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An obligate parasite or holoparasite is a
parasitic 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 (biology), adapted structurally to this w ...
organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological ...

organism
that cannot complete its life-cycle without exploiting a suitable
host A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places *Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman *Michel Host (1 ...
. If an obligate parasite cannot obtain a host it will fail to
reproduce Reproduction (or procreation or breeding) is the biological process Biological processes are those processes that are vital for an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is ...

reproduce
. This is opposed to a
facultative parasiteA facultative parasite is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synonym for "Out ...
, which can act as a parasite but does not rely on its host to continue its life-cycle. Obligate parasites have
evolved Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, ...

evolved
a variety of parasitic strategies to exploit their hosts. Holoparasites and some
hemiparasite A parasitic plant is a plant Plants are mainly multicellular organisms, predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy th ...
s are obligate. It is advantageous for the parasite to preserve the health of their host when this is compatible with their nutritional and reproductive requirements, except when the death of the host is necessary for transmission.Combes, C. (1997) Fitness of Parasites: Pathology and Selection ''International Journal for Parasitology'' 27 (1): 1–10.


Species

Obligate parasitism is exhibited in a range of organisms, with examples in
virus A virus is a submicroscopic infectious agent In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecu ...

virus
es,
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

bacteria
,
fungi A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ...

fungi
,
plants Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Docto ...

plants
, and
animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

animal
s.Balashov, Yu.S. (2011) Parasitism and Ecological Parasitology. ''Entomological Review'' 91 (9): 1216–1223. They are unable to complete their development without passing through at least one parasitic stage which is necessary to their life-cycle. Whether one regards
virus A virus is a submicroscopic infectious agent In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecu ...

virus
es as living organisms or not, they cannot reproduce except by means of resources within living cells. Accordingly, it is convenient and customary to regard them as obligate
intracellular parasite Intracellular parasites are microparasites that are capable of growing and reproducing inside the cells of a Host (biology), host. Types of parasites There are two main types of intracellular parasites: Facultative and Obligate. Facultative int ...
s. Among the
Vespidae The Vespidae are a large (nearly 5000 species), diverse, cosmopolitan Cosmopolitan may refer to: Food and drink * Cosmopolitan (cocktail), also known as a "Cosmo" History * Rootless cosmopolitan, a Soviet derogatory epithet during Joseph St ...

Vespidae
family, ''
Vespula austriaca ''Vespula austriaca'' is an obligate parasitic wasp, parasitizing the nests of other species in the genus '' Vespula'' in the Old World The "Old World" is a retronym A retronym is a newer name for an existing thing that differentiates ...

Vespula austriaca
'' is an example of an obligate reproductive parasite; its common host is '' Vespula acadica''. In the genus ''
Bombus A bumblebee (or bumble bee, bumble-bee, or humble-bee) is any of over 250 species in the genus ''Bombus'', part of Apidae, one of the bee families. This genus is the only Extant taxon, extant group in the tribe Bombini, though a few extinct ...

Bombus
'', ''
B. bohemicus
B. bohemicus
'' is an obligate parasite of ''B. locurum'', ''B. cryptarum'', and ''B. terrestris.''


Host-parasite interaction


Life-cycle

Parasitic life cycle In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...
s involve the exploitation of at least one host. Parasites that infect a single species are said to have direct life-cycles.May, R. M. & Anderson, R. M. (1979) Population biology of infectious diseases. ''Nature'' 280: 455–461. For example, the
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, ...
species ''
Necator americanus ''Necator'' may refer to: * ''Necator'' (fungus), a genus of fungus in the family Corticiaceae * ''Necator'' (nematode), a genus of nematodes in the family Ancylostomatidae {{genus disambiguation ...
''. Parasites that infect more than one host are said to have a complex or indirect life-cycle. For example, the
malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms Signs and symptoms are the observed or detectable signs, and experienced symptoms of an illness, injury, or condition. A sign fo ...

malaria
plasmodium.


Intermediate or final host

An intermediate or secondary host is exploited by the parasite only for a short transition period. A final or primary host is exploited by the parasite and is the only location in which the parasite is able to reach maturity and if possible, reproduce sexually. For example, '' Ribeiroia ondatrae'' uses
Ramshorn snail The tiny hamlet of Ramsor (Methodist spelling) in North Staffordshire played a significant part in the origins of Primitive Methodism. Listed in the Domesday Book as Ramshorn, this ancient hamlet is a typical example of the depopulation of the ...
as its first intermediate host, amphibians and fish as second intermediate hosts and birds as definitive hosts.


Parasitic permanence

Obligate parasites may not necessarily spend all of their time behaving as parasites. When a parasite is permanent, a number of generations occur in or on the host of an infested individual.
Head lice The head louse (''Pediculus humanus capitis'') is an obligate{{wiktionary, obligate As an adjective, obligate means "by necessity" (antonym '' facultative'') and is used mainly in biology in phrases such as: * Obligate aerobe 300px, Aerobic and ...

Head lice
are an example of this. Temporary parasites are organisms whose parasitic mode of life is limited to a few or even one stage of development. An example of this is the larval stage of
harvest mite Trombiculidae (; commonly referred to as chiggers, but also known as berry bugs, harvest mites, red bugs or scrub-itch mites, are a family of mites. Chiggers are often confused with Tunga penetrans, jiggers - a type of flea. Several species of Tr ...
s, while the adult stage is non-parasitic.


Location on host

The parasite may live outside of the host
ectoparasite Parasitism is a close relationship between species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined ...

ectoparasite
; for example, a
tick Ticks (suborder Ixodida) are parasitic arachnid Arachnida () is a class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of in ...

tick
. Alternatively, the parasite may live within the host
endoparasite Parasitism is a Symbiosis, symbiotic biological interactions, 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 (biology), ad ...

endoparasite
; for example, the fluke. An obligate parasite that does not live directly in or on the host, but rather acts at a distance for example, a
cuckoo Cuckoos are bird Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological ...

cuckoo
which hatches and is raised by non-relatives is known as a
brood parasite Brood parasites are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...
.


Invasion strategies

In order to establish infestation in a susceptible host, obligate parasites must evade defences before, during and after entry into the host. Due to the wide range of obligate parasite types, it is impossible to identify a general invasion strategy.
Intracellular parasite Intracellular parasites are microparasites that are capable of growing and reproducing inside the cells of a Host (biology), host. Types of parasites There are two main types of intracellular parasites: Facultative and Obligate. Facultative int ...
s use various strategies to invade cells and subvert cellular signalling pathways. Most bacteria and viruses undergo passive uptake, where they rely on the host cell for uptake. However,
apicomplexa The Apicomplexa (also called Apicomplexia) are a large phylum of parasitic alveolates. Most of them possess a unique form of organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a sp ...
ns engage in active entry. One obligate wasp parasite,'' Polistes atrimandibularis'', infiltrates their hosts' colony by modifying their chemical signature to match that of the hosts'. This tricks the host wasps into thinking the parasite is one of their own.


Evasion of host defences

A number of obligate intracellular parasites have evolved mechanisms for evading their hosts' cellular defences, including the ability to survive in distinct
cellular compartment Cellular compartments in cell biology comprise all of the closed parts within the cytosol The cytosol, also known as intracellular fluid (ICF) or cytoplasmic matrix, or groundplasm, is the liquid found inside cells. It is separated into com ...
s. One of the mechanisms that hosts employ in their attempt to reduce the replication and spread of pathogens is apoptosis (programmed cell death). Some obligate parasites have developed ways to suppress this phenomenon, for example ''Toxoplasma gondii'' although the mechanism is not yet fully understood.


Manipulation of host behaviour

Changes in a host’s behaviour following infection with obligate parasites are extremely common. Unusual behaviour observed in infected individuals is noted, and if its complexity suggests that this behaviour will benefit the transmission of the parasite, then this is said to be an example of adaptive behaviour, adaptive manipulation.Hughes, D. (2013) Pathways to understanding the extended phenotype of parasites in their hosts. ''The Journal of Experimental Biology'' 216: 142–147. However, there is a difficulty in demonstrating changes in behaviour are the result of a selective process favouring transmission of the parasite. It has been suggested that these changes may merely be a side-effect of infection. Most behaviour changes have not been demonstrated to lead to fitness (biology), fitness gains in either the host or the parasite. An example of this behaviour is the attraction of rats to cat urine after infection with ''Toxoplasma gondii''. However, the "scientific metaphors, including anthropomorphisms" sometimes used in "popular media and the scientific literature" to describe the manipulation of host behavior have been described as "catchy, yet misleading".


Extended phenotype

In some cases the behaviour we observe in an organism is not due to the expression of their genes, but rather to the genes of parasites infecting them. This behaviour is an The Extended Phenotype, extended phenotype.


Evolution of host behaviour manipulation

Three main evolutionary routes have been suggested for the appearance of host behaviour manipulation by parasites. The first is a ''parasite driven'' scenario of manipulation, while the second and third are ''host driven'' scenarios of manipulation. #Manipulation ''sensu stricto'' (extended phenotype- abhorrent behaviour displayed by parasitised hosts results from the expression of the parasites genes) this capacity could have been the product of natural selection in an ancestral parasite with the trait.Adamo, S. A. (2012) The strings of the puppet master: How parasites change host behaviour in Hughes, D.P., Brodeur, J. & Thomas, F. (Eds.), ''Host Manipulation by Parasites'' (pp. 36–51).Oxford, UK: Oxford University Press. #The mafia-like strategy- retaliation for non-compliance (eg.great spotted cuckoo and magpie) magpies that eject the cuckoos eggs from their nest suffer a much greater rate of cuckoo predation. #The exploitation of compensatory responses induce host compensatory responses since these may at least partially match with the transmission routes of parasites. E.g. the sexually transmitted ectoparasite ''Chrysomelobia labidomerae'', parasitizing the leaf beetle host ''Labidomera clivicollis''~ infected males exhibit increased sexual behaviour and as a result enhance inter- and intra- sexual contacts (copulation and competition) which provide more opportunities for parasite transmission.Abbot, P. & Dll, L. M. (2001). Sexually transmitted parasites and sexual selection in the milkweed leaf beetle, ''Labidomera clivicollis''. ''Oikos'' 92: 91–100 It has been suggested that The Extended Phenotype, extended phenotype behaviours are not adaptive, but are exaptation, Exaptative. While they may have a benefit for the parasitic organism, they did not arise with the intention of this benefit.


Parasitic mimicry in brood parasites

The cowbird and
cuckoo Cuckoos are bird Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological ...

cuckoo
require the nests and parental care of other passerines in order for their young to fledge. These are known as
brood parasite Brood parasites are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...
s. The parasitic bird species mimics egg patterns and colours of the host species, which reduces egg rejection. The chicks of some species are able to manipulate host behaviour by making rapid calls that mimic the sound made by up to four of the host chicks. Mimicry of the host species also occurs in the paper wasp species ''Polistes semenowi'' and ''Polistes sulcifer'' and the bumblebee species ''Bombus bohemicus'', with the parasite changing its proportions of cuticular hydrocarbons, species- and colony-specific identifying chemicals, to match that of the usurped host species.Kreuter, Kirsten; Elfi Bunk (November 23, 2011). "How the social parasitic bumblebee Bombus bohemicus sneaks into power of reproduction". ''Behavioral Ecology and Sociobiology'' 66 (3): 475–486. doi:10.1007/s00265-011-1294-z. Retrieved 21 September 2015. Several butterfly species will also exhibit brood parasitic behavior. An example is ''Niphanda fusca,'' a butterfly that will release cuticular hydrocarbons (CHCs) to trick the host ant, ''C. japonicus,'' into adopting the larva as their own in their own nest. The ant will then raise the larva of the butterfly, feeding it directly from mouth-to-mouth, until it pupates. It is proposed that this mimicry has evolved through two processes: either as coevolutionary responses to host defences against brood parasites or modifying pre-existing host provisioning strategies. Competition between the parasite and host young for parental resources might lead to exaggeration of the aspects of the signal that most effectively exploit host parents. The parasitic young are likely to experience stronger selection for exaggerated signals than host young, because they are unrelated to the other chicks in the nest and therefore under selection to behave more selfishly.


Evolution of obligate parasitism

Current theory in evolutionary biology indicates that host-parasite relationships may evolve towards equilibrial states of severe disease.Ewald, P.W. (1983). Host-parasite relations, vectors, and the evolution of disease severity. ''Annual Review of Ecology and Systematics'' 14:465–485. This differs from the conventional belief that commensalism is the ideal equilibrium for both the host and parasite.


See also

*Obligate intracellular parasite *Parasitoid


References

{{reflist, 28em Ecology Parasites Parasitism