Editing Hydra (genus)

{{short description|Genus of cnidarians}}
{{About|the aquatic animal|the mythological monster|Lernaean Hydra|other uses|Hydra (disambiguation)}}

{{Use dmy dates|date=April 2020}}
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{{Automatic taxobox
| image = Hydra-Foto.jpg
| image_caption = ''Hydra'' [[budding]]
| parent_authority = Dana, 1846
| taxon = Hydra
| authority = [[Carl Linnaeus|Linnaeus]], [[10th edition of Systema Naturae|1758]]<ref name="worse">{{cite WoRMS|vauthors=Schuchert P |year=2011|title=''Hydra'' Linnaeus, 1758|db=hydrozoa|id=267491|access-date=2011-12-20}}</ref>
| subdivision_ranks = Species
| subdivision_ref = <ref name=worse/>
| subdivision = {{Collapsible list 
 |* ''Hydra baikalensis'' <small>Swarczewsky, 1923</small>
 |* ''Hydra beijingensis'' <small>Fan, 2003</small>
 |* ''Hydra canadensis'' <small>Rowan, 1930</small>
 |* ''Hydra cauliculata'' <small>Hyman, 1938</small>
 |* ''Hydra circumcincta'' <small>Schulze, 1914</small>
 |* ''Hydra daqingensis'' <small>Fan, 2000</small>
 |* ''Hydra ethiopiae'' <small>Hickson, 1930</small>
 |* ''Hydra hadleyi'' <small>(Forrest, 1959)</small>
 |* ''Hydra harbinensis'' <small>Fan & Shi, 2003</small>
 |* ''Hydra hymanae'' <small>Hadley & Forrest, 1949</small>
 |* ''Hydra iheringi'' <small>Cordero, 1939</small>
 |* ''Hydra intaba'' <small>Ewer, 1948</small>
 |* ''Hydra intermedia'' <small>De Carvalho Wolle, 1978</small>
 |* ''Hydra japonica'' <small>Itô, 1947</small>
 |* ''Hydra javanica'' <small>Schulze, 1929</small>
 |* ''Hydra liriosoma'' <small>Campbell, 1987</small>
 |* ''Hydra madagascariensis'' <small>Campbell, 1999</small>
 |* ''Hydra magellanica'' <small>Schulze, 1927</small>
 |* ''Hydra mariana'' <small>Cox & Young, 1973</small>
 |* ''Hydra minima'' <small>Forrest, 1963</small>
 |* ''Hydra mohensis'' <small>Fan & Shi, 1999</small>
 |* ''[[Hydra oligactis]]'' <small>Pallas, 1766</small>
 |* ''Hydra oregona'' <small>Griffin & Peters, 1939</small>
 |* ''Hydra oxycnida'' <small>Schulze, 1914</small>
 |* ''Hydra paludicola'' <small>Itô, 1947</small>
 |* ''Hydra paranensis'' <small>Cernosvitov, 1935</small>
 |* ''Hydra parva'' <small>Itô, 1947</small>
 |* ''Hydra plagiodesmica'' <small>Dioni, 1968</small>
 |* ''Hydra polymorpha'' <small>Chen & Wang, 2008</small>
 |* ''Hydra robusta'' <small>(Itô, 1947)</small>
 |* ''Hydra rutgersensis'' <small>Forrest, 1963</small>
 |* ''Hydra salmacidis'' <small>Lang da Silveira et al., 1997</small>
 |* ''Hydra sinensis'' <small>Wang et al., 2009</small>
 |* ''Hydra thomseni'' <small>Cordero, 1941</small>
 |* ''Hydra umfula'' <small>Ewer, 1948</small>
 |* ''Hydra utahensis'' <small>Hyman, 1931</small>
 |* ''[[Hydra viridissima]]'' <small>Pallas, 1766</small>
 |* ''[[Hydra vulgaris]]'' <small>Pallas, 1766</small>
 |* ''Hydra zeylandica'' <small>Burt, 1929</small>
 |* ''Hydra zhujiangensis'' <small>Liu & Wang, 2010</small>}}
| synonyms = *''Chlorohydra''
}}

'''''Hydra''''' ({{IPAc-en|ˈ|h|aɪ|d|r|ə}} {{Respell|HY|drə}}) is a [[genus]] of small [[freshwater ecosystem|freshwater]] [[hydrozoan]]s of the [[phylum]] [[Cnidaria]]. They are solitary, carnivorous jellyfish-like animals,<ref name = mc>{{cite book| title = Exploring Life Sciences| volume = 6|pages = 428-428|ISBN = 0-7614-7141-3|publisher = Marshall Cavendish}}</ref> native to the [[temperate climate|temperate]] and [[tropical climate|tropical]] regions.<ref>{{cite book |last=Gilberson |first=Lance |title=Zoology Lab Manual |date=1999 |publisher=Primis Custom Publishing |edition=4th |name-list-style=vanc}}</ref><ref>{{cite book |title=Biology |vauthors=Solomon E, Berg L, Martin D |date=2002 |publisher=[[Cengage|Brooks/Cole]] |edition=6th}}</ref> The genus was named by [[Linnaeus]] in 1758 after the [[Hydra (mythology)|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 [[regeneration (biology)|regenerative ability]]; they do not appear to die of old age, or to [[senescence|age]] at all.

==Habitat==

Hydras are often found in [[freshwater]] bodies, but some hydras are found in open water. They live attached to submerged rocks using a sticky secretion from their base.<ref name = mc/>

==Morphology==

[[File:Hydra nematocyst firing 01.png|thumb|left|Schematic drawing of a discharging nematocyst]]

''Hydra'' has a tubular, [[symmetry (biology)#Radial symmetry|radially symmetric]] body up to {{convert|10|mm|abbr=on}} 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 [[tentacle]]s. Each tentacle, or cnida (plural: cnidae), is clothed with highly specialised stinging cells called [[cnidocyte]]s. Cnidocytes contain specialized structures called [[nematocyst]]s, which look like miniature light bulbs with 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 [[neurotoxin]]s 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]], a gel-like substance. The outer layer is the [[Squamous epithelium|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]]<ref>{{Cite journal |vauthors=Jung S, Dingley AJ, Augustin R, Anton-Erxleben F, Stanisak M, Gelhaus C, Gutsmann T, Hammer MU, Podschun R, Bonvin AM, Leippe M, Bosch TC, Grötzinger J |date=January 2009 |title=Hydramacin-1, structure and antibacterial activity of a protein from the basal metazoan Hydra |journal=The Journal of Biological Chemistry |volume=284 |issue=3 |pages=1896–905 |doi=10.1074/jbc.M804713200 |pmid=19019828|s2cid=3887876 |doi-access=free }}</ref> 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 the body column''.''<ref name=":2">{{cite journal | vauthors = Tomczyk S, Fischer K, Austad S, Galliot B | title = Hydra, a powerful model for aging studies | journal = Invertebrate Reproduction & Development | volume = 59 | issue = sup1 | pages = 11–16 | date = January 2015 | pmid = 26120246 | pmc = 4463768 | doi = 10.1080/07924259.2014.927805 | bibcode = 2015InvRD..59S..11T }}</ref> ''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".<ref name=":0">{{cite book |last=Gilbert|first=Scott F. | name-list-style = vanc |date=2000 |chapter=Regeneration |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK9971/ |title = Developmental Biology |publisher=Sinauer Associates | edition = 6th }}</ref>

Respiration and excretion occur by [[diffusion]] throughout the surface of the [[Epidermis (zoology)|epidermis]], while larger excreta are discharged through the mouth.<ref>{{Cite web|url=http://olympus.magnet.fsu.edu/micd/galleries/moviegallery/pondscum/coelenterata/hydra/index.html|title=Olympus Microscopy Resource Center {{!}} Pond Life Video Gallery – Hydra (Coelenterata)|website=olympus.magnet.fsu.edu|access-date=2019-09-21}}</ref><ref>{{Cite journal|last1=Cantor|first1=Marvin H.|last2=Rahat|first2=Menachem|year=1982|title=Regulation of Respiration and Photosynthesis in Hydra viridis and in Its Separate Cosymbionts: Effect of Nutrients|journal=Physiological Zoology|publisher=The University of Chicago Press|volume=55|issue=3|pages=281–288|issn=0031-935X|jstor=30157891|doi=10.1086/physzool.55.3.30157891|s2cid=86961916}}</ref>

==Nervous system==
The nervous system of ''Hydra'' is a [[nerve net]], which is structurally simple compared to [[Primitive (phylogenetics)|more derived]] animal nervous systems. ''Hydra'' does not have a recognizable [[brain]] or true [[muscle]]s. Nerve nets connect sensory [[Eyespot apparatus|photoreceptor]]s and touch-sensitive nerve cells located in the body wall and tentacles.

The structure of the nerve net has two levels: 
*level 1 – sensory cells or internal cells; and
*level 2 – interconnected ganglion cells connected via synapse to epithelial or motor cells.

Some have only two sheets of [[neuron]]s.<ref name="pmid28441559">{{cite journal | vauthors = Ji N, Flavell SW | title = Hydra: Imaging Nerve Nets in Action | journal = Current Biology | volume = 27 | issue = 8 | pages = R294–R295 | date = April 2017 | pmid = 28441559 | doi = 10.1016/j.cub.2017.03.040 | doi-access = free | bibcode = 2017CBio...27.R294J | hdl = 1721.1/114954 | hdl-access = free }}</ref>

==Motion and locomotion==
[[File:Hydras (8).JPG|thumb|upright|''Hydra'' attached to a substrate]]
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 [[Wiktionary:sedentary|sedentary]] or [[Sessility (zoology)|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 the [[Wiktionary:substrate|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.&nbsp;100&nbsp;mm) in a day. ''Hydra'' may also move by [[Amoeboid movement|amoeboid motion]] of their bases or by detaching from the substrate and floating away in the current.

==Reproduction and life cycle==
[[File:Hydra Budding.svg|thumb|left|upright=1.35|''Hydra'' ''[[budding]]'': {{olist |Non-reproducing |Creating a bud |Daughter growing out |Beginning to cleave |Daughter broken off |Daughter [[Clone (cell biology)|clone]] of parent}}]]
Most hydra species do not have any gender system. Instead, when food is plentiful, many ''Hydra'' [[Asexual reproduction|reproduce asexually]] by [[budding]]. A section of the body wall and an extension of the digestive cavity develops, creating a bud.<ref name = mc/> The buds grow into miniature adults and break away when mature.

When a hydra is well fed, a new bud can form every two days.<ref>{{cite book |vauthors=Patton WK |chapter=Hydra (coelenterate) |title=Grolier Multimedia Encyclopedia. |publisher=Grolier Online |date=August 2014}}</ref> When conditions are harsh, often before winter or in poor feeding conditions, [[sexual reproduction]] occurs in some ''Hydra''. Swellings in the body wall develop into either ovaries or testes. The testes release free-swimming [[gamete]]s 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 [[hermaphrodite]]s<ref>{{cite book |vauthors=Holstein T, Emschermann P |date=1995 |title=Cnidaria: Hydrozoa Süsswasserfauna von Mitteleuropa. Bd 1/2+ 3 |location=Stuttgart |publisher=Spektrum Akademischer Verlag |isbn=978-3-8274-0836-5}}</ref> and may produce both testes and ovaries at the same time.

Many members of the [[Hydrozoa]] go through a body change from a [[polyp (zoology)|polyp]] to an adult form called a [[Medusa (biology)|medusa]], which is usually the life stage where sexual reproduction occurs, but ''Hydra'' do not progress beyond the polyp phase.<ref>{{Cite book|author=Hickman, Cleveland P. Jr. |title=Integrated principles of zoology|year=2019|isbn=978-1-260-20519-0|edition=Eighteenth|location=New York, NY|oclc=1097367369}}</ref>

==Feeding==
''Hydra'' mainly feed on aquatic invertebrates such as ''[[Daphnia]]'' and ''[[Cyclops (genus)|Cyclops]]''.

The mouth of the hydra is surrounded by four to eight tentacles.<ref name = mc/> When feeding, ''Hydra'' extend their body to maximum length and then slowly extend their tentacles. Despite their simple construction, 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 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.{{citation needed |date=March 2019}}

The hydra's mouth is not permanent, as when the hydra closes its mouth, the cells surrounding the open mouth fuse together. These joints are then broken when the hydra feeds again.<ref name = mc/>

The feeding behaviour of ''Hydra'' demonstrates the sophistication of what appears to be a simple nervous system.

Some species of ''Hydra'' exist in a [[Mutualism (biology)|mutual relationship]] with various types of unicellular [[algae]]. The algae are protected from predators by ''Hydra''; in return, [[photosynthesis|photosynthetic]] products from the algae are beneficial as a food source to ''Hydra''<ref>{{Cite journal |last1=Thorington |first1=Glyne |last2=Margulis |first2=Lynn |date=1981 |title=Hydra viridis; transfer of metabolites between Hydra and symbiotic algae |url=https://www.journals.uchicago.edu/doi/10.2307/1540911 |journal=The Biological Bulletin |language=en |volume=160 |issue=1 |pages=175–188 |doi=10.2307/1540911 |jstor=1540911 |pmid=6164406 |s2cid=21008864 |issn=0006-3185}}</ref><ref>{{Cite journal |last1=Muscatine |first1=Leonard |last2=Lenhoff |first2=Howard M. |date=1963-11-15 |title=Symbiosis: On the Role of Algae Symbiotic with Hydra |url=https://www.science.org/doi/10.1126/science.142.3594.956 |journal=Science |language=en |volume=142 |issue=3594 |pages=956–958 |doi=10.1126/science.142.3594.956 |pmid=17753799 |bibcode=1963Sci...142..956M |s2cid=28578967 |issn=0036-8075}}</ref> and even help to maintain the ''Hydra'' microbiome.<ref>{{Cite journal |last1=Bathia |first1=Jay |last2=Schröder |first2=Katja |last3=Fraune |first3=Sebastian |last4=Lachnit |first4=Tim |last5=Rosenstiel |first5=Philip |last6=Bosch |first6=Thomas C. G. |date=6 June 2022 |title=Symbiotic Algae of Hydra viridissima Play a Key Role in Maintaining Homeostatic Bacterial Colonization |journal=Front. Microbiol. |volume=13 |pages=869666 |doi=10.3389/fmicb.2022.869666 |pmid=35733963 |pmc=9207534 |doi-access=free }}</ref>

===Measuring the feeding response===

[[File:Fig1 TentacleSpread wiki.png|thumb|Reduction of glutathione causes reduction in the tentacle spread in hydra.]]
The feeding response in ''Hydra'' is induced by [[glutathione]] (specifically in the reduced state as GSH) released from damaged tissue of injured prey.<ref>{{cite journal | vauthors = Loomis WF | title = Glutathione control of the specific feeding reactions of hydra. | journal = Annals of the New York Academy of Sciences | date = October 1955 | volume = 62 | issue = 9 | pages = 211–27 | doi = 10.1111/j.1749-6632.1955.tb35372.x | bibcode = 1955NYASA..62..211L | s2cid = 85570550 }}</ref> There are several methods conventionally used for quantification of the feeding response. In some, the duration for which the mouth remains open is measured.<ref name="pmid7957948">{{cite journal | vauthors = Bellis SL, Laux DC, Rhoads DE | title = Affinity purification of Hydra glutathione binding proteins | journal = FEBS Letters | volume = 354 | issue = 3 | pages = 320–4 | date = November 1994 | pmid = 7957948 | doi = 10.1016/0014-5793(94)01154-0| s2cid = 29262166 | doi-access = free | bibcode = 1994FEBSL.354..320B }}</ref> Other methods rely on counting the number of ''Hydra'' among a small population showing the feeding response after addition of glutathione.<ref name="pmid2888575">{{cite journal | vauthors = Venturini G | title = The hydra GSH receptor. Pharmacological and radioligand binding studies | journal = Comparative Biochemistry and Physiology. C, Comparative Pharmacology and Toxicology | volume = 87 | issue = 2 | pages = 321–4 |year = 1987 | pmid = 2888575 | doi = 10.1016/0742-8413(87)90015-6}}</ref> Recently, an assay for measuring the feeding response in hydra has been developed.<ref name="kulk">{{cite journal | vauthors = Kulkarni R, Galande S | title = Measuring glutathione-induced feeding response in hydra | journal = Journal of Visualized Experiments | issue = 93 | pages = e52178 | date = November 2014 | pmid = 25490534 | pmc = 4354099 | doi = 10.3791/52178 }}</ref> In this method, the linear two-dimensional distance between the tip of the tentacle and 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.<ref name="kulk" />

== Predators ==
The species ''[[Hydra oligactis]]'' is preyed upon by the [[flatworm]] ''[[Microstomum lineare]]''.<ref>{{Cite journal|last=Krohne|first=Georg|date=2018|title=Organelle survival in a foreign organism: Hydra nematocysts in the flatworm Microstomum lineare|url=https://pubmed.ncbi.nlm.nih.gov/29661512/|journal=European Journal of Cell Biology|volume=97|issue=4|pages=289–299|doi=10.1016/j.ejcb.2018.04.002|issn=1618-1298|pmid=29661512}}</ref><ref>{{Cite journal|last=Krohne|first=Georg|date=2020|title=Hydra nematocysts in the flatworm Microstomum lineare: in search for alterations preceding their disappearance from the new host|url=https://pubmed.ncbi.nlm.nih.gov/31848750/|journal=Cell and Tissue Research|volume=379|issue=1|pages=63–71|doi=10.1007/s00441-019-03149-w|issn=1432-0878|pmid=31848750|s2cid=209380951}}</ref>

==Tissue regeneration==

''Hydras'' undergo [[morphallaxis]] (tissue regeneration) when injured or severed. Typically, ''Hydras'' reproduce by just budding off 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".<ref name=":0" /> The polarity of 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.<ref name=":1">{{cite journal | vauthors = Fujisawa T | title = Hydra regeneration and epitheliopeptides | journal = Developmental Dynamics | volume = 226 | issue = 2 | pages = 182–9 | date = February 2003 | pmid = 12557197 | doi = 10.1002/dvdy.10221 | s2cid = 26953455 | doi-access = free }}</ref> 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 the bud forms is where the gradients are low for both the head and foot.<ref name=":0" />

''Hydras'' are capable of regenerating from pieces of tissue from the body and additionally after tissue dissociation from reaggregates.<ref name=":1" /> 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 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.<ref name="Fujisawa-2003">{{cite journal | last=Fujisawa | first=Toshitaka | title=Hydra regeneration and epitheliopeptides | journal=[[Developmental Dynamics]] | publisher=[[American Association for Anatomy]] ([[Wiley publishing|Wiley]]) | volume=226 | issue=2 | date=2003-01-29 | issn=1058-8388 | doi=10.1002/dvdy.10221 | pages=182–189| pmid=12557197 | s2cid=26953455 | doi-access=free }}</ref>

==Non-senescence==
Daniel Martinez claimed in a 1998 article in ''Experimental Gerontology'' that ''Hydra'' are [[Biological immortality|biologically immortal]].<ref>{{cite journal | vauthors = Martínez DE | title = Mortality patterns suggest lack of senescence in hydra | journal = Experimental Gerontology | volume = 33 | issue = 3 | pages = 217–25 | date = May 1998 | pmid = 9615920 | doi = 10.1016/S0531-5565(97)00113-7 | s2cid = 2009972 }}</ref> This publication has been widely cited as evidence that ''Hydra'' do not [[senescence|senesce]] (do not age), and that they are proof of 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.<ref>{{cite journal | vauthors = Estep PW | title = Declining asexual reproduction is suggestive of senescence in hydra: comment on Martinez, D., "Mortality patterns suggest lack of senescence in hydra." Exp Gerontol 33, 217–25 | journal = Experimental Gerontology | volume = 45 | issue = 9 | pages = 645–6 | date = September 2010 | pmid = 20398746 | doi = 10.1016/j.exger.2010.03.017 | s2cid = 35408542 }}</ref>

The controversial unlimited lifespan of ''Hydra'' has attracted much attention from scientists. Research today appears to confirm Martinez' study.<ref name="B–K">{{cite journal | vauthors = Boehm AM, Khalturin K, Anton-Erxleben F, Hemmrich G, Klostermeier UC, Lopez-Quintero JA, Oberg HH, Puchert M, Rosenstiel P, Wittlieb J, Bosch TC | title = FoxO is a critical regulator of stem cell maintenance in immortal Hydra | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 109 | issue = 48 | pages = 19697–702 | date = November 2012 | pmid = 23150562 | pmc = 3511741 | doi = 10.1073/pnas.1209714109 | bibcode = 2012PNAS..10919697B | doi-access = free }}</ref> ''Hydra'' stem cells have a capacity for indefinite self-renewal. The [[transcription factor]] "[[FOX proteins|forkhead box O]]" (FoxO) has been identified as a critical driver of the continuous self-renewal of ''Hydra''.<ref name="B–K" /> In experiments, a drastically reduced population growth resulted from FoxO [[down-regulation]].<ref name="B–K" />

In bilaterally symmetrical organisms ([[Bilateria]]), the transcription factor FoxO affects stress response, lifespan, and increase in stem cells. If this transcription factor is knocked down in bilaterian model organisms, such as [[Drosophila melanogaster|fruit flies]] and [[nematode]]s, their lifespan is significantly decreased. In experiments on ''[[Hydra vulgaris|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.<ref name=":2" />

==DNA repair==

Hydra are capable of two types of [[DNA repair]]: [[nucleotide excision repair]] and [[base excision repair]].<ref name="Barve2021">{{Cite journal |doi=10.3389/fgene.2021.670695 |doi-access=free |pmc=8117345 |pmid=33995496|title=DNA Repair Repertoire of the Enigmatic Hydra |year=2021 |last1=Barve |first1=Apurva |last2=Galande |first2=Alisha A. |last3=Ghaskadbi |first3=Saroj S. |last4=Ghaskadbi |first4=Surendra |journal=Frontiers in Genetics |volume=12 |page=670695 }}</ref> The repair pathways facilitate DNA replication by removing DNA damage. Their identification in hydra 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.<ref name = Barve2021/>

==Genomics==
An [[ortholog]] comparison analysis done within the last decade{{As of?|reason=Which decade?|date=September 2024}} demonstrated that ''Hydra'' share a minimum of 6,071 [[gene]]s with humans. ''Hydra'' is becoming an increasingly better model system as more genetic approaches become available.<ref name=":2" /> [[Transgenic hydra]] have become attractive model organisms to study the [[evolution]] of [[Immunity (medical)|immunity]].<ref>{{cite web|title= Transgenic Hydra Facility, University of Kiel (Germany)| url=http://transgenic-hydra.org/lines.htm/ }}</ref> A draft of the [[genome]] of ''[[Hydra magnipapillata]]'' was [[List of sequenced animal genomes#Cnidaria|reported in 2010]].<ref>{{cite journal | vauthors = Chapman JA, Kirkness EF, Simakov O, Hampson SE, Mitros T, Weinmaier T, etal | title = The dynamic genome of Hydra | journal = Nature | volume = 464 | issue = 7288 | pages = 592–6 | date = March 2010 | pmid = 20228792 | pmc = 4479502 | doi = 10.1038/nature08830 | bibcode = 2010Natur.464..592C }}</ref>

The genomes of [[cnidarians]] are usually less than 500 Mb ([[megabase]]s) in size, as in the ''[[Hydra viridissima]]'', which has a genome size of approximately 300 Mb. In contrast, the genomes of [[Hydra oligactis|brown hydras]] are approximately 1 Gb in size. This is because the brown hydra genome is the result of an expansion event involving [[Long interspersed nuclear element|LINEs]], 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.<ref>{{cite journal |last1=Wong |first1=WY |last2=Simakov |first2=O |last3=Bridge |first3=DM |last4=Cartwright |first4=P |last5=Bellantuono |first5=AJ |last6=Kuhn |first6=A |last7=Holstein |first7=TW |last8=David |first8=CN |last9=Steele |first9=RE |last10=Martínez |first10=DE |title=Expansion of a single transposable element family is associated with genome-size increase and radiation in the genus Hydra |journal=Proc Natl Acad Sci U S A |year=2019 |volume=116 |issue=46 |pages=22915–22917 |doi=10.1073/pnas.1910106116 |pmid=31659034|pmc=6859323 |bibcode=2019PNAS..11622915W |doi-access=free }}</ref>

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 the ancestral function is still unknown.

Hydra genome shows a preference towards proximal [[Promoter (genetics)|promoters]]. Thanks to this feature, many [[Reporter gene|reporter cell lines]] have been created with regions around 500 to 2000 bases upstream of the gene of interest. Its cis-regulatory elements ([[Cis-regulatory element|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 a Rabl configuration. There are interactions between the [[centromere]]s of different chromosomes and the centromeres and [[telomere]]s 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|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]].<ref name="A chromosome-scale epigenetic map o">{{cite journal |last1=J. F. |first1=Cazet |last2=S. |first2=Siebert |last3=H. M. |first3=Little |last4=P. |first4=Bertemes |last5=A. S. |first5=Primack |last6=P. |first6=Ladurner |last7=M. |first7=Achrainer |last8=M. T. |first8=Fredriksen |last9=R. T. |first9=Moreland |last10=S. |first10=Singh |last11=S. |first11=Zhang |last12=T. G. |first12=Wolfsberg |last13=T. G. |first13=Schnitzler |last14=A. D. |first14=Baxevanis |last15=O. |first15=Simakov |last16=B. |first16=Hobmayer |last17=C. E. |first17=Juliano |title=A chromosome-scale epigenetic map of the Hydra genome reveals conserved regulators of cell state. |journal=Genome Research |date=2023 |volume=33 |issue=2 |pages=283–298 |doi=10.1101/gr.277040.122 |pmid=36639202 |pmc=10069465 |doi-access=free }}</ref>

==Transcriptomics==
Different Hydra cell types express gene families of different evolutionary ages. [[Progenitor cell]]s (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 the base of metazoans, like gland and neuronal cells, and others express genes from newer families, originating from the base of [[cnidaria]] or [[medusozoa]], like nematocysts. Interstitial cells contain translation factors with a function that has been conserved for at least 400 million years.<ref name="A chromosome-scale epigenetic map o"/>

== See also ==
{{Commons category|Hydra}}
* [[Lernaean Hydra]], a Greek mythological aquatic creature after which the genus is named
* ''[[Turritopsis dohrnii]]'', another cnidarian (a jellyfish) that scientists believe to be immortal

== References ==
{{Reflist|35em}}

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[[Category:Hydridae]]
[[Category:Negligibly senescent organisms]]