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Modes of asexual reproduction

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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 gametesand 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 conjugationtransformation and transduction can be "Modes of asexual reproduction" to sexual reproduction in the sense of genetic recombination in meiosis.

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.

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 protistse.

The nucleus of the parent cell divides several times by mitosisproducing several nuclei. The cytoplasm then separates, creating multiple daughter cells. In apicomplexansmultiple fission, or schizogony appears either as merogonysporogony or gametogony. Merogony results in merozoiteswhich are multiple daughter cells, that originate within the same cell membrane, Modes of asexual reproduction [11] sporogony results in sporozoitesand gametogony results in micro gametes.

Some cells split via budding for example baker's yeastresulting 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 hydrawhich 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. Also, budding external or internal is present in some worm like Taenia or Echinococci ; these worm produce cyst and then produce invaginated Modes of asexual reproduction evaginated protoscolex with budding.

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

Other plants reproduce by forming bulbs or tubers for example tulip bulbs and dahlia tubers.

Advantages of Asexual Reproduction

Some plants produce adventitious shoots and may form a clonal colonywhere all the individuals are clones, and the clones may cover a large area. Many multicellular organisms form spores during their biological life cycle in a process called sporogenesis.

Exceptions are animals and some protists, who Modes of asexual reproduction 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 multicellular 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 fertilizationspore 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 Modes of asexual reproduction develop into a new organism after dispersal. This method of reproduction is found for example in conidial fungi and the red algae Polysiphoniaand 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 Basidiomycotaand many algae, are produced by meiosis. 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 planariansmany 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 lichenswhich 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 sorediadust-like particles consisting of fungal hyphen wrapped around photobiont cells.

Clonal Fragmentation in multicellular 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 echinodermsthis method of reproduction is usually known as fissiparity. Agamogenesis is any form of reproduction that does not involve a male gamete.

Examples are parthenogenesis Modes of asexual reproduction apomixis. Parthenogenesis is a form of agamogenesis "Modes of asexual reproduction" which an unfertilized egg develops into a new individual. Parthenogenesis occurs naturally in many plants, invertebrates e. In plants, apomixis may or may not involve parthenogenesis.

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 agamospermythe 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 embryonythe 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 Modes of asexual reproduction, where the genetic material of the embryo are derived entirely from pollen.

Some species can alternate between sexual and asexual strategies, an ability known as heterogamydepending on conditions. Alternation is observed in several rotifer species cyclical parthenogenesis e. The cape bee Apis mellifera subsp.

Asexual reproduction can be defined...

A few species of amphibians, reptilesand 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 Brachionuswhich reproduce via cyclical parthenogenesis: 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 multicellular slug which then forms a fruiting body with asexually generated spores.

Modes of asexual reproduction 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. 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. For example, in the rotifer Brachionus calyciflorus asexual reproduction obligate parthenogenesis can be inherited by a recessive allele, which leads to loss of Modes of asexual reproduction reproduction in homozygous offspring.

Asexual reproduction is a mode...

Asexual reproduction is found in nearly half of the animal phyla. The New Mexico whiptail is another example.

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