Asexual reproduction

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Not to be confused with Asexuality. Asexual reproduction_sentence_0

Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. Asexual reproduction_sentence_1

The offspring that arise by asexual reproduction, either from from a single cell or from a multicellular organism, inherit the genes of their single parent. Asexual reproduction_sentence_2

Asexual reproduction is the primary form of reproduction for single-celled organisms such as archaea and bacteria. Asexual reproduction_sentence_3

Many eukaryotic organisms including plants, animals, and fungi can also reproduce asexually. Asexual reproduction_sentence_4

In vertebrates, the most common form of asexual reproduction is parthenogenesis which is typically used as an alternative to sexual reproduction in times when reproductive opportunities are limited. Asexual reproduction_sentence_5

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. Asexual reproduction_sentence_6

Types Asexual reproduction_section_0

Fission Asexual reproduction_section_1

Main article: Fission (biology) Asexual reproduction_sentence_7

Prokaryotes (Archaea and Bacteria) reproduce asexually through binary fission, in which the parent organism divides in two to produce two genetically identical daughter organisms. Asexual reproduction_sentence_8

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. Asexual reproduction_sentence_9

Multiple fission at the cellular level occurs in many protists, e.g. sporozoans and algae. Asexual reproduction_sentence_10

The nucleus of the parent cell divides several times by mitosis, producing several nuclei. Asexual reproduction_sentence_11

The cytoplasm then separates, creating multiple daughter cells. Asexual reproduction_sentence_12

In apicomplexans, multiple fission, or schizogony appears either as merogony, sporogony or gametogony. Asexual reproduction_sentence_13

Merogony results in merozoites, which are multiple daughter cells, that originate within the same cell membrane, sporogony results in sporozoites, and gametogony results in microgametes. Asexual reproduction_sentence_14

Budding Asexual reproduction_section_2

Main article: Budding Asexual reproduction_sentence_15

Some cells divide by budding (for example baker's yeast), resulting in a "mother" and a "daughter" cell that is initially smaller than the parent. Asexual reproduction_sentence_16

Budding is also known on a multicellular level; an animal example is the hydra, which reproduces by budding. Asexual reproduction_sentence_17

The buds grow into fully matured individuals which eventually break away from the parent organism. Asexual reproduction_sentence_18

Internal budding is a process of asexual reproduction, favoured by parasites such as Toxoplasma gondii. Asexual reproduction_sentence_19

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. Asexual reproduction_sentence_20

Also, budding (external or internal) occurs in some worms like Taenia or Echinococcus; these worms produce cysts and then produce (invaginated or evaginated) protoscolex with budding. Asexual reproduction_sentence_21

Vegetative propagation Asexual reproduction_section_3

Main article: Vegetative propagation Asexual reproduction_sentence_22

Vegetative propagation is a type of asexual reproduction found in plants where new individuals are formed without the production of seeds or spores and thus without syngamy or meiosis. Asexual reproduction_sentence_23

Examples of vegetative reproduction include the formation of miniaturized plants called plantlets on specialized leaves, for example in kalanchoe (Bryophyllum daigremontianum) and many produce new plants from rhizomes or stolon (for example in strawberry). Asexual reproduction_sentence_24

Other plants reproduce by forming bulbs or tubers (for example tulip bulbs and Dahlia tubers). Asexual reproduction_sentence_25

Some plants produce adventitious shoots and may form a clonal colony. Asexual reproduction_sentence_26

In these examples, all the individuals are clones, and the clonal population may cover a large area. Asexual reproduction_sentence_27

Spore formation Asexual reproduction_section_4

Main article: Sporogenesis Asexual reproduction_sentence_28

Many multicellular organisms form spores during their biological life cycle in a process called sporogenesis. Asexual reproduction_sentence_29

Exceptions are animals and some protists, which undergo meiosis immediately followed by fertilization. Asexual reproduction_sentence_30

Plants and many algae on the other hand undergo sporic meiosis where meiosis leads to the formation of haploid spores rather than gametes. Asexual reproduction_sentence_31

These spores grow into multicellular individuals (called gametophytes in the case of plants) without a fertilization event. Asexual reproduction_sentence_32

These haploid individuals give rise to gametes through mitosis. Asexual reproduction_sentence_33

Meiosis and gamete formation therefore occur in separate generations or "phases" of the life cycle, referred to as alternation of generations. Asexual reproduction_sentence_34

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. Asexual reproduction_sentence_35

However, both events (spore formation and fertilization) are necessary to complete sexual reproduction in the plant life cycle. Asexual reproduction_sentence_36

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. Asexual reproduction_sentence_37

This method of reproduction is found for example in conidial fungi and the red algae Polysiphonia, and involves sporogenesis without meiosis. Asexual reproduction_sentence_38

Thus the chromosome number of the spore cell is the same as that of the parent producing the spores. Asexual reproduction_sentence_39

However, mitotic sporogenesis is an exception and most spores, such as those of plants, most Basidiomycota, and many algae, are produced by meiosis. Asexual reproduction_sentence_40

Fragmentation Asexual reproduction_section_5

Main article: Fragmentation (reproduction) Asexual reproduction_sentence_41

Fragmentation is a form of asexual reproduction where a new organism grows from a fragment of the parent. Asexual reproduction_sentence_42

Each fragment develops into a mature, fully grown individual. Asexual reproduction_sentence_43

Fragmentation is seen in many organisms. Asexual reproduction_sentence_44

Animals that reproduce asexually include planarians, many annelid worms including polychaetes and some oligochaetes, turbellarians and sea stars. Asexual reproduction_sentence_45

Many fungi and plants reproduce asexually. Asexual reproduction_sentence_46

Some plants have specialized structures for reproduction via fragmentation, such as gemmae in liverworts. Asexual reproduction_sentence_47

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. Asexual reproduction_sentence_48

These fragments can take the form of soredia, dust-like particles consisting of fungal hyphen wrapped around photobiont cells. Asexual reproduction_sentence_49

Clonal Fragmentation in multicellular or colonial organisms is a form of asexual reproduction or cloning where an organism is split into fragments. Asexual reproduction_sentence_50

Each of these fragments develop into mature, fully grown individuals that are clones of the original organism. Asexual reproduction_sentence_51

In echinoderms, this method of reproduction is usually known as fissiparity. Asexual reproduction_sentence_52

Due to many environmental and epigenetic differences, clones originating from the same ancestor might actually be genetically and epigenetically different. Asexual reproduction_sentence_53

Agamogenesis Asexual reproduction_section_6

Agamogenesis is any form of reproduction that does not involve a male gamete. Asexual reproduction_sentence_54

Examples are parthenogenesis and apomixis. Asexual reproduction_sentence_55

Parthenogenesis Asexual reproduction_section_7

Main article: Parthenogenesis Asexual reproduction_sentence_56

Parthenogenesis is a form of agamogenesis in which an unfertilized egg develops into a new individual. Asexual reproduction_sentence_57

It has been documented in over 2,000 species. Asexual reproduction_sentence_58

Parthenogenesis occurs in the wild in many invertebrates (e.g. water fleas, rotifers, aphids, stick insects, some ants, bees and parasitic wasps) and vertebrates (mostly reptiles, amphibians, and fish). Asexual reproduction_sentence_59

It has also been documented in domestic birds and in genetically altered lab mice. Asexual reproduction_sentence_60

Plants can engage in parthenogenesis as well through a process called apomixis. Asexual reproduction_sentence_61

However this process is considered by many to not be an independent reproduction method, but instead a breakdown of the mechanisms behind sexual reproduction . Asexual reproduction_sentence_62

Parthenogenetic organisms can be split into two main categories: facultative and obligate. Asexual reproduction_sentence_63

Facultative Parthenogenesis Asexual reproduction_section_8

In facultative parthenogenesis, females can reproduce both sexually and asexually. Asexual reproduction_sentence_64

Because of the many advantages of sexual reproduction, most facultative parthenotes only reproduce asexually when forced to. Asexual reproduction_sentence_65

This typically occurs in instances when finding a mate becomes difficult. Asexual reproduction_sentence_66

For example, female Zebra Sharks will reproduce asexually if they are unable to find a mate in their ocean habitats. Asexual reproduction_sentence_67

Parthenogenesis was previously believed to rarely occur in vertebrates, and only be possible in very small animals. Asexual reproduction_sentence_68

However, it has been discovered in many more species in recent years. Asexual reproduction_sentence_69

Today, the largest species that has been documented reproducing parthenogenically is the Komodo Dragon at 10 feet long and over 300 pounds. Asexual reproduction_sentence_70

Heterogony is a form of facultative parthenogenesis where females alternate between sexual and asexual reproduction at regular intervals (see Alternation between sexual and asexual reproduction). Asexual reproduction_sentence_71

Aphids are one group of organism that engages in this type of reproduction. Asexual reproduction_sentence_72

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. Asexual reproduction_sentence_73

However, some aphid species are obligate parthenotes. Asexual reproduction_sentence_74

Obligate Parthenogenesis Asexual reproduction_section_9

In obligate parthenogenesis, females only reproduce asexually. Asexual reproduction_sentence_75

One example of this is the Desert Grassland Whiptail Lizard, a hybrid of two other species. Asexual reproduction_sentence_76

Typically hybrids are infertile but through parthenogenesis this species has been able to develop stable populations. Asexual reproduction_sentence_77

Gynogenesis is a form of obligate parthenogenesis where a sperm cell is used to initiate reproduction. Asexual reproduction_sentence_78

However, the sperm's genes never get incorporated into the egg cell. Asexual reproduction_sentence_79

The best known example of this is the Amazon Molly. Asexual reproduction_sentence_80

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. Asexual reproduction_sentence_81

Apomixis and nucellar embryony Asexual reproduction_section_10

Main articles: Apomixis and Nucellar embryony Asexual reproduction_sentence_82

Apomixis in plants is the formation of a new sporophyte without fertilization. Asexual reproduction_sentence_83

It is important in ferns and in flowering plants, but is very rare in other seed plants. Asexual reproduction_sentence_84

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. Asexual reproduction_sentence_85

An example of an apomictic plant would be the triploid European dandelion. Asexual reproduction_sentence_86

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. Asexual reproduction_sentence_87

In nucellar embryony, the embryo is formed from the diploid nucellus tissue surrounding the embryo sac. Asexual reproduction_sentence_88

Nucellar embryony occurs in some citrus seeds. Asexual reproduction_sentence_89

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. Asexual reproduction_sentence_90

Alternation between sexual and asexual reproduction Asexual reproduction_section_11

See also: Plant reproduction § Sexual reproduction Asexual reproduction_sentence_91

Some species can alternate between sexual and asexual strategies, an ability known as heterogamy, depending on many conditions. Asexual reproduction_sentence_92

Alternation is observed in several rotifer species (cyclical parthenogenesis e.g. in Brachionus species) and a few types of insects. Asexual reproduction_sentence_93

One example of this is aphids which can engage in heterogony. Asexual reproduction_sentence_94

In this system, females are born pregnant and produce only female offspring. Asexual reproduction_sentence_95

This cycle allows them to reproduce very quickly. Asexual reproduction_sentence_96

However, most species reproduce sexually once a year. Asexual reproduction_sentence_97

This switch it triggered by environmental changes in the fall and causes females to develop eggs instead of embryos. Asexual reproduction_sentence_98

This dynamic reproductive cycle allows them to produce specialized offspring with polyphenism, a type of polymorphism where different phenotypes have evolved to carry out specific tasks. Asexual reproduction_sentence_99

The cape bee Apis mellifera subsp. Asexual reproduction_sentence_100 capensis can reproduce asexually through a process called thelytoky. Asexual reproduction_sentence_101

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. Asexual reproduction_sentence_102

Monogonont rotifers of the genus Brachionus reproduce via cyclical parthenogenesis: at low population densities females produce asexually and at higher densities a chemical cue accumulates and induces the transition to sexual reproduction. Asexual reproduction_sentence_103

Many protists and fungi alternate between sexual and asexual reproduction. Asexual reproduction_sentence_104

A few species of amphibians, reptiles, and birds have a similar ability. Asexual reproduction_sentence_105

The slime mold Dictyostelium undergoes binary fission (mitosis) as single-celled amoebae under favorable conditions. Asexual reproduction_sentence_106

However, when conditions turn unfavorable, the cells aggregate and follow one of two different developmental pathways, depending on conditions. Asexual reproduction_sentence_107

In the social pathway, they form a multi-cellular slug which then forms a fruiting body with asexually generated spores. Asexual reproduction_sentence_108

In the sexual pathway, two cells fuse to form a giant cell that develops into a large cyst. Asexual reproduction_sentence_109

When this macrocyst germinates, it releases hundreds of amoebic cells that are the product of meiotic recombination between the original two cells. Asexual reproduction_sentence_110

The hyphae of the common mold (Rhizopus) are capable of producing both mitotic as well as meiotic spores. Asexual reproduction_sentence_111

Many algae similarly switch between sexual and asexual reproduction. Asexual reproduction_sentence_112

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. Asexual reproduction_sentence_113

Inheritance in asexual species Asexual reproduction_section_12

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. Asexual reproduction_sentence_114

Inheritance of asexual reproduction by a single recessive locus has also been found in the parasitoid wasp Lysiphlebus fabarum. Asexual reproduction_sentence_115

Examples in animals Asexual reproduction_section_13

Asexual reproduction is found in nearly half of the animal phyla. Asexual reproduction_sentence_116

Parthenogenesis occurs in the hammerhead shark and the blacktip shark. Asexual reproduction_sentence_117

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. Asexual reproduction_sentence_118

The New Mexico whiptail is another example. Asexual reproduction_sentence_119

Some reptiles use the ZW sex-determination system, which produces either males (with ZZ sex chromosomes) or females (with ZW or WW sex chromosomes). Asexual reproduction_sentence_120

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. Asexual reproduction_sentence_121

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. Asexual reproduction_sentence_122

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. Asexual reproduction_sentence_123

Within animals, this phenomenon has been best studied in the parasitic Hymenoptera. Asexual reproduction_sentence_124

In the 9-banded armadillos, this process is obligatory and usually gives rise to genetically identical quadruplets. Asexual reproduction_sentence_125

In other mammals, monozygotic twinning has no apparent genetic basis, though its occurrence is common. Asexual reproduction_sentence_126

There are at least 10 million identical human twins and triplets in the world today. Asexual reproduction_sentence_127

Bdelloid rotifers reproduce exclusively asexually, and all individuals in the class Bdelloidea are females. Asexual reproduction_sentence_128

Asexuality evolved in these animals millions of years ago and has persisted since. Asexual reproduction_sentence_129

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. Asexual reproduction_sentence_130

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. Asexual reproduction_sentence_131

In the grass thrips genus Aptinothrips there have been several transitions to asexuality, likely due to different causes. Asexual reproduction_sentence_132

Adaptive significance of asexual reproduction Asexual reproduction_section_14

A complete lack of sexual reproduction is relatively rare among multicellular organisms, particularly animals. Asexual reproduction_sentence_133

It is not entirely understood why the ability to reproduce sexually is so common among them. Asexual reproduction_sentence_134

Current hypotheses suggest that asexual reproduction may have short term benefits when rapid population growth is important or in stable environments, while sexual reproduction offers a net advantage by allowing more rapid generation of genetic diversity, allowing adaptation to changing environments. Asexual reproduction_sentence_135

Developmental constraints may underlie why few animals have relinquished sexual reproduction completely in their life-cycles. Asexual reproduction_sentence_136

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. Asexual reproduction_sentence_137

See also Asexual reproduction_section_15

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: reproduction.