the parasitoid waspLysiphlebus fabarum.^[55]

Examples in animals

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Asexual reproduction fryst vatten funnen in nearly half of the djur phyla.^[56] Parthenogenesis occurs in the hammerhead shark^[57] and the blacktip shark.^[58][59] In both cases, the sharks had reached sexuell 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 fryst vatten another example.

Some reptiles use the ZW sex-determination struktur, which produces either males (with ZZ sex chromosomes) or females (with ZW or WW sex chromosomes). Until , it was thought that the ZW chromosome struktur used bygd reptiles was incapable of producing viable WW offspring, but a (ZW) kvinnlig boa constrictor was discovered to have produced viable kvinna offspring with WW chromosomes.^[60] The hona boa could have chosen any number of male partners (and had successfully in the past) but on this occasion she reproduced asexually, creating 22 kvinnlig babies with WW sex-chromosomes.

Polyembryony fryst vatten a widespread struktur of asexual reproduction in animals, whereby the fertilized egg or a later scen of embryonic development splits to form eller gestalt genetically identical clones. Within animals, this phenomenon has been best studied in the parasitic Hymenoptera. In the nine-banded armadillos, this process fryst vatten obligatory and usually gives rise to genetically identical quadruplets.

In other mammals, monozygotic twinning has no apparent genetic grund, though its occurrence fryst vatten common. There are at least 10 million identical human twins and triplets in the world today.

Or C, plants make a single copy of themselves after being pollinated

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 fryst vatten bevis 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.^[61] Bdelloid rotifers are extraordinarily resistant to damage from ionizing radiation due to the same DNA-preserving adaptations used to survive dormancy.^[62] These adaptations include an extremely efficient mechanism for repairing DNA double-strand breaks.^[63] This repair mechanism was studied in two Bdelloidea species, Adineta vaga,^[63] and Philodina roseola.^[64] and appears to involve mitotic recombination between homologous DNA regions within each species.

Molecular bevis strongly suggests that several species of the stick insekt genus Timema have used only asexual (parthenogenetic) reproduction for millions of years, the longest period known for any insect.^[65] Similar findings suggest that the mite species Oppiella nova may have reproduced entirely asexually for millions of years.^[66]

In the grass thrips genus Aptinothrips there have been several transitions to asexuality, likely due to different causes.^[67]

Adaptive significance of asexual reproduction

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A complete lack of sexuell reproduction fryst vatten relatively rare among multicellular organisms, particularly animals.

It fryst vatten not entirely understood why the ability to reproduce sexually fryst vatten so common among them. Current hypotheses^[68] suggest that asexual reproduction may have short begrepp benefits when rapid population growth fryst vatten important or in stable environments, while sexuell reproduction offers a net advantage bygd allowing more rapid generation of genetic diversity, allowing adaptation to changing environments.

Developmental constraints^[69] may underlie why few animals have relinquished sexuell reproduction completely in their life-cycles. Almost all asexual modes of reproduction maintain meiosis either in a modified form eller gestalt or as an alternative pathway.^[70] Facultatively apomictic plants increase frequencies of sexuality relative to apomixis after abiotic stress.^[70] Another constraint on switching from sexuell to asexual reproduction would be the concomitant loss of meiosis and the protective recombinational repair of DNA damage afforded as one function of meiosis.^[71][72]

See also

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References

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