![Roof rat-(rattus rattus)](https://upload.wikimedia.org/wikipedia/commons/a/a3/Roof_rat-%28rattus_rattus%29.jpg)
In
biology
Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary i ...
and
medicine
Medicine is the science and practice of caring for a patient, managing the diagnosis, prognosis, prevention, treatment, palliation of their injury or disease, and promoting their health. Medicine encompasses a variety of health care pract ...
, a host is a larger
organism
In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and ...
that harbours a smaller
organism
In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and ...
;
whether a
parasitic
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has c ...
, a
mutualistic, or a
commensalist ''guest'' (
symbiont
Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...
). The guest is typically provided with nourishment and shelter. Examples include
animal
Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...
s playing host to parasitic
worm
Worms are many different distantly related bilateral animals that typically have a long cylindrical tube-like body, no limbs, and no eyes (though not always).
Worms vary in size from microscopic to over in length for marine polychaete wor ...
s (e.g.
nematodes
The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a broa ...
),
cell
Cell most often refers to:
* Cell (biology), the functional basic unit of life
Cell may also refer to:
Locations
* Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...
s harbouring
pathogen
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
ic (disease-causing)
virus
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Since Dmitri Ivanovsky's 1 ...
es, a
bean
A bean is the seed of several plants in the family Fabaceae, which are used as vegetables for human or animal food. They can be cooked in many different ways, including boiling, frying, and baking, and are used in many traditional dishes th ...
plant hosting mutualistic (helpful)
nitrogen-fixing bacteria
Nitrogen fixation is a chemical process by which molecular nitrogen (), with a strong triple covalent bond, in the air is converted into ammonia () or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. Atmos ...
. More specifically in
botany
Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the Ancient Greek w ...
, a host plant supplies
food resources to micropredators, which have an
evolutionarily stable relationship with their hosts similar to
ectoparasitism
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...
. The host range is the collection of hosts that an organism can use as a partner.
Symbiosis
Symbiosis
Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...
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.
In
parasitism
Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted structurally to this way of lif ...
, the parasite benefits at the host's expense.
[ In ]commensalism
Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fro ...
, the two live together without harming each other,[ while in mutualism, both parties benefit.][
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.
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.][ In contrast, a ]parasitoid
In evolutionary ecology, a parasitoid is an organism that lives in close association with its host (biology), host at the host's expense, eventually resulting in the death of the host. Parasitoidism is one of six major evolutionarily stable str ...
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 on predation
Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill the ...
. Generally, the host is kept alive until the parasitoid is fully grown and ready to pass on to its next life stage. A guest's relationship with its host may be intermittent or temporary, perhaps associated with multiple hosts, making the relationship equivalent to the herbivory
A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpart ...
of a wild-living animal. Another possibility is that the host–guest relationship may have no permanent physical contact, as in the brood parasitism
Brood parasites are animals that rely on others to raise their young. The strategy appears among birds, insects and fish. The brood parasite manipulates a host, either of the same or of another species, to raise its young as if it were its own ...
of the cuckoo
Cuckoos are birds in the Cuculidae family, the sole taxon in the order Cuculiformes . The cuckoo family includes the common or European cuckoo, roadrunners, koels, malkohas, couas, coucals and anis. The coucals and anis are sometimes separ ...
.
Hosts to parasites
Parasites follow a wide variety of evolutionary strategies, placing their hosts in an equally wide range of relationships.[ Parasitism implies ]host–parasite coevolution
Host–parasite coevolution is a special case of coevolution, where a host and a parasite continually adapt to each other. This can create an evolutionary arms race between them. A more benign possibility is of an evolutionary trade-off between ...
, including the maintenance of gene polymorphism
A gene is said to be polymorphic if more than one allele occupies that gene's locus within a population. In addition to having more than one allele at a specific locus, each allele must also occur in the population at a rate of at least 1% to gen ...
s 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.
Types of hosts
*Definitive or primary host – an organism in which the parasite
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...
reaches the adult stage and reproduces sexually, if possible. This is the final host.
*Secondary or intermediate host – an organism that harbors the sexually immature parasite and is required by the parasite to undergo development and complete its life cycle. It often acts as a vector of the parasite to reach its definitive host. For example, ''Dirofilaria immitis
''Dirofilaria immitis'', also known as heartworm or dog heartworm, is a parasitic roundworm that is a type of filarial worm, a small thread-like worm, that causes dirofilariasis. It is spread from host to host through the bites of mosquitoes ...
'', the heartworm of dogs, uses the mosquito as its intermediate host until it matures into the infective L3 larval stage.
It is not always easy or even possible to identify which host is definitive and which secondary. As the life cycles of many parasites are not well understood, sometimes the subjectively more important organism is arbitrarily labelled as definitive, and this designation may continue even after it is found to be incorrect. For example, sludge worm
''Tubifex tubifex,'' also called the sludge worm, "Boogie Worm", or sewage worm, is a species of tubificid segmented worm which inhabits the sediments of lakes and rivers on several continents. ''Tubifex'' probably includes several species, bu ...
s are sometimes considered "intermediate hosts" for salmonid
Salmonidae is a family of ray-finned fish that constitutes the only currently extant family in the order Salmoniformes . It includes salmon (both Atlantic and Pacific species), trout (both ocean-going and landlocked), chars, freshwater whitefis ...
whirling disease
''Myxobolus cerebralis'' is a myxosporean parasite of salmonids (salmon and trout species) that causes whirling disease in farmed salmon and trout and also in wild fish populations. It was first described in rainbow trout in Germany in 1893, b ...
, even though the myxosporean
Myxosporea is a class of microscopic parasites, belonging to the Myxozoa clade within Cnidaria. They have a complex life cycle which comprises vegetative forms in two hosts, an aquatic invertebrate (generally an annelid but sometimes a bryozoa ...
parasite reproduces sexually inside them. In trichinosis
Trichinosis, also known as trichinellosis, is a parasitic disease caused by roundworms of the ''Trichinella'' type. During the initial infection, invasion of the intestines can result in diarrhea, abdominal pain, and vomiting. Migration of larv ...
, a disease caused by roundworm
The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a broa ...
s, the host has reproductive adults in its digestive tract and immature juveniles in its muscle
Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...
s, and is therefore both an intermediate and a definitive host.
*Paratenic or transport host – an organism that harbors the sexually immature parasite but is not necessary for the parasite's development cycle
In software engineering, a software development process is a process of dividing software development work into smaller, parallel, or sequential steps or sub-processes to improve design, product management. It is also known as a software devel ...
to progress. Paratenic hosts serve as "dumps" for non-mature stages of a parasite in which they can accumulate in high numbers. The trematode ''Alaria americana
''Alaria americana'' is a species of a trematode in a family Diplostomidae. All of these species infect carnivorous mammals by living in their small intestines as mature worms. ''A. americana'' are most frequently found in temperate regions, pre ...
'' may serve as an example: the so-called mesocercarial stages of this parasite reside in tadpole
A tadpole is the larval stage in the biological life cycle of an amphibian. Most tadpoles are fully aquatic, though some species of amphibians have tadpoles that are terrestrial. Tadpoles have some fish-like features that may not be found i ...
s, which are rarely eaten by the definitive canine host. The tadpoles are more frequently preyed on by snakes
Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes . Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more joi ...
, in which the mesocercariae may not undergo further development. However, the parasites may accumulate in the snake paratenic host and infect the definitive host once the snake is consumed by a canid. The nematode '' Skrjabingylus nasicola'' is another example, with slugs as the intermediate hosts, shrews and rodents as the paratenic hosts, and mustelids as the definitive hosts.
*Dead-end, incidental, or accidental host – an organism that generally does not allow transmission to the definitive host, thereby preventing the parasite from completing its development. For example, humans and horses are dead-end hosts for West Nile virus
West Nile virus (WNV) is a single-stranded RNA virus that causes West Nile fever. It is a member of the family ''Flaviviridae'', from the genus ''Flavivirus'', which also contains the Zika virus, dengue virus, and yellow fever virus. The virus ...
, whose life cycle is normally between culicine mosquito
Mosquitoes (or mosquitos) are members of a group of almost 3,600 species of small flies within the family Culicidae (from the Latin ''culex'' meaning " gnat"). The word "mosquito" (formed by ''mosca'' and diminutive ''-ito'') is Spanish for "li ...
es and birds. People and horses can become infected, but the level of virus in their blood does not become high enough to pass on the infection to mosquitoes that bite them.
* Reservoir host – an organism that harbors a pathogen
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
but suffers no ill effects. However, it serves as a source of infection to other species that are susceptible, with important implications for disease
A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that a ...
control. A single reservoir host may be reinfected several times.
Plant hosts of micropredators
Micropredation is an evolutionarily stable strategy
An evolutionarily stable strategy (ESS) is a strategy (or set of strategies) that is ''impermeable'' when adopted by a population in adaptation to a specific environment, that is to say it cannot be displaced by an alternative strategy (or set of ...
within parasitism, in which a small predator lives parasitically on a much larger host plant, eating parts of it.[
The range of ]plant
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...
s on which a herbivorous
A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpart ...
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 are monophagous
A generalist species is able to thrive in a wide variety of environmental conditions and can make use of a variety of different resources (for example, a heterotroph with a varied diet). A specialist species can thrive only in a narrow range of e ...
, feeding on a single plant. The silkworm
The domestic silk moth (''Bombyx mori''), is an insect from the moth family Bombycidae. It is the closest relative of ''Bombyx mandarina'', the wild silk moth. The silkworm is the larva or caterpillar of a silk moth. It is an economically imp ...
larva is one of these, with mulberry
''Morus'', a genus of flowering plants in the family Moraceae, consists of diverse species of deciduous trees commonly known as mulberries, growing wild and under cultivation in many temperate world regions. Generally, the genus has 64 identif ...
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. The diamondback moth
The diamondback moth (''Plutella xylostella''), sometimes called the cabbage moth, is a moth species of the family Plutellidae and genus '' Plutella''. The small, grayish-brown moth sometimes has a cream-colored band that forms a diamond along ...
is an example of this, feeding exclusively on brassica
''Brassica'' () is a genus of plants in the cabbage and mustard family (Brassicaceae). The members of the genus are informally known as cruciferous vegetables, cabbages, or mustard plants. Crops from this genus are sometimes called ''cole cr ...
s, and the larva of the potato tuber moth feeds on potatoes, tomatoes and tobacco, all members of the same plant family, Solanaceae
The Solanaceae , or nightshades, are a family of flowering plants that ranges from annual and perennial herbs to vines, lianas, epiphytes, shrubs, and trees, and includes a number of agricultural crops, medicinal plants, spices, weeds, and orn ...
. Herbivorous insects with a wide range of hosts in various different plant families are known as polyphagous
Feeding is the process by which organisms, typically animals, obtain food. Terminology often uses either the suffixes -vore, -vory, or -vorous from Latin ''vorare'', meaning "to devour", or -phage, -phagy, or -phagous from Greek φαγε ...
. One example is the buff ermine
The buff ermine (''Spilarctia luteum'') is a moth of the family Erebidae. It is sometimes placed in the genus '' Spilosoma''. The species was first described by Johann Siegfried Hufnagel in 1766. It is found throughout the temperate belt of the ...
moth whose larvae feed on alder
Alders are trees comprising the genus ''Alnus'' in the birch family Betulaceae. The genus comprises about 35 species of monoecious trees and shrubs, a few reaching a large size, distributed throughout the north temperate zone with a few sp ...
, mint
MiNT is Now TOS (MiNT) is a free software alternative operating system kernel for the Atari ST system and its successors. It is a multi-tasking alternative to TOS and MagiC. Together with the free system components fVDI device drivers, XaA ...
, plantain
Plantain may refer to:
Plants and fruits
* Cooking banana, banana cultivars in the genus ''Musa'' whose fruits are generally used in cooking
** True plantains, a group of cultivars of the genus ''Musa''
* ''Plantaginaceae'', a family of flowerin ...
, oak
An oak is a tree or shrub in the genus ''Quercus'' (; Latin "oak tree") of the beech family, Fagaceae. There are approximately 500 extant species of oaks. The common name "oak" also appears in the names of species in related genera, notably ''L ...
, rhubarb
Rhubarb is the fleshy, edible stalks ( petioles) of species and hybrids (culinary rhubarb) of ''Rheum'' in the family Polygonaceae, which are cooked and used for food. The whole plant – a herbaceous perennial growing from short, thick rhizo ...
, currant, blackberry
The blackberry is an edible fruit produced by many species in the genus ''Rubus'' in the family Rosaceae, hybrids among these species within the subgenus ''Rubus'', and hybrids between the subgenera ''Rubus'' and ''Idaeobatus''. The taxonomy of ...
, dock
A dock (from Dutch language, Dutch ''dok'') is the area of water between or next to one or a group of human-made structures that are involved in the handling of boats or ships (usually on or near a shore) or such structures themselves. The ex ...
, ragwort
''Jacobaea vulgaris'', syn. ''Senecio jacobaea'', is a very common wild flower in the family Asteraceae that is native to northern Eurasia, usually in dry, open places, and has also been widely distributed as a weed elsewhere.
Common names inc ...
, nettle
{{redirect, Nettle
Nettle refers to plants with stinging hairs, particularly those of the genus ''Urtica''. It can also refer to plants which resemble ''Urtica'' species in appearance but do not have stinging hairs. Plants called "nettle" include ...
and honeysuckle
Honeysuckles are arching shrubs or twining vines in the genus ''Lonicera'' () of the family Caprifoliaceae, native to northern latitudes in North America and Eurasia. Approximately 180 species of honeysuckle have been identified in both contin ...
.
Plants often produce toxic or unpalatable secondary metabolite
Secondary metabolites, also called specialised metabolites, toxins, secondary products, or natural products, are organic compounds produced by any lifeform, e.g. bacteria, fungi, animals, or plants, which are not directly involved in the norm ...
s 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.
Some insect micropredators migrate regularly from one host to another. The hawthorn-carrot aphid overwinters on its primary host, a hawthorn
Hawthorn or Hawthorns may refer to:
Plants
* '' Crataegus'' (hawthorn), a large genus of shrubs and trees in the family Rosaceae
* ''Rhaphiolepis'' (hawthorn), a genus of about 15 species of evergreen shrubs and small trees in the family Rosace ...
tree, and migrates during the summer to its secondary host, a plant in the carrot family.
Host range
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 the epidemiology
Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and determinants of health and disease conditions in a defined population.
It is a cornerstone of public health, and shapes policy decisions and evidenc ...
of the parasitism or disease.
Host range of viruses
For instance, the production of antigenic shift
Antigenic shift is the process by which two or more different strains of a virus, or strains of two or more different viruses, combine to form a new subtype having a mixture of the surface antigens of the two or more original strains. The term is ...
s in Influenza A virus
''Influenza A virus'' (''IAV'') causes influenza in birds and some mammals, and is the only species of the genus ''Alphainfluenzavirus'' of the virus family ''Orthomyxoviridae''. Strain (biology)#Microbiology or virology, Strains of all subtypes ...
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. An influenza vaccine
Influenza vaccines, also known as flu shots, are vaccines that protect against infection by influenza viruses. New versions of the vaccines are developed twice a year, as the influenza virus rapidly changes. While their effectiveness varies fr ...
produced against an existing viral strain
In biology, a strain is a genetic variant, a subtype or a culture within a biological species. Strains are often seen as inherently artificial concepts, characterized by a specific intent for genetic isolation. This is most easily observed in mic ...
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.
Non-parasitic associations
Mutualistic hosts
Some hosts participate in fully mutualistic interactions with both organisms being completely dependent on the other. For example, termite
Termites are small insects that live in colonies and have distinct castes (eusocial) and feed on wood or other dead plant matter. Termites comprise the infraorder Isoptera, or alternatively the epifamily Termitoidae, within the order Blattode ...
s are hosts to the protozoa
Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...
that live in their gut and which digest cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...
,[ and the human ]gut flora
Gut microbiota, gut microbiome, or gut flora, are the microorganisms, including bacteria, archaea, fungi, and viruses that live in the digestive tracts of animals. The gastrointestinal metagenome is the aggregate of all the genomes of the gut mi ...
is essential for efficient digestion
Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery blood plasma. In certain organisms, these smaller substances are absorbed through the small intest ...
. Many corals and other marine invertebrates house zooxanthellae
Zooxanthellae is a colloquial term for single-celled dinoflagellates that are able to live in symbiosis with diverse marine invertebrates including demosponges, corals, jellyfish, and nudibranchs. Most known zooxanthellae are in the genus ''Sy ...
, 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 by photosynthesis
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
which supplement its diet. ''Lamellibrachia luymesi
''Lamellibrachia luymesi'' is a species of tube worms in the family Siboglinidae. It lives at deep-sea cold seeps where hydrocarbons (oil and methane) are leaking out of the seafloor. It is entirely reliant on internal, sulfide-oxidizing bacteria ...
'', 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. Some hermit crab
Hermit crabs are anomuran decapod crustaceans of the superfamily Paguroidea that have adapted to occupy empty scavenged mollusc shells to protect their fragile exoskeletons. There are over 800 species of hermit crab, most of which possess an a ...
s place pieces of sponge
Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through t ...
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.
An important hosting relationship is mycorrhiza
A mycorrhiza (from Greek μύκης ', "fungus", and ῥίζα ', "root"; pl. mycorrhizae, mycorrhiza or mycorrhizas) is a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant ...
, 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. Another such relationship is between leguminous plants and certain nitrogen-fixing bacteria called rhizobia
Rhizobia are diazotrophic bacteria that fix nitrogen after becoming established inside the root nodules of legumes (Fabaceae). To express genes for nitrogen fixation, rhizobia require a plant host; they cannot independently fix nitrogen. In gene ...
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 as bean
A bean is the seed of several plants in the family Fabaceae, which are used as vegetables for human or animal food. They can be cooked in many different ways, including boiling, frying, and baking, and are used in many traditional dishes th ...
s, pea
The pea is most commonly the small spherical seed or the seed-pod of the flowering plant species ''Pisum sativum''. Each pod contains several peas, which can be green or yellow. Botanically, pea pods are fruit, since they contain seeds and d ...
s, chickpea
The chickpea or chick pea (''Cicer arietinum'') is an annual legume of the family Fabaceae, subfamily Faboideae. Its different types are variously known as gram" or Bengal gram, garbanzo or garbanzo bean, or Egyptian pea. Chickpea seeds are high ...
s and alfalfa
Alfalfa () (''Medicago sativa''), also called lucerne, is a perennial flowering plant in the legume family Fabaceae. It is cultivated as an important forage crop in many countries around the world. It is used for grazing, hay, and silage, as w ...
are able to fix nitrogen in this way, and mixing clover
Clover or trefoil are common names for plants of the genus ''Trifolium'' (from Latin ''tres'' 'three' + ''folium'' 'leaf'), consisting of about 300 species of flowering plants in the legume or pea family Fabaceae originating in Europe. The genus ...
with grass
Poaceae () or Gramineae () is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos and the grasses of natural grassland and species cultivated in lawns an ...
es increases the yield of pastures.
Neurotransmitter tyramine
Tyramine ( ) (also spelled tyramin), also known under several other names, is a naturally occurring trace amine derived from the amino acid tyrosine. Tyramine acts as a catecholamine releasing agent. Notably, it is unable to cross the blood-b ...
produced by commensal '' Providencia'' bacteria, which colonize the gut of the nematode ''Caenorhabditis elegans
''Caenorhabditis elegans'' () is a free-living transparent nematode about 1 mm in length that lives in temperate soil environments. It is the type species of its genus. The name is a blend of the Greek ''caeno-'' (recent), ''rhabditis'' (ro ...
'', bypasses the requirement for its host to biosynthesise tyramine. This product is then probably converted to octopamine
Octopamine (molecular formula C8H11NO2; also known as OA, and also norsynephrine, ''para''-octopamine and others) is an organic chemical closely related to norepinephrine, and synthesized biologically by a homologous pathway. Octopamine is ofte ...
by the host enzyme tyramine β-hydroxylase and manipulates a host sensory decision.
Hosts in cleaning symbiosis
Hosts of many species are involved in cleaning symbiosis
Cleaning symbiosis is a mutually beneficial association between individuals of two species, where one (the cleaner) removes and eats parasites and other materials from the surface of the other (the client). Cleaning symbiosis is well-known amon ...
, 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. 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.
Commensal hosts
Remora
The remora (), sometimes called suckerfish, is any of a family (Echeneidae) of ray-finned fish in the order Carangiformes. Depending on species, they grow to long. Their distinctive first dorsal fins take the form of a modified oval, sucker-li ...
s (also called suckerfish) can swim freely but have evolved suckers that enable them to adhere to smooth surfaces, gaining a free ride (phoresis
Phoresis or phoresy is a non-permanent, commensalistic interaction in which one organism (a phoront or phoretic) attaches itself to another (the host) solely for the purpose of travel. Phoresis has been observed directly in ticks and mites s ...
), and they spend most of their lives clinging to a host animal such as a whale, turtle or shark. However, the relationship may be mutualistic, as remoras, though not generally considered to be cleaner fish
Cleaner fish are fish that show a specialist feeding strategy by providing a service to other species, referred to as clients, by removing dead skin, ectoparasites, and infected tissue from the surface or gill chambers. This example of cleaning ...
, often consume parasitic copepod
Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (inhabiting sea waters), some are benthos, benthic (living on the ocean floor) ...
s: for example, these are found in the stomach contents of 70% of the common remora
The common remora (''Remora remora'') is a pelagic marine fish belonging to family Echeneidae. The dorsal fin, which has 22 to 26 soft rays, acts as a suction cup, creating a vacuum to allow it to attach to larger marine animals, such as whales ...
. Many mollusc
Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...
s, barnacle
A barnacle is a type of arthropod constituting the subclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in eros ...
s and polychaete worms
Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are m ...
attach themselves to the carapace of the Atlantic horseshoe crab
The Atlantic horseshoe crab (''Limulus polyphemus''), also known as the American horseshoe crab, is a species of marine and brackish chelicerate arthropod. Despite their name, horseshoe crabs are more closely related to spiders, ticks, and scorp ...
; for some this is a convenient arrangement, but for others it is an obligate form of commensalism and they live nowhere else.
History
The first host to be noticed in ancient times was human: human parasite
Human parasites include various protozoa and worms.
Human parasites are divided into endoparasites, which cause infection inside the body, and ectoparasites, which cause infection superficially within the skin.
The cysts and eggs of endoparasit ...
s such as hookworm
Hookworms are intestinal, blood-feeding, parasitic roundworms that cause types of infection known as helminthiases. Hookworm infection is found in many parts of the world, and is common in areas with poor access to adequate water, sanitation, an ...
are recorded from ancient Egypt from 3000 BC onwards, while in ancient Greece
Ancient Greece ( el, Ἑλλάς, Hellás) was a northeastern Mediterranean civilization, existing from the Greek Dark Ages of the 12th–9th centuries BC to the end of classical antiquity ( AD 600), that comprised a loose collection of cult ...
, the Hippocratic Corpus
The Hippocratic Corpus (Latin: ''Corpus Hippocraticum''), or Hippocratic Collection, is a collection of around 60 early Ancient Greek medical works strongly associated with the physician Hippocrates and his teachings. The Hippocratic Corpus cove ...
describes human bladder worm. The medieval
In the history of Europe, the Middle Ages or medieval period lasted approximately from the late 5th to the late 15th centuries, similar to the Post-classical, post-classical period of World history (field), global history. It began with t ...
Persian physician Avicenna
Ibn Sina ( fa, ابن سینا; 980 – June 1037 CE), commonly known in the West as Avicenna (), was a Persian polymath who is regarded as one of the most significant physicians, astronomers, philosophers, and writers of the Islamic G ...
recorded human and animal parasites including roundworms, threadworms, the Guinea worm and tapeworms.[ In Early Modern times, ]Francesco Redi
Francesco Redi (18 February 1626 – 1 March 1697) was an Italian physician, naturalist, biologist, and poet. He is referred to as the "founder of experimental biology", and as the "father of modern parasitology". He was the first person to cha ...
recorded animal parasites, while the microscopist Antonie van Leeuwenhoek
Antonie Philips van Leeuwenhoek ( ; ; 24 October 1632 – 26 August 1723) was a Dutch microbiologist and microscopist in the Golden Age of Dutch science and technology. A largely self-taught man in science, he is commonly known as " the ...
observed and illustrated the protozoan ''Giardia lamblia
''Giardia duodenalis'', also known as ''Giardia intestinalis'' and ''Giardia lamblia'', is a flagellated parasitic microorganism of the genus '' Giardia'' that colonizes the small intestine, causing a diarrheal condition known as giardiasis. ...
'' from "his own loose stools".[
Hosts to mutualistic symbionts were recognised more recently, when in 1877 ]Albert Bernhard Frank
Albert Bernhard Frank (January 17, 1839 – September 27, 1900) was a German botanist, plant pathologist, and mycologist, born in Dresden. He is credited with coining the term ''mycorrhiza'' in his 1885 paper "Über die auf Wurzelsymbiose beruhen ...
described the mutualistic relationship between a fungus
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from th ...
and an alga
Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mi ...
in lichen
A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among filaments of multiple fungi species in a mutualistic relationship.[PHI-base
The Pathogen-Host Interactions database (PHI-base) is a biological database that contains curated information on genes experimentally proven to affect the outcome of pathogen-host interactions. The database is maintained by researchers at Rotham ...]
(Pathogen-Host Interaction database)
* Generalist and specialist species
A generalist species is able to thrive in a wide variety of environmental conditions and can make use of a variety of different resources (for example, a heterotroph with a varied diet). A specialist species can thrive only in a narrow range of env ...
* Host cell protein
Host cell proteins (HCPs) are process-related protein impurities that are produced by the host organism during biotherapeutic manufacturing and production. During the purification process, a majority of produced HCPs are removed from the final pro ...
References
{{DEFAULTSORT:Host (Biology)
Biological interactions
Parasitology
Disease ecology