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Inbiology andmedicine, ahost is a largerorganism that harbours a smallerorganism;[1] whether aparasitic, amutualistic, or acommensalistguest (symbiont). The guest is typically provided with nourishment and shelter. Examples includeanimals playing host to parasiticworms (e.g.nematodes),cells harbouringpathogenic (disease-causing)viruses, or abean plant hosting mutualistic (helpful)nitrogen-fixing bacteria. More specifically inbotany, a host plant suppliesfood resources to micropredators, which have anevolutionarily stable relationship with their hosts similar toectoparasitism. Thehost range is the collection of hosts that an organism can use as a partner.
Symbiosis spans a wide variety of possible relationships between organisms, differing in their permanence and their effects on the two parties. If one of the partners in an association is much larger than the other, it is generally known as the host.[1] Inparasitism, the parasite benefits at the host's expense.[2] Incommensalism, the two live together without harming each other,[3] while inmutualism, both parties benefit.[4]
Most parasites are only parasitic for part of their life cycle. By comparing parasites with their closest free-living relatives, parasitism has been shown to have evolved on at least 233 separate occasions. Some organisms live in close association with a host and only become parasitic when environmental conditions deteriorate.[5]
A parasite may have a long-term relationship with its host, as is the case with all endoparasites. The guest seeks out the host and obtains food or another service from it, but does not usually kill it.[6] In contrast, aparasitoid spends a large part of its life within or on a single host, ultimately causing the host's death, with some of the strategies involved verging onpredation. Generally, the host is kept alive until the parasitoid is fully grown and ready to pass on to its next life stage.[7] A guest's relationship with its host may be intermittent or temporary, perhaps associated with multiple hosts, making the relationship equivalent to theherbivory of a wild-living animal. Another possibility is that the host–guest relationship may have no permanent physical contact, as in thebrood parasitism of thecuckoo.[6]
Parasites follow a wide variety of evolutionary strategies, placing their hosts in an equally wide range of relationships.[2] Parasitism implieshost–parasite coevolution, including the maintenance ofgene polymorphisms in the host, where there is a trade-off between the advantage of resistance to a parasite and a cost such as disease caused by the gene.[8]
It is not always easy or even possible to identify which host is definitive and which secondary. The life cycles of many parasites are not well understood, and the subjectively or economically more important organism may initially be designated incorrectly as primary. Mislabelling may continue even after the error becomes known. For example trout and salmon are sometimes said to be "primary hosts" forsalmonidwhirling disease, even though themyxosporean parasite reproduces sexually inside thesludge worm.[9] And where the host harbors the different parasite's phases at different sites within its body, the host is both intermediate and definitive: for exampletrichinosis, a disease caused byroundworms, where the host has immature juveniles in itsmuscles and reproductive adults in its digestive tract.[10]
Micropredation is anevolutionarily stable strategy within parasitism, in which a small predator lives parasitically on a much larger host plant, eating parts of it.[2]
The range ofplants on which aherbivorous insect feeds is known as its host range. This can be wide or narrow, but it never includes all plants. A small number of insects aremonophagous, feeding on a single plant. Thesilkworm larva is one of these, withmulberry leaves being the only food consumed. More often, an insect with a limited host range is oligophagous, being restricted to a few closely related species, usually in the same plant family.[15] Thediamondback moth is an example of this, feeding exclusively onbrassicas,[16] and the larva of thepotato tuber moth feeds on potatoes, tomatoes and tobacco, all members of the same plant family,Solanaceae.[17] Herbivorous insects with a wide range of hosts in various different plant families are known aspolyphagous. One example is thebuff ermine moth whose larvae feed onalder,mint,plantain,oak,rhubarb,currant,blackberry,dock,ragwort,nettle andhoneysuckle.[18]
Plants often produce toxic or unpalatablesecondary metabolites to deter herbivores from feeding on them. Monophagous insects have developed specific adaptations to overcome those in their specialist hosts, giving them an advantage over polyphagous species. However, this puts them at greater risk of extinction if their chosen hosts suffer setbacks. Monophagous species are able to feed on the tender young foliage with high concentrations of damaging chemicals on which polyphagous species cannot feed, having to make do with older leaves. There is a trade off between offspring quality and quantity; the specialist maximises the chances of its young thriving by paying great attention to the choice of host, while the generalist produces larger numbers of eggs in sub-optimal conditions.[19]
Some insect micropredators migrate regularly from one host to another. Thehawthorn-carrot aphid overwinters on its primary host, ahawthorn tree, and migrates during the summer to its secondary host, a plant in thecarrot family.[20]
The host range is the set of hosts that a parasite can use as a partner. In the case of human parasites, the host range influences theepidemiology of the parasitism or disease.
For instance, the production ofantigenic shifts inInfluenza A virus can result from pigs being infected with the virus from several different hosts (such as human and bird). This co-infection provides an opportunity for mixing of the viral genes between existing strains, thereby producing a new viral strain. Aninfluenza vaccine produced against an existingviral strain might not be effective against this new strain, which then requires a new influenza vaccine to be prepared for the protection of the human population.[21]
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Some hosts participate in fully mutualistic interactions with both organisms being completely dependent on the other. For example,termites are hosts to theprotozoa that live in their gut and which digestcellulose,[22] and the humangut flora is essential for efficientdigestion.[23] Many corals and other marine invertebrates housezooxanthellae, single-celled algae, in their tissues. The host provides a protected environment in a well-lit position for the algae, while benefiting itself from the nutrients produced byphotosynthesis which supplement its diet.[24]Lamellibrachia luymesi, a deep sea giant tubeworm, has an obligate mutualistic association with internal, sulfide-oxidizing, bacterial symbionts. The tubeworm extracts the chemicals that the bacteria need from the sediment, and the bacteria supply the tubeworm, which has no mouth, with nutrients.[25] Somehermit crabs place pieces ofsponge on the shell in which they are living. These grow over and eventually dissolve away the mollusc shell; the crab may not ever need to replace its abode again and is well-camouflaged by the overgrowth of sponge.[26]
An important hosting relationship ismycorrhiza, a symbiotic association between a fungus and the roots of a vascular host plant. The fungus receives carbohydrates, the products of photosynthesis, while the plant receives phosphates and nitrogenous compounds acquired by the fungus from the soil. Over 95% of plant families have been shown to have mycorrhizal associations.[27] Another such relationship is betweenleguminous plants and certain nitrogen-fixing bacteria calledrhizobia that form nodules on the roots of the plant. The host supplies the bacteria with the energy needed for nitrogen fixation and the bacteria provide much of the nitrogen needed by the host. Such crops asbeans,peas,chickpeas andalfalfa are able to fix nitrogen in this way,[28] and mixingclover withgrasses increases the yield of pastures.[29]
Neurotransmittertyramine produced by commensalProvidencia bacteria, which colonize the gut of the nematodeCaenorhabditis elegans, bypasses the requirement for its host to biosynthesise tyramine. This product is then probably converted tooctopamine by the host enzyme tyramine β-hydroxylase and manipulates a host sensory decision.[30]
Hosts of many species are involved incleaning symbiosis, both in the sea and on land, making use of smaller animals to clean them of parasites. Cleaners include fish, shrimps and birds; hosts or clients include a much wider range of fish, marine reptiles including turtles and iguanas, octopus, whales, and terrestrial mammals.[4] The host appears to benefit from the interaction, but biologists have disputed whether this is a truly mutualistic relationship or something closer to parasitism by the cleaner.[31][32]
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Remoras (also called suckerfish) can swim freely but have evolved suckers that enable them to adhere to smooth surfaces, gaining a free ride (phoresis), and they spend most of their lives clinging to a host animal such as a whale, turtle or shark.[3] However, the relationship may be mutualistic, as remoras, though not generally considered to becleaner fish, often consume parasiticcopepods: for example, these are found in the stomach contents of 70% of thecommon remora.[33] Manymolluscs,barnacles andpolychaete worms attach themselves to the carapace of theAtlantic horseshoe crab; for some this is a convenient arrangement, but for others it is an obligate form of commensalism and they live nowhere else.[22]
The first host to be noticed in ancient times was human:human parasites such ashookworm are recorded fromancient Egypt from 3000 BC onwards, while inancient Greece, theHippocratic Corpus describes humanbladder worm.[34] Themedieval Persian physicianAvicenna recorded human and animal parasites including roundworms, threadworms, the Guinea worm and tapeworms.[34] InEarly Modern times,Francesco Redi recorded animal parasites, while the microscopistAntonie van Leeuwenhoek observed and illustrated the protozoanGiardia lamblia from "his own loose stools".[34]
Hosts to mutualistic symbionts were recognised more recently, when in 1877Albert Bernhard Frank described the mutualistic relationship between afungus and analga inlichens.[35]
