Editing Parasitism (section)

=== Host defences ===

Hosts have evolved a variety of defensive measures against their parasites, including physical barriers like the skin of vertebrates,<ref name=Colorado/> the immune system of mammals,<ref name=Maizels2009/> insects actively removing parasites,<ref name=Jeanne1979/> and defensive chemicals in plants.<ref name=Runyon2010/>

The evolutionary biologist [[W. D. Hamilton]] suggested that [[sexual reproduction]] could have evolved to help to defeat multiple parasites by enabling [[genetic recombination]], the shuffling of genes to create varied combinations. Hamilton showed by mathematical modelling that sexual reproduction would be evolutionarily stable in different situations, and that the theory's predictions matched the actual ecology of sexual reproduction.<ref name="Hamilton1990">{{cite journal |last1=Hamilton |first1=W. D. |author1-link=W. D. Hamilton|last2=Axelrod |first2=R. |last3=Tanese |first3=R. |title=Sexual reproduction as an adaptation to resist parasites (a review). |journal=Proceedings of the National Academy of Sciences |volume=87 |issue=9 |date=May 1990 |doi=10.1073/pnas.87.9.3566 |pmid=2185476 |pmc=53943 |pages=3566–3573 |bibcode=1990PNAS...87.3566H |doi-access=free }}</ref><ref name="EbertHamilton1996">{{cite journal |last1=Ebert |first1=Dieter |last2=Hamilton |first2=William D. |author2-link=W. D. Hamilton |title=Sex against virulence: the coevolution of parasitic diseases |journal=Trends in Ecology & Evolution |volume=11 |issue=2 |year=1996 |doi=10.1016/0169-5347(96)81047-0 |pmid=21237766 |pages=79–82|bibcode=1996TEcoE..11...79E }}</ref> However, there may be a trade-off between [[immunocompetence]] and breeding male vertebrate hosts' [[secondary sex characteristic]]s, such as the plumage of [[peacock]]s and the manes of [[lion]]s. This is because the male hormone [[testosterone]] encourages the growth of secondary sex characteristics, favouring such males in [[sexual selection]], at the price of reducing their immune defences.<ref name="FolstadKarter1992">{{cite journal |last1=Folstad |first1=Ivar |last2=Karter |first2=Andrew John |url=https://www.academia.edu/17248251 |title=Parasites, Bright Males, and the Immunocompetence Handicap |journal=The American Naturalist |volume=139 |issue=3 |year=1992 |pages=603–622 |doi=10.1086/285346 |bibcode=1992ANat..139..603F |s2cid=85266542 }}{{Dead link|date=July 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

==== Vertebrates ====

[[File:Short Horned Lizard (4457945238).jpg|thumb|left|The dry [[skin]] of vertebrates such as the [[short-horned lizard]] prevents the entry of many parasites.]]

The physical barrier of the tough and often dry and waterproof [[skin]] of reptiles, birds and mammals keeps invading microorganisms from entering the body. [[Human skin]] also secretes [[sebum]], which is toxic to most microorganisms.<ref name=Colorado>{{cite web |title=Host–Parasite Interactions Innate Defenses of the Host |publisher=University of Colorado |url=http://www.colorado.edu/outreach/BSI/k12activities/interactive/innatedefenses.pdf |access-date=7 May 2014 |archive-url=https://web.archive.org/web/20160304000852/http://www.colorado.edu/outreach/BSI/k12activities/interactive/innatedefenses.pdf |archive-date=4 March 2016 |url-status=dead }}</ref> On the other hand, larger parasites such as [[trematode]]s detect chemicals produced by the skin to locate their hosts when they enter the water. Vertebrate [[saliva]] and tears contain [[lysozyme]], an enzyme that breaks down the [[Bacterial cell structure#Cell wall|cell walls]] of invading bacteria.<ref name=Colorado/> Should the organism pass the mouth, the [[stomach]] with its [[hydrochloric acid]], toxic to most microorganisms, is the next line of defence.<ref name=Colorado/> Some intestinal parasites have a thick, tough outer coating which is digested slowly or not at all, allowing the parasite to pass through the stomach alive, at which point they enter the intestine and begin the next stage of their life. Once inside the body, parasites must overcome the [[immune system]]'s [[serum proteins]] and [[pattern recognition receptor]]s, intracellular and cellular, that trigger the adaptive immune system's [[lymphocyte]]s such as [[T cell]]s and antibody-producing [[B cell]]s. These have receptors that recognise parasites.<ref name=Maizels2009>{{cite journal |last=Maizels |first=R. M. |title=Parasite immunomodulation and polymorphisms of the immune system |journal=J. Biol. |year=2009 |volume=8 |issue=7 |pages=62 |pmid=19664200 |pmc=2736671 |doi=10.1186/jbiol166 |doi-access=free }}</ref>

==== Insects ====

[[File:Leaf Spot on Oak in Gunnersbury Triangle.jpg|thumb|[[Leaf spot]] on [[oak]]. The spread of the parasitic fungus is limited by defensive chemicals produced by the tree, resulting in circular patches of damaged tissue.]]

Insects often adapt their nests to reduce parasitism. For example, one of the key reasons why the wasp ''[[Polistes canadensis]]'' nests across multiple [[honeycomb|combs]], rather than building a single comb like much of the rest of its genus, is to avoid infestation by [[tineid moth]]s. The tineid moth lays its eggs within the wasps' nests and then these eggs hatch into larvae that can burrow from cell to cell and prey on wasp pupae. Adult wasps attempt to remove and kill moth eggs and larvae by chewing down the edges of cells, coating the cells with an oral secretion that gives the nest a dark brownish appearance.<ref name=Jeanne1979>{{cite journal |last=Jeanne |first=Robert L. |year=1979 |url=https://www.researchgate.net/publication/226226828 |title=Construction and Utilization of Multiple Combs in Polistes canadensis in Relation to the Biology of a Predaceous Moth |journal=Behavioral Ecology and Sociobiology |volume=4 |issue=3|pages=293–310 |doi=10.1007/bf00297649|bibcode=1979BEcoS...4..293J |s2cid=36132488 }}</ref>

==== Plants ====

Plants respond to parasite attack with a series of chemical defences, such as [[polyphenol oxidase]], under the control of the [[jasmonic acid]]-insensitive (JA) and [[salicylic acid]] (SA) signalling pathways.<ref name=Runyon2010>{{cite journal |last=Runyon |first=J. B. |author2=Mescher, M. C. |author3=De Moraes, C. M. |title=Plant defenses against parasitic plants show similarities to those induced by herbivores and pathogens |journal=Plant Signal Behav |volume=5 |issue=8 |pages=929–31 |year=2010 |pmid=20495380 |pmc=3115164 |doi=10.4161/psb.5.8.11772|bibcode=2010PlSiB...5..929R }}</ref><ref name=Thaler2002>{{cite journal |last1=Thaler |first1=Jennifer S. |last2=Karban |first2=Richard |last3=Ullman |first3=Diane E. |last4=Boege |first4=Karina |last5=Bostock |first5=Richard M. |url=https://www.researchgate.net/publication/226325654 |title=Cross-talk between jasmonate and salicylate plant defense pathways: effects on several plant parasites |journal=Oecologia |volume=131 |issue=2 |year=2002 |doi=10.1007/s00442-002-0885-9 |pmid=28547690 |pages=227–235|bibcode=2002Oecol.131..227T |s2cid=25912204 }}</ref> The different biochemical pathways are activated by different attacks, and the two pathways can interact positively or negatively. In general, plants can either initiate a specific or a non-specific response.<ref name=Thaler2002/><ref name="Frank_2000">{{cite journal |last=Frank |first=S. A. |url=https://stevefrank.org/reprints-pdf/00JTB-Defense.pdf |archive-url=https://web.archive.org/web/20010614080530/http://stevefrank.org/reprints-pdf/00JTB-Defense.pdf |archive-date=14 June 2001 |url-status=live |title=Specific and non-specific defense against parasitic attack |journal=J. Theor. Biol. |volume=202 |issue=4 |pages=283–304 |year=2000 |pmid=10666361 |doi=10.1006/jtbi.1999.1054|bibcode=2000JThBi.202..283F |citeseerx=10.1.1.212.7024 }}</ref> Specific responses involve recognition of a parasite by the plant's cellular receptors, leading to a strong but localised response: defensive chemicals are produced around the area where the parasite was detected, blocking its spread, and avoiding wasting defensive production where it is not needed.<ref name="Frank_2000"/> Non-specific defensive responses are systemic, meaning that the responses are not confined to an area of the plant, but spread throughout the plant, making them costly in energy. These are effective against a wide range of parasites.<ref name="Frank_2000"/> When damaged, such as by [[lepidoptera]]n [[caterpillar]]s, leaves of plants including [[maize]] and [[cotton]] release increased amounts of volatile chemicals such as [[terpene]]s that signal they are being attacked; one effect of this is to attract parasitoid wasps, which in turn attack the caterpillars.<ref name="Paré Tumlinson pp. 325–332">{{cite journal |last1=Paré |first1=Paul W. |last2=Tumlinson |first2=James H. |title=Plant Volatiles as a Defense against Insect Herbivores |journal=Plant Physiology |volume=121 |issue=2 |date=1 October 1999|doi=10.1104/pp.121.2.325 |pmid=10517823 |pages=325–332|pmc=1539229 }}</ref>
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