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74-613: Rotaria citrina Rotaria macrura Rotaria magnacalcarata Rotaria neptunia Rotaria neptunoida Rotaria rotatoria Rotaria socialis Rotaria sordida Rotaria tardigrada Rotaria sp. CWB-2005 Rotaria is a genus of asexual , microscopic animal known as a bdelloid rotifer . Analysis published in 2007 of morphology and DNA sequence data of species from the genus confirmed that despite their asexual mechanism of reproduction , two fundamental properties of species , independent evolution and ecological divergence by natural selection occurred. This demonstrates that sex
148-535: A mutual relationship with various types of unicellular algae . The algae are protected from predators by Hydra ; in return, photosynthetic products from the algae are beneficial as a food source to Hydra , and even help to maintain the Hydra microbiome. The feeding response in Hydra is induced by glutathione (specifically in the reduced state as GSH) released from damaged tissue of injured prey. There are several methods conventionally used for quantification of
222-419: 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 of soredia , dust-like particles consisting of fungal hyphae wrapped around photobiont cells. Clonal Fragmentation in multicellular or colonial organisms is a form of asexual reproduction or cloning where an organism
296-830: A Rabl configuration. There are interactions between the centromeres of different chromosomes and the centromeres and telomeres of the same chromosome. It presents a great number of intercentromeric interactions when compared to other cnidarians, probably due to the loss of multiple subunits of condensin II . It is organized in domains that span dozens to hundreds of megabases, containing epigenetically co-regulated genes and flanked by boundaries located within heterochromatin . Different Hydra cell types express gene families of different evolutionary ages. Progenitor cells (stem cells, neuron and nematocyst precursors, and germ cells) express genes from families that predate metazoans . Among differentiated cells some express genes from families that date from
370-448: A capacity for indefinite self-renewal. The transcription factor " forkhead box O " (FoxO) has been identified as a critical driver of the continuous self-renewal of Hydra . In experiments, a drastically reduced population growth resulted from FoxO down-regulation . In bilaterally symmetrical organisms ( Bilateria ), the transcription factor FoxO affects stress response, lifespan, and increase in stem cells. If this transcription factor
444-497: A chemical cue accumulates and induces the transition to sexual reproduction. Many protists and fungi alternate between sexual and asexual reproduction. A few species of amphibians, reptiles, and birds have a similar ability. 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
518-470: A coiled thread inside. At the narrow outer edge of the cnidocyte is a short trigger hair called a cnidocil. Upon contact with prey, the contents of the nematocyst are explosively discharged, firing a dart-like thread containing neurotoxins into whatever triggered the release. This can paralyze the prey, especially if many hundreds of nematocysts are fired. Hydra has two main body layers, which makes it " diploblastic ". The layers are separated by mesoglea ,
592-414: A fertilization event. These haploid individuals produce 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 sexual reproduction is often more narrowly defined as the fusion of gametes ( fertilization ), spore formation in plant sporophytes and algae might be considered
666-418: 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
740-567: 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 . 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
814-514: A gel-like substance. The outer layer is the epidermis , and the inner layer is called the gastrodermis , because it lines the stomach. The cells making up these two body layers are relatively simple. Hydramacin is a bactericide recently discovered in Hydra ; it protects the outer layer against infection. A single Hydra is composed of 50,000 to 100,000 cells which consist of three specific stem cell populations that create many different cell types. These stem cells continually renew themselves in
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#1733085929572888-564: A male gamete. Examples are parthenogenesis and apomixis . Parthenogenesis is a form of agamogenesis in which an unfertilized egg develops into a new individual. It has been documented in over 2,000 species. 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). It has also been documented in domestic birds and in genetically altered lab mice. Plants can engage in parthenogenesis as well through
962-415: A mate becomes difficult. For example, female zebra sharks will reproduce asexually if they are unable to find a mate in their ocean habitats. 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
1036-442: A modified form or as an alternative pathway. Facultatively apomictic plants increase frequencies of sexuality relative to apomixis after abiotic stress. 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. Hydra (genus) Hydra ( / ˈ h aɪ d r ə / HY -drə )
1110-429: 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 and many algae, are produced by meiosis . Fragmentation
1184-402: A phenomenon known as "egg parasitism." This method of reproduction has been found in several species of the clam genus Corbicula , many plants like, Cupressus dupreziana , Lomatia tasmanica , Pando and recently in the fish Squalius alburnoides . Other species where androgenesis has been observed naturally are the stick insects Bacillus rossius and Bassillus Grandii ,
1258-538: A process called apomixis . However this process is considered by many to not be an independent reproduction method, but instead a breakdown of the mechanisms behind sexual reproduction. Parthenogenetic organisms can be split into two main categories: facultative and obligate. In facultative parthenogenesis, females can reproduce both sexually and asexually. Because of the many advantages of sexual reproduction, most facultative parthenotes only reproduce asexually when forced to. This typically occurs in instances when finding
1332-402: A process called thelytoky . 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. Monogonont rotifers of the genus Brachionus reproduce via cyclical parthenogenesis: at low population densities females produce asexually and at higher densities
1406-516: A sperm cell is used to initiate reproduction. However, the sperm's genes never get incorporated into the egg cell. The best known example of this is 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. Apomixis in plants is the formation of a new sporophyte without fertilization. It
1480-566: A tubular, radially symmetric body up to 10 mm (0.39 in) long when extended, secured by a simple adhesive foot known as the basal disc. Gland cells in the basal disc secrete a sticky fluid that accounts for its adhesive properties. At the free end of the body is a mouth opening surrounded by one to twelve thin, mobile tentacles . Each tentacle, or cnida (plural: cnidae), is clothed with highly specialised stinging cells called cnidocytes . Cnidocytes contain specialized structures called nematocysts , which look like miniature light bulbs with
1554-577: A type of transposable elements , in particular, a single family of the CR1 class. This expansion is unique to this subgroup of the genus Hydra and is absent in the green hydra, which has a repeating landscape similar to other cnidarians. These genome characteristics make Hydra attractive for studies of transposon-driven speciations and genome expansions. Due to the simplicity of their life cycle when compared to other hydrozoans, hydras have lost many genes that correspond to cell types or metabolic pathways of which
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#17330859295721628-402: A whole new individual; the bud occurs around two-thirds of the way down the body axis. When a Hydra is cut in half, each half regenerates and forms into a small Hydra ; the "head" regenerates a "foot" and the "foot" regenerates a "head". This regeneration occurs without cell division. If the Hydra is sliced into many segments, the middle slices form both a "head" and a "foot". The polarity of
1702-400: A year. This switch is triggered by environmental changes in the fall and causes females to develop eggs instead of embryos. 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. The cape bee Apis mellifera subsp. capensis can reproduce asexually through
1776-520: Is a genus of small freshwater hydrozoans of the phylum Cnidaria . They are native to the temperate and tropical regions. The genus was named by Linnaeus in 1758 after the Hydra , which was the many-headed beast of myth defeated by Heracles , as when the animal has a part severed, it will regenerate much like the mythical hydra's heads. Biologists are especially interested in Hydra because of their regenerative ability ; they do not appear to die of old age, or to age at all. Hydra has
1850-548: 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 and some oligochaetes , 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
1924-559: 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 . 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 ). Some plants reproduce by forming bulbs or tubers , for example tulip bulbs and Dahlia tubers. In these examples, all
1998-471: 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 nine-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
2072-402: 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
2146-418: Is derived entirely from pollen . Androgenesis occurs when a zygote is produced with only paternal nuclear genes . During standard sexual reproduction , one female and one male parent each produce haploid gametes (such as a sperm or egg cell, each containing only a single set of chromosomes ), which recombine to create offspring with genetic material from both parents. However, in androgenesis, there
2220-633: Is genetically and physically similar to the parent or an exact clone of the parent. Asexual reproduction is the primary form of reproduction for single-celled organisms such as archaea and bacteria . Many eukaryotic organisms including plants , animals , and fungi can also reproduce asexually. 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. Some monitor lizards , including Komodo dragons , can reproduce asexually. While all prokaryotes reproduce without
2294-414: 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,
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2368-650: Is knocked down in bilaterian model organisms, such as fruit flies and nematodes , their lifespan is significantly decreased. In experiments on H. vulgaris (a radially symmetrical member of phylum Cnidaria ), when FoxO levels were decreased, there was a negative effect on many key features of the Hydra , but no death was observed, thus it is believed other factors may contribute to the apparent lack of aging in these creatures. Hydra are capable of two types of DNA repair : nucleotide excision repair and base excision repair . The repair pathways facilitate DNA replication by removing DNA damage. Their identification in hydra
2442-766: Is no recombination of maternal and paternal chromosomes, and only the paternal chromosomes are passed down to the offspring (the inverse of this is gynogenesis , where only the maternal chromosomes are inherited, which is more common than androgenesis). The offspring produced in androgenesis will still have maternally inherited mitochondria , as is the case with most sexually reproducing species. Androgenesis occurs in nature in many invertebrates (for example, clams, stick insects, some ants, bees, flies and parasitic wasps ) and vertebrates (mainly amphibians and fish ). The androgenesis has also been seen in genetically modified laboratory mice. One of two things can occur to produce offspring with exclusively paternal genetic material:
2516-412: Is not a necessary condition for speciation. Asexual reproduction Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes . The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the full set of genes of their single parent and thus the newly created individual
2590-667: 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 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 may underlie why few animals have relinquished sexual reproduction completely in their life-cycles. Almost all asexual modes of reproduction maintain meiosis either in
2664-441: 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 . Due to many environmental and epigenetic differences, clones originating from the same ancestor might actually be genetically and epigenetically different. Agamogenesis is any form of reproduction that does not involve
2738-521: Is the Komodo dragon at 10 feet long and over 300 pounds. 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 ). 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
2812-661: The Meselson effect that have allowed them to survive better in periods of dehydration. Bdelloid rotifers are extraordinarily resistant to damage from ionizing radiation due to the same DNA-preserving adaptations used to survive dormancy. These adaptations include an extremely efficient mechanism for repairing DNA double-strand breaks. This repair mechanism was studied in two Bdelloidea species, Adineta vaga , and Philodina roseola . and appears to involve mitotic recombination between homologous DNA regions within each species. Molecular evidence strongly suggests that several species of
2886-487: The evolution of immunity . A draft of the genome of Hydra magnipapillata was reported in 2010 . The genomes of cnidarians are usually less than 500 Mb ( megabases ) in size, as in the Hydra viridissima , which has a genome size of approximately 300 Mb. In contrast, the genomes of brown hydras are approximately 1 Gb in size. This is because the brown hydra genome is the result of an expansion event involving LINEs ,
2960-488: The hammerhead shark and the blacktip shark . 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
3034-431: 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. Inheritance of asexual reproduction by a single recessive locus has also been found in the parasitoid wasp Lysiphlebus fabarum . Asexual reproduction is found in nearly half of the animal phyla. Parthenogenesis occurs in
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3108-457: The stick insect genus Timema have used only asexual (parthenogenetic) reproduction for millions of years, the longest period known for any insect. Similar findings suggest that the mite species Oppiella nova may have reproduced entirely asexually for millions of years. In the grass thrips genus Aptinothrips there have been several transitions to asexuality, likely due to different causes. A complete lack of sexual reproduction
3182-429: The substrate with the mouth and tentacles and then relocate the foot, which provides the usual attachment, this process is called looping. In somersaulting, the body then bends over and makes a new place of attachment with the foot. By this process of "looping" or "somersaulting", a Hydra can move several inches (c. 100 mm) in a day. Hydra may also move by amoeboid motion of their bases or by detaching from
3256-444: The ancestral function is still unknown. Hydra genome shows a preference towards proximal promoters . Thanks to this feature, many reporter cell lines have been created with regions around 500 to 2000 bases upstream of the gene of interest. Its cis-regulatory elements ( CRE ) are mostly located less than 2000 base pairs upstream from the closest transcription initiation site, but there are CREs located further away. Its chromatin has
3330-417: The body column . Hydras have two significant structures on their body: the "head" and the "foot". When a Hydra is cut in half, each half regenerates and forms into a small Hydra ; the "head" regenerates a "foot" and the "foot" regenerates a "head". If the Hydra is sliced into many segments then the middle slices form both a "head" and a "foot". Respiration and excretion occur by diffusion throughout
3404-551: The body wall develop into either ovaries or testes. The testes release free-swimming gametes into the water, and these can fertilize the egg in the ovary of another individual. The fertilized eggs secrete a tough outer coating, and, as the adult dies (due to starvation or cold), these resting eggs fall to the bottom of the lake or pond to await better conditions, whereupon they hatch into nymph Hydra . Some Hydra species, like Hydra circumcincta and Hydra viridissima , are hermaphrodites and may produce both testes and ovaries at
3478-494: The bud forms is where the gradients are low for both the head and foot. Hydras are capable of regenerating from pieces of tissue from the body and additionally after tissue dissociation from reaggregates. This process takes place not only in the pieces of tissue excised from the body column, but also from re-aggregates of dissociated single cells. It was found that in these aggregates, cells initially distributed randomly undergo sorting and form two epithelial cell layers, in which
3552-476: 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 in the Saharan Cypress Cupressus dupreziana , where the genetic material of the embryo
3626-614: The endodermal epithelial cells play more active roles in the process. Active mobility of these endodermal epithelial cells forms two layers in both the re-aggregate and the re-generating tip of the excised tissue. As these two layers are established, a patterning process takes place to form heads and feet. Daniel Martinez claimed in a 1998 article in Experimental Gerontology that Hydra are biologically immortal . This publication has been widely cited as evidence that Hydra do not senesce (do not age), and that they are proof of
3700-453: The existence of non-senescing organisms generally. In 2010, Preston Estep published (also in Experimental Gerontology ) a letter to the editor arguing that the Martinez data refutes the hypothesis that Hydra do not senesce. The controversial unlimited lifespan of Hydra has attracted much attention from scientists. Research today appears to confirm Martinez' study. Hydra stem cells have
3774-438: The fall to lay eggs for the next season. However, some aphid species are obligate parthenotes. In obligate parthenogenesis, females only reproduce asexually. One example of this is 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. Gynogenesis is a form of obligate parthenogenesis where
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#17330859295723848-401: The feeding response. In some, the duration for which the mouth remains open is measured. Other methods rely on counting the number of Hydra among a small population showing the feeding response after addition of glutathione. Recently, an assay for measuring the feeding response in hydra has been developed. In this method, the linear two-dimensional distance between the tip of the tentacle and
3922-495: 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 . Prokaryotes ( Archaea and Bacteria ) reproduce asexually through binary fission , in which the parent organism divides in two to produce two genetically identical daughter organisms. Eukaryotes (such as protists and unicellular fungi ) may reproduce in
3996-527: The individuals are clones, and the clonal population may cover a large area. Many multicellular organisms produce spores during their biological life cycle in a process called sporogenesis . Exceptions are animals and some protists, which 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 multicellular individuals called gametophytes , without
4070-502: The little fire ant Wasmannia auropunctata , Vollenhovia emeryi , Paratrechina longicornis , occasionally in Apis mellifera , the Hypseleotris carp gudgeons, the parasitoid Venturia canescens , and occasionally in fruit flies Drosophila melanogaster carrying a specific mutant allele. It has also been induced in many crops and fish via irradiation of an egg cell to destroy
4144-404: The little fire ant Wasmannia auropunctata , Vollenhovia emeryi , Paratrechina longicornis , occasionally in Apis mellifera , the Hypseleotris carp gudgeons, the parasitoid Venturia canescens , and occasionally in fruit flies Drosophila melanogaster carrying a specific mutant allele. It has also been induced in many crops and fish via irradiation of an egg cell to destroy
4218-412: The maternal nuclear genome can be eliminated from the zygote, or the female can produce an egg with no nucleus , resulting in an embryo developing with only the genome of the male gamete. Other type of androgenesis is the male apomixis or paternal apomixis is a reproductive process in which a plant develops from a sperm cell (male gamete) without the participation of a female cell (ovum). In this process,
4292-415: The maternal nuclear genome. Obligate androgenesis is the process in which males are capable of producing both eggs and sperm, however, the eggs have no genetic contribution and the offspring come only from the sperm, which allows these individuals to self-fertilize and produce clonal offspring without the need for females. They are also capable of interbreeding with sexual and other androgenetic lineages in
4366-591: The maternal nuclear genome. 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. One example of this is aphids which can engage in heterogony. In this system, females are born pregnant and produce only female offspring. This cycle allows them to reproduce very quickly. However, most species reproduce sexually once
4440-428: The more common apomixis, where development occurs without fertilization, but with genetic material only from the mother. There are also clonal species that reproduce through vegetative reproduction like Lomatia tasmanica and Pando , where the genetic material is exclusively male. Other species where androgenesis has been observed naturally are the stick insects Bacillus rossius and Bassillus Grandii ,
4514-450: The mouth of hydra was shown to be a direct measure of the extent of the feeding response. This method has been validated using a starvation model, as starvation is known to cause enhancement of the Hydra feeding response. The species Hydra oligactis is preyed upon by the flatworm Microstomum lineare . Hydras undergo morphallaxis (tissue regeneration) when injured or severed. Typically, Hydras reproduce by just budding off
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#17330859295724588-620: The nerve net has two levels: Some have only two sheets of neurons . If Hydra are alarmed or attacked, the tentacles can be retracted to small buds, and the body column itself can be retracted to a small gelatinous sphere. Hydra generally react in the same way regardless of the direction of the stimulus, and this may be due to the simplicity of the nerve nets. Hydra are generally sedentary or sessile , but do occasionally move quite readily, especially when hunting. They have two distinct methods for moving – 'looping' and 'somersaulting'. They do this by bending over and attaching themselves to
4662-557: 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) occurs in some worms like Taenia or Echinococcus ; these worms produce cysts and then produce (invaginated or evaginated) protoscolex with budding . Vegetative propagation
4736-434: The regeneration is explained by two pairs of positional value gradients. There is both a head and foot activation and inhibition gradient. The head activation and inhibition works in an opposite direction of the pair of foot gradients. The evidence for these gradients was shown in the early 1900s with grafting experiments. The inhibitors for both gradients have shown to be important to block the bud formation. The location where
4810-446: The same cell membrane, sporogony results in sporozoites , and gametogony results in micro gametes . 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. Budding is also known on a multicellular level; an animal example is the hydra , which reproduces by budding. The buds grow into fully matured individuals which eventually break away from
4884-512: The same time. Many members of the Hydrozoa go through a body change from a polyp to an adult form called a medusa , which is usually the life stage where sexual reproduction occurs, but Hydra do not progress beyond the polyp phase. Hydra mainly feed on aquatic invertebrates such as Daphnia and Cyclops . While feeding, Hydra extend their body to maximum length and then slowly extend their tentacles. Despite their simple construction,
4958-740: 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. 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. 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. In
5032-404: The struggling prey. Within two minutes, the tentacles surround the prey and move it into the open mouth aperture. Within ten minutes, the prey is engulfed within the body cavity, and digestion commences. Hydra can stretch their body wall considerably. The feeding behaviour of Hydra demonstrates the sophistication of what appears to be a simple nervous system. Some species of Hydra exist in
5106-471: The substrate and floating away in the current. Most hydra species do not have any gender system. Instead, when food is plentiful, many Hydra reproduce asexually by budding . The buds form from the body wall, grow into miniature adults and break away when mature. When a hydra is well fed, a new bud can form every two days. When conditions are harsh, often before winter or in poor feeding conditions, sexual reproduction occurs in some Hydra . Swellings in
5180-417: The surface of the epidermis , while larger excreta are discharged through the mouth. The nervous system of Hydra is a nerve net , which is structurally simple compared to more derived animal nervous systems. Hydra does not have a recognizable brain or true muscles . Nerve nets connect sensory photoreceptors and touch-sensitive nerve cells located in the body wall and tentacles. The structure of
5254-400: The tentacles of Hydra are extraordinarily extensible and can be four to five times the length of the body. Once fully extended, the tentacles are slowly maneuvered around waiting for contact with a suitable prey animal. Upon contact, nematocysts on the tentacle fire into the prey, and the tentacle itself coils around the prey. Most of the tentacles join in the attack within 30 seconds to subdue
5328-489: The zygote is formed solely with genetic material from the father, resulting in offspring genetically identical to the male organism. This has been noted in many plants like Nicotiana , Capsicum frutescens , Cicer arietinum , Poa arachnifera , Solanum verrucosum , Phaeophyceae , Pripsacum dactyloides , Zea mays , and occurs as the regular reproductive method in Cupressus dupreziana . This contrasts with
5402-484: Was based, in part, on the presence in its genome of genes homologous to ones present in other genetically well studied species playing key roles in these DNA repair pathways. An ortholog comparison analysis done within the last decade demonstrated that Hydra share a minimum of 6,071 genes with humans. Hydra is becoming an increasingly better model system as more genetic approaches become available. Transgenic hydra have become attractive model organisms to study
5476-475: 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. The female boa could have chosen any number of male partners (and had successfully in the past) but on this occasion she reproduced asexually, creating 22 female babies with WW sex-chromosomes. Polyembryony
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