This is an accepted version of this page
109-459: Dicyemida , also known as Rhombozoa , is a phylum of tiny parasites that live in the renal appendages of cephalopods . Classification is controversial. Traditionally, dicyemids have been grouped with the Orthonectida in the phylum Mesozoa and, from 2017, molecular evidence appears to confirm this. However, other molecular phylogenies have placed the dicyemids more closely related to
218-438: A holoparasite such as Cuscuta derives all of its nutrients from another plant. Parasitic plants make up about one per cent of angiosperms and are in almost every biome in the world. All these plants have modified roots, haustoria , which penetrate the host plants, connecting them to the conductive system—either the xylem , the phloem , or both. This provides them with the ability to extract water and nutrients from
327-419: A symbiotic union of a fungus and photosynthetic algae or cyanobacteria , reproduce through fragmentation to ensure that new individuals contain both symbionts. These fragments can take the form of soredia , dust-like particles consisting of fungal hyphae wrapped around photobiont cells. Clonal Fragmentation in multicellular or colonial organisms is a form of asexual reproduction or cloning where an organism
436-511: A cause of gastroenteritis , is spread by the fecal–oral route from animals, or by eating insufficiently cooked poultry , or by contaminated water. Haemophilus influenzae , an agent of bacterial meningitis and respiratory tract infections such as influenza and bronchitis , is transmitted by droplet contact. Treponema pallidum , the cause of syphilis , is spread by sexual activity . Viruses are obligate intracellular parasites, characterised by extremely limited biological function, to
545-497: A chemical cue accumulates and induces the transition to sexual reproduction. Many protists and fungi alternate between sexual and asexual reproduction. A few species of amphibians, reptiles, and birds have a similar ability. The slime mold Dictyostelium undergoes binary fission (mitosis) as single-celled amoebae under favorable conditions. However, when conditions turn unfavorable, the cells aggregate and follow one of two different developmental pathways, depending on conditions. In
654-627: A faster rate than their hosts. Classic examples include interactions between vertebrate hosts and tapeworms , flukes , and those between the malaria-causing Plasmodium species, and fleas . Parasites reduce host fitness by general or specialised pathology , that ranges from parasitic castration to modification of host behaviour . Parasites increase their own fitness by exploiting hosts for resources necessary for their survival, in particular by feeding on them and by using intermediate (secondary) hosts to assist in their transmission from one definitive (primary) host to another. Although parasitism
763-414: A fertilization event. These haploid individuals produce gametes through mitosis . Meiosis and gamete formation therefore occur in separate multicellular generations or "phases" of the life cycle, referred to as alternation of generations . Since sexual reproduction is often more narrowly defined as the fusion of gametes ( fertilization ), spore formation in plant sporophytes and algae might be considered
872-495: A few closely related hosts. [REDACTED] Data related to Rhombozoa at Wikispecies 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 characterised parasites as "predators that eat prey in units of less than one". Parasites include single-celled protozoans such as
981-418: A form of asexual reproduction (agamogenesis) despite being the result of meiosis and undergoing a reduction in ploidy . However, both events (spore formation and fertilization) are necessary to complete sexual reproduction in the plant life cycle. Fungi and some algae can also utilize true asexual spore formation, which involves mitosis giving rise to reproductive cells called mitospores that develop into
1090-567: A functionally similar manner by mitosis ; most of these are also capable of sexual reproduction. Multiple fission at the cellular level occurs in many protists , e.g. sporozoans and algae . The nucleus of the parent cell divides several times by mitosis , producing several nuclei. The cytoplasm then separates, creating multiple daughter cells . In apicomplexans , multiple fission, or schizogony appears either as merogony , sporogony or gametogony . Merogony results in merozoites , which are multiple daughter cells, that originate within
1199-531: A handful of times. In a study done on octopuses, it was found that Dicyemida that had similarly shaped calottes rarely coexisted in the same individual host, which suggested a strong level of competition for habitat. In Japan, two types of dicyemid parasites, D. misakiense and D. japonicum , have often been discovered living in the same host. In 1938, when the two species were initially discovered, scientists did not classify them as separate species due to their large amount of morphological similarities. In fact,
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#17327719954051308-595: A hormone or by diverting nutrients. For example, the trematode Zoogonus lasius , whose sporocysts lack mouths, castrates the intertidal marine snail Tritia obsoleta chemically, developing in its gonad and killing its reproductive cells. Directly transmitted parasites, not requiring a vector to reach their hosts, include such parasites of terrestrial vertebrates as lice and mites; marine parasites such as copepods and cyamid amphipods; monogeneans ; and many species of nematodes, fungi, protozoans, bacteria, and viruses. Whether endoparasites or ectoparasites, each has
1417-564: A male gamete. Examples are parthenogenesis and apomixis . Parthenogenesis is a form of agamogenesis in which an unfertilized egg develops into a new individual. It has been documented in over 2,000 species. Parthenogenesis occurs in the wild in many invertebrates (e.g. water fleas, rotifers , aphids, stick insects , some ants, bees and parasitic wasps) and vertebrates (mostly reptiles, amphibians, and fish). It has also been documented in domestic birds and in genetically altered lab mice. Plants can engage in parthenogenesis as well through
1526-440: A mammal species hosts four species of nematode, two of trematodes, and two of cestodes. Humans have 342 species of helminth parasites, and 70 species of protozoan parasites. Some three-quarters of the links in food webs include a parasite, important in regulating host numbers. Perhaps 40 per cent of described species are parasitic. Asexual reproduction This is an accepted version of this page Asexual reproduction
1635-415: A mate becomes difficult. For example, female zebra sharks will reproduce asexually if they are unable to find a mate in their ocean habitats. Parthenogenesis was previously believed to rarely occur in vertebrates, and only be possible in very small animals. However, it has been discovered in many more species in recent years. Today, the largest species that has been documented reproducing parthenogenically
1744-467: A more inclusive name for the phylum or class. Adult dicyemids range in length from 0.5 to 7 millimetres (0.020 to 0.276 in), and they can be easily viewed through a light microscope. They display eutely , a condition in which each adult individual of a given species has the same number of cells, making cell number a useful identifying character. Dicyemida lack respiratory, circulatory, excretory, digestive, and nervous systems. The organism's structure
1853-429: A new organism after dispersal. This method of reproduction is found for example in conidial fungi and the red algae Polysiphonia , and involves sporogenesis without meiosis. Thus the chromosome number of the spore cell is the same as that of the parent producing the spores. However, mitotic sporogenesis is an exception and most spores, such as those of plants and many algae, are produced by meiosis . Fragmentation
1962-836: A parasite employs to identify and approach a potential host are known as "host cues". Such cues can include, for example, vibration, exhaled carbon dioxide , skin odours, visual and heat signatures, and moisture. Parasitic plants can use, for example, light, host physiochemistry, and volatiles to recognize potential hosts. There are six major parasitic strategies , namely parasitic castration ; directly transmitted parasitism; trophically -transmitted parasitism; vector -transmitted parasitism; parasitoidism ; and micropredation. These apply to parasites whose hosts are plants as well as animals. These strategies represent adaptive peaks ; intermediate strategies are possible, but organisms in many different groups have consistently converged on these six, which are evolutionarily stable. A perspective on
2071-635: A parasitic alien species. First used in English in 1539, the word parasite comes from the Medieval French parasite , from the Latinised form parasitus , from Ancient Greek παράσιτος (parasitos) 'one who eats at the table of another' in turn from παρά (para) 'beside, by' and σῖτος (sitos) 'wheat, food'. The related term parasitism appears in English from 1611. Parasitism
2180-402: A phenomenon known as "egg parasitism." This method of reproduction has been found in several species of the clam genus Corbicula , many plants like, Cupressus dupreziana , Lomatia tasmanica , Pando and recently in the fish Squalius alburnoides . Other species where androgenesis has been observed naturally are the stick insects Bacillus rossius and Bassillus Grandii ,
2289-432: A phenomenon termed the biotrophy-necrotrophy switch . Pathogenic fungi are well-known causative agents of diseases on animals as well as humans. Fungal infections ( mycosis ) are estimated to kill 1.6 million people each year. One example of a potent fungal animal pathogen are Microsporidia - obligate intracellular parasitic fungi that largely affect insects, but may also affect vertebrates including humans, causing
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#17327719954052398-538: A predator, the European sparrowhawk , giving her time to lay her eggs in the host's nest unobserved. Host species often combat parasitic egg mimicry through egg polymorphism , having two or more egg phenotypes within a single population of a species. Multiple phenotypes in host eggs decrease the probability of a parasitic species accurately "matching" their eggs to host eggs. In kleptoparasitism (from Greek κλέπτης ( kleptēs ), "thief"), parasites steal food gathered by
2507-538: A process called apomixis . However this process is considered by many to not be an independent reproduction method, but instead a breakdown of the mechanisms behind sexual reproduction. Parthenogenetic organisms can be split into two main categories: facultative and obligate. In facultative parthenogenesis, females can reproduce both sexually and asexually. Because of the many advantages of sexual reproduction, most facultative parthenotes only reproduce asexually when forced to. This typically occurs in instances when finding
2616-402: A process called thelytoky . The freshwater crustacean Daphnia reproduces by parthenogenesis in the spring to rapidly populate ponds, then switches to sexual reproduction as the intensity of competition and predation increases. Monogonont rotifers of the genus Brachionus reproduce via cyclical parthenogenesis: at low population densities females produce asexually and at higher densities
2725-399: A recorded case of two separate species of dicyemida existing in the same host and having exactly the same calotte . Species that share similar or even identical calottes have been found on occasion, but have never been found within the same host. Because of the constant variation in calotte size between species (even within one given host) there is very rarely observable competition between
2834-573: A single host-species. Within that species, most individuals are free or almost free of parasites, while a minority carry a large number of parasites; this is known as an aggregated distribution . Trophically -transmitted parasites are transmitted by being eaten by a host. They include trematodes (all except schistosomes ), cestodes , acanthocephalans , pentastomids , many roundworms , and many protozoa such as Toxoplasma . They have complex life cycles involving hosts of two or more species. In their juvenile stages they infect and often encyst in
2943-525: A sperm cell is used to initiate reproduction. However, the sperm's genes never get incorporated into the egg cell. The best known example of this is the Amazon molly . Because they are obligate parthenotes, there are no males in their species so they depend on males from a closely related species (the Sailfin molly ) for sperm. Apomixis in plants is the formation of a new sporophyte without fertilization. It
3052-430: A suitable fungus soon after germinating. Parasitic fungi derive some or all of their nutritional requirements from plants, other fungi, or animals. Plant pathogenic fungi are classified into three categories depending on their mode of nutrition: biotrophs, hemibiotrophs and necrotrophs. Biotrophic fungi derive nutrients from living plant cells, and during the course of infection they colonise their plant host in such
3161-399: A variety of calotte shapes, which means they are infected with multiple different species. On the occasion that similar (but not identical) calotte shapes happen to be present within one host’s body, one species usually ends up dominating the other, indicating that it has adapted more readily to the environment within the host. However, this occurrence is very rare and has only been observed
3270-1047: A variety of methods to infect animal hosts, including physical contact, the fecal–oral route , free-living infectious stages, and vectors, suiting their differing hosts, life cycles, and ecological contexts. Examples to illustrate some of the many possible combinations are given in the table. social behaviour (grooming) Among the many variations on parasitic strategies are hyperparasitism, social parasitism, brood parasitism, kleptoparasitism, sexual parasitism, and adelphoparasitism. Hyperparasites feed on another parasite, as exemplified by protozoa living in helminth parasites, or facultative or obligate parasitoids whose hosts are either conventional parasites or parasitoids. Levels of parasitism beyond secondary also occur, especially among facultative parasitoids. In oak gall systems, there can be up to five levels of parasitism. Hyperparasites can control their hosts' populations, and are used for this purpose in agriculture and to some extent in medicine . The controlling effects can be seen in
3379-469: A variety of routes. To give a few examples, Bacillus anthracis , the cause of anthrax , is spread by contact with infected domestic animals ; its spores , which can survive for years outside the body, can enter a host through an abrasion or may be inhaled. Borrelia , the cause of Lyme disease and relapsing fever , is transmitted by vectors, ticks of the genus Ixodes , from the diseases' reservoirs in animals such as deer . Campylobacter jejuni ,
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3488-399: A very distinctive morphology, swimming about with ciliated rings that resemble headlights. It has long been assumed that this sexually produced infusoriform, which is released when the host eliminates urine from the kidneys, is both the dispersal and the infectious stage. The mechanism of infection, however, remains unknown, as are the effects, if any, of dicyemids on their hosts. Some part of
3597-474: A way as to keep it alive for a maximally long time. One well-known example of a biotrophic pathogen is Ustilago maydis , causative agent of the corn smut disease. Necrotrophic pathogens on the other hand, kill host cells and feed saprophytically , an example being the root-colonising honey fungi in the genus Armillaria . Hemibiotrophic pathogens begin their colonising their hosts as biotrophs, and subsequently killing off host cells and feeding as necrotrophs,
3706-400: A year. This switch is triggered by environmental changes in the fall and causes females to develop eggs instead of embryos. This dynamic reproductive cycle allows them to produce specialized offspring with polyphenism , a type of polymorphism where different phenotypes have evolved to carry out specific tasks. The cape bee Apis mellifera subsp. capensis can reproduce asexually through
3815-548: Is a form of asexual reproduction where a new organism grows from a fragment of the parent. Each fragment develops into a mature, fully grown individual. Fragmentation is seen in many organisms. Animals that reproduce asexually include planarians , many annelid worms including polychaetes and some oligochaetes , turbellarians and sea stars . Many fungi and plants reproduce asexually. Some plants have specialized structures for reproduction via fragmentation, such as gemmae in mosses and liverworts . Most lichens , which are
3924-563: Is a kind of symbiosis , a close and persistent long-term biological interaction between a parasite and its host. Unlike saprotrophs , parasites feed on living hosts, though some parasitic fungi, for instance, may continue to feed on hosts they have killed. Unlike commensalism and mutualism , the parasitic relationship harms the host, either feeding on it or, as in the case of intestinal parasites, consuming some of its food. Because parasites interact with other species, they can readily act as vectors of pathogens, causing disease . Predation
4033-414: Is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes . The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the full set of genes of their single parent and thus the newly created individual is genetically and physically similar to the parent or an exact clone of the parent. Asexual reproduction
4142-559: Is a type of asexual reproduction found in plants where new individuals are formed without the production of seeds or spores and thus without syngamy or meiosis . Examples of vegetative reproduction include the formation of miniaturized plants called plantlets on specialized leaves, for example in kalanchoe ( Bryophyllum daigremontianum ) and many produce new plants from rhizomes or stolon (for example in strawberry ). Some plants reproduce by forming bulbs or tubers , for example tulip bulbs and Dahlia tubers. In these examples, all
4251-471: Is a widespread form of asexual reproduction in animals, whereby the fertilized egg or a later stage of embryonic development splits to form genetically identical clones. Within animals, this phenomenon has been best studied in the parasitic Hymenoptera . In the nine-banded armadillos , this process is obligatory and usually gives rise to genetically identical quadruplets. In other mammals, monozygotic twinning has no apparent genetic basis, though its occurrence
4360-675: Is aggregated. Coinfection by multiple parasites is common. Autoinfection , where (by exception) the whole of the parasite's life cycle takes place in a single primary host, can sometimes occur in helminths such as Strongyloides stercoralis . Vector-transmitted parasites rely on a third party, an intermediate host, where the parasite does not reproduce sexually, to carry them from one definitive host to another. These parasites are microorganisms, namely protozoa , bacteria , or viruses , often intracellular pathogens (disease-causers). Their vectors are mostly hematophagic arthropods such as fleas, lice, ticks, and mosquitoes. For example,
4469-443: Is by definition not a symbiosis, as the interaction is brief, but the entomologist E. O. Wilson has characterised parasites as "predators that eat prey in units of less than one". Within that scope are many possible strategies. Taxonomists classify parasites in a variety of overlapping schemes, based on their interactions with their hosts and on their life cycles , which can be complex. An obligate parasite depends completely on
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4578-402: Is common. There are at least 10 million identical human twins and triplets in the world today. Bdelloid rotifers reproduce exclusively asexually, and all individuals in the class Bdelloidea are females. Asexuality evolved in these animals millions of years ago and has persisted since. There is evidence to suggest that asexual reproduction has allowed the animals to evolve new proteins through
4687-418: Is derived entirely from pollen . Androgenesis occurs when a zygote is produced with only paternal nuclear genes . During standard sexual reproduction , one female and one male parent each produce haploid gametes (such as a sperm or egg cell, each containing only a single set of chromosomes ), which recombine to create offspring with genetic material from both parents. However, in androgenesis, there
4796-645: Is found in the ant Tetramorium inquilinum , an obligate parasite which lives exclusively on the backs of other Tetramorium ants. A mechanism for the evolution of social parasitism was first proposed by Carlo Emery in 1909. Now known as " Emery's rule ", it states that social parasites tend to be closely related to their hosts, often being in the same genus. Intraspecific social parasitism occurs in parasitic nursing, where some individual young take milk from unrelated females. In wedge-capped capuchins , higher ranking females sometimes take milk from low ranking females without any reciprocation. In brood parasitism ,
4905-414: Is important in ferns and in flowering plants, but is very rare in other seed plants. In flowering plants, the term "apomixis" is now most often used for agamospermy , the formation of seeds without fertilization, but was once used to include vegetative reproduction . An example of an apomictic plant would be the triploid European dandelion . Apomixis mainly occurs in two forms: In gametophytic apomixis,
5014-766: Is no recombination of maternal and paternal chromosomes, and only the paternal chromosomes are passed down to the offspring (the inverse of this is gynogenesis , where only the maternal chromosomes are inherited, which is more common than androgenesis). The offspring produced in androgenesis will still have maternally inherited mitochondria , as is the case with most sexually reproducing species. Androgenesis occurs in nature in many invertebrates (for example, clams, stick insects, some ants, bees, flies and parasitic wasps ) and vertebrates (mainly amphibians and fish ). The androgenesis has also been seen in genetically modified laboratory mice. One of two things can occur to produce offspring with exclusively paternal genetic material:
5123-585: Is often unambiguous, it is part of a spectrum of interactions between species , grading via parasitoidism into predation, through evolution into mutualism , and in some fungi, shading into being saprophytic . Human knowledge of parasites such as roundworms and tapeworms dates back to ancient Egypt , Greece , and Rome . In early modern times, Antonie van Leeuwenhoek observed Giardia lamblia with his microscope in 1681, while Francesco Redi described internal and external parasites including sheep liver fluke and ticks . Modern parasitology developed in
5232-667: Is relatively rare among multicellular organisms , particularly animals . It is not entirely understood why the ability to reproduce sexually is so common among them. Current hypotheses suggest that asexual reproduction may have short term benefits when rapid population growth is important or in stable environments, while sexual reproduction offers a net advantage by allowing more rapid generation of genetic diversity, allowing adaptation to changing environments. Developmental constraints may underlie why few animals have relinquished sexual reproduction completely in their life-cycles. Almost all asexual modes of reproduction maintain meiosis either in
5341-509: Is reminiscent of the asexual reproduction of sporocysts or rediae in larval trematode infections of snails . As with the trematode asexual stages, a few nematogens can usually be found in older hosts. Their function may be to increase the population of the parasite to keep up with the growth of the host. Rhombogens contain hermaphroditic gonads developed within the axial cell. These gonads, more correctly termed infusorigens , self-fertilise to produce infusoriform larvae. These larvae possess
5450-419: Is seen in some species of anglerfish , such as Ceratias holboelli , where the males are reduced to tiny sexual parasites , wholly dependent on females of their own species for survival, permanently attached below the female's body, and unable to fend for themselves. The female nourishes the male and protects him from predators, while the male gives nothing back except the sperm that the female needs to produce
5559-458: Is simple: a single axial cell is surrounded by a jacket of twenty to thirty ciliated cells. The anterior region of the organism is termed a calotte and functions to attach the parasite to folds on the surface of its host 's renal appendages . When more than one species of dicyemida exist within the same host, they have distinctly shaped calottes , which range in shape from conical to disk shaped, or cap shaped. To this day, there has never been
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#17327719954055668-441: Is split into fragments. Each of these fragments develop into mature, fully grown individuals that are clones of the original organism. In echinoderms , this method of reproduction is usually known as fissiparity . Due to many environmental and epigenetic differences, clones originating from the same ancestor might actually be genetically and epigenetically different. Agamogenesis is any form of reproduction that does not involve
5777-525: Is the Komodo dragon at 10 feet long and over 300 pounds. Heterogony is a form of facultative parthenogenesis where females alternate between sexual and asexual reproduction at regular intervals (see Alternation between sexual and asexual reproduction ). Aphids are one group of organism that engages in this type of reproduction. They use asexual reproduction to reproduce quickly and create winged offspring that can colonize new plants and reproduce sexually in
5886-527: Is the primary form of reproduction for single-celled organisms such as archaea and bacteria . Many eukaryotic organisms including plants , animals , and fungi can also reproduce asexually. In vertebrates , the most common form of asexual reproduction is parthenogenesis , which is typically used as an alternative to sexual reproduction in times when reproductive opportunities are limited. Some monitor lizards , including Komodo dragons , can reproduce asexually. While all prokaryotes reproduce without
5995-469: Is then carried to a nest, sometimes alongside other prey if it is not large enough to support a parasitoid throughout its development. An egg is laid on top of the prey and the nest is then sealed. The parasitoid develops rapidly through its larval and pupal stages, feeding on the provisions left for it. Koinobiont parasitoids, which include flies as well as wasps, lay their eggs inside young hosts, usually larvae. These are allowed to go on growing, so
6104-666: The Meselson effect that have allowed them to survive better in periods of dehydration. Bdelloid rotifers are extraordinarily resistant to damage from ionizing radiation due to the same DNA-preserving adaptations used to survive dormancy. These adaptations include an extremely efficient mechanism for repairing DNA double-strand breaks. This repair mechanism was studied in two Bdelloidea species, Adineta vaga , and Philodina roseola . and appears to involve mitotic recombination between homologous DNA regions within each species. Molecular evidence strongly suggests that several species of
6213-491: The hammerhead shark and the blacktip shark . In both cases, the sharks had reached sexual maturity in captivity in the absence of males, and in both cases the offspring were shown to be genetically identical to the mothers. The New Mexico whiptail is another example. Some reptiles use the ZW sex-determination system , which produces either males (with ZZ sex chromosomes) or females (with ZW or WW sex chromosomes). Until 2010, it
6322-431: The rotifer Brachionus calyciflorus asexual reproduction (obligate parthenogenesis ) can be inherited by a recessive allele, which leads to loss of sexual reproduction in homozygous offspring. Inheritance of asexual reproduction by a single recessive locus has also been found in the parasitoid wasp Lysiphlebus fabarum . Asexual reproduction is found in nearly half of the animal phyla. Parthenogenesis occurs in
6431-555: The roundworms . Additional molecular evidence suggests that this phylum is derived from the Lophotrochozoa . The phylum (or class if retained within Mesozoa) contains three families, Conocyemidae , Dicyemidae and Kantharellidae , which have sometimes been further grouped into orders . Authors who treat Dicyemida as an order and separate the family Conocyemidae into a different order ( Heterocyemida ) prefer 'Rhombozoa' as
6540-505: The snubnosed eel is probably a facultative endoparasite (i.e., it is semiparasitic) that opportunistically burrows into and eats sick and dying fish. Plant-eating insects such as scale insects , aphids , and caterpillars closely resemble ectoparasites, attacking much larger plants; they serve as vectors of bacteria, fungi and viruses which cause plant diseases . As female scale insects cannot move, they are obligate parasites, permanently attached to their hosts. The sensory inputs that
6649-457: The stick insect genus Timema have used only asexual (parthenogenetic) reproduction for millions of years, the longest period known for any insect. Similar findings suggest that the mite species Oppiella nova may have reproduced entirely asexually for millions of years. In the grass thrips genus Aptinothrips there have been several transitions to asexuality, likely due to different causes. A complete lack of sexual reproduction
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#17327719954056758-511: The 19th century. In human culture, parasitism has negative connotations. These were exploited to satirical effect in Jonathan Swift 's 1733 poem "On Poetry: A Rhapsody", comparing poets to hyperparasitical "vermin". In fiction, Bram Stoker 's 1897 Gothic horror novel Dracula and its many later adaptations featured a blood-drinking parasite. Ridley Scott 's 1979 film Alien was one of many works of science fiction to feature
6867-635: The agents of malaria , sleeping sickness , and amoebic dysentery ; animals such as hookworms , lice , mosquitoes , and vampire bats ; fungi such as honey fungus and the agents of ringworm ; and plants such as mistletoe , dodder , and the broomrapes . There are six major parasitic strategies of exploitation of animal hosts, namely parasitic castration , directly transmitted parasitism (by contact), trophically-transmitted parasitism (by being eaten), vector-transmitted parasitism, parasitoidism , and micropredation. One major axis of classification concerns invasiveness: an endoparasite lives inside
6976-711: The air or soil given off by host shoots or roots , respectively. About 4,500 species of parasitic plant in approximately 20 families of flowering plants are known. Species within the Orobanchaceae (broomrapes) are among the most economically destructive of all plants. Species of Striga (witchweeds) are estimated to cost billions of dollars a year in crop yield loss, infesting over 50 million hectares of cultivated land within Sub-Saharan Africa alone. Striga infects both grasses and grains, including corn , rice , and sorghum , which are among
7085-864: The deer tick Ixodes scapularis acts as a vector for diseases including Lyme disease , babesiosis , and anaplasmosis . Protozoan endoparasites, such as the malarial parasites in the genus Plasmodium and sleeping-sickness parasites in the genus Trypanosoma , have infective stages in the host's blood which are transported to new hosts by biting insects. Parasitoids are insects which sooner or later kill their hosts, placing their relationship close to predation. Most parasitoids are parasitoid wasps or other hymenopterans ; others include dipterans such as phorid flies . They can be divided into two groups, idiobionts and koinobionts, differing in their treatment of their hosts. Idiobiont parasitoids sting their often-large prey on capture, either killing them outright or paralysing them immediately. The immobilised prey
7194-521: The dicyemid life cycle may be tied to temperate benthic environments, where they occur in greatest abundance. While dicyemids have occasionally been found in the tropics, the infection rates are typically quite low, and many potential host species are not infected. Dicyemids have never been reported from truly oceanic cephalopods, who instead host a parasitic ciliate fauna. Most dicyemid species are recovered from only one or two host species. While not strictly host specific, most dicyemids are only found in
7303-487: The embryo arises from an unfertilized egg within a diploid embryo sac that was formed without completing meiosis. In nucellar embryony , the embryo is formed from the diploid nucellus tissue surrounding the embryo sac. Nucellar embryony occurs in some citrus seeds. Male apomixis can occur in rare cases, such as in the Saharan Cypress Cupressus dupreziana , where the genetic material of the embryo
7412-409: The energy that would have gone into reproduction into host and parasite growth, sometimes causing gigantism in the host. The host's other systems remain intact, allowing it to survive and to sustain the parasite. Parasitic crustaceans such as those in the specialised barnacle genus Sacculina specifically cause damage to the gonads of their many species of host crabs . In the case of Sacculina ,
7521-491: The evolutionary options can be gained by considering four key questions: the effect on the fitness of a parasite's hosts; the number of hosts they have per life stage; whether the host is prevented from reproducing; and whether the effect depends on intensity (number of parasites per host). From this analysis, the major evolutionary strategies of parasitism emerge, alongside predation. Parasitic castrators partly or completely destroy their host's ability to reproduce, diverting
7630-438: The fall to lay eggs for the next season. However, some aphid species are obligate parthenotes. In obligate parthenogenesis, females only reproduce asexually. One example of this is the desert grassland whiptail lizard , a hybrid of two other species. Typically hybrids are infertile but through parthenogenesis this species has been able to develop stable populations. Gynogenesis is a form of obligate parthenogenesis where
7739-444: The family Cuculidae , over 40% of cuckoo species are obligate brood parasites, while others are either facultative brood parasites or provide parental care. The eggs of some brood parasites mimic those of their hosts, while some cowbird eggs have tough shells, making them hard for the hosts to kill by piercing, both mechanisms implying selection by the hosts against parasitic eggs. The adult female European cuckoo further mimics
7848-495: The formation and fusion of gametes, mechanisms for lateral gene transfer such as conjugation , transformation and transduction can be likened to sexual reproduction in the sense of genetic recombination in meiosis . Prokaryotes ( Archaea and Bacteria ) reproduce asexually through binary fission , in which the parent organism divides in two to produce two genetically identical daughter organisms. Eukaryotes (such as protists and unicellular fungi ) may reproduce in
7957-424: The host and parasitoid develop together for an extended period, ending when the parasitoids emerge as adults, leaving the prey dead, eaten from inside. Some koinobionts regulate their host's development, for example preventing it from pupating or making it moult whenever the parasitoid is ready to moult. They may do this by producing hormones that mimic the host's moulting hormones ( ecdysteroids ), or by regulating
8066-442: The host to complete its life cycle, while a facultative parasite does not. Parasite life cycles involving only one host are called "direct"; those with a definitive host (where the parasite reproduces sexually) and at least one intermediate host are called "indirect". An endoparasite lives inside the host's body; an ectoparasite lives outside, on the host's surface. Mesoparasites—like some copepods , for example—enter an opening in
8175-422: The host's body and remain partly embedded there. Some parasites can be generalists, feeding on a wide range of hosts, but many parasites, and the majority of protozoans and helminths that parasitise animals, are specialists and extremely host-specific. An early basic, functional division of parasites distinguished microparasites and macroparasites. These each had a mathematical model assigned in order to analyse
8284-453: The host's body; an ectoparasite lives outside, on the host's surface. Like predation, parasitism is a type of consumer–resource interaction , but unlike predators , parasites, with the exception of parasitoids, are much smaller than their hosts, do not kill them, and often live in or on their hosts for an extended period. Parasites of animals are highly specialised , each parasite species living on one given animal species, and reproduce at
8393-656: The host's endocrine system. A micropredator attacks more than one host, reducing each host's fitness by at least a small amount, and is only in contact with any one host intermittently. This behavior makes micropredators suitable as vectors, as they can pass smaller parasites from one host to another. Most micropredators are hematophagic , feeding on blood. They include annelids such as leeches , crustaceans such as branchiurans and gnathiid isopods, various dipterans such as mosquitoes and tsetse flies , other arthropods such as fleas and ticks, vertebrates such as lampreys , and mammals such as vampire bats . Parasites use
8502-425: The host. A parasitic plant is classified depending on where it latches onto the host, either the stem or the root, and the amount of nutrients it requires. Since holoparasites have no chlorophyll and therefore cannot make food for themselves by photosynthesis , they are always obligate parasites, deriving all their food from their hosts. Some parasitic plants can locate their host plants by detecting chemicals in
8611-477: The host. The parasitism is often on close relatives, whether within the same species or between species in the same genus or family. For instance, the many lineages of cuckoo bees lay their eggs in the nest cells of other bees in the same family. Kleptoparasitism is uncommon generally but conspicuous in birds; some such as skuas are specialised in pirating food from other seabirds, relentlessly chasing them down until they disgorge their catch. A unique approach
8720-423: The hosts suffer increased parental investment and energy expenditure to feed parasitic young, which are commonly larger than host young. The growth rate of host nestlings is slowed, reducing the host's fitness. Brood parasites include birds in different families such as cowbirds , whydahs , cuckoos , and black-headed ducks . These do not build nests of their own, but leave their eggs in nests of other species . In
8829-527: The individuals are clones, and the clonal population may cover a large area. Many multicellular organisms produce spores during their biological life cycle in a process called sporogenesis . Exceptions are animals and some protists, which undergo meiosis immediately followed by fertilization. Plants and many algae on the other hand undergo sporic meiosis where meiosis leads to the formation of haploid spores rather than gametes. These spores grow into multicellular individuals called gametophytes , without
8938-472: The intermediate host. When the intermediate-host animal is eaten by a predator, the definitive host, the parasite survives the digestion process and matures into an adult; some live as intestinal parasites . Many trophically transmitted parasites modify the behaviour of their intermediate hosts, increasing their chances of being eaten by a predator. As with directly transmitted parasites, the distribution of trophically transmitted parasites among host individuals
9047-471: The intestinal infection microsporidiosis . Protozoa such as Plasmodium , Trypanosoma , and Entamoeba are endoparasitic. They cause serious diseases in vertebrates including humans—in these examples, malaria, sleeping sickness, and amoebic dysentery —and have complex life cycles. Many bacteria are parasitic, though they are more generally thought of as pathogens causing disease. Parasitic bacteria are extremely diverse, and infect their hosts by
9156-441: The large blue butterfly, Phengaris arion , its larvae employing ant mimicry to parasitise certain ants, Bombus bohemicus , a bumblebee which invades the hives of other bees and takes over reproduction while their young are raised by host workers, and Melipona scutellaris , a eusocial bee whose virgin queens escape killer workers and invade another colony without a queen. An extreme example of interspecific social parasitism
9265-579: The largest group is the parasitoid wasps in the Hymenoptera. The phyla and classes with the largest numbers of parasitic species are listed in the table. Numbers are conservative minimum estimates. The columns for Endo- and Ecto-parasitism refer to the definitive host, as documented in the Vertebrate and Invertebrate columns. A hemiparasite or partial parasite such as mistletoe derives some of its nutrients from another living plant, whereas
9374-482: The latter in mature hosts. The asexual stage is termed a nematogen ; it produces vermiform larvae within the axial cell. These mature through direct development to form more nematogens. Nematogens proliferate in young cephalopods, filling the kidneys. As the infection ages, perhaps as the nematogens reach a certain density, vermiform larvae mature to form rhombogens , the sexual life stage, rather than more nematogens. This sort of density-responsive reproductive cycle
9483-512: The little fire ant Wasmannia auropunctata , Vollenhovia emeryi , Paratrechina longicornis , occasionally in Apis mellifera , the Hypseleotris carp gudgeons, the parasitoid Venturia canescens , and occasionally in fruit flies Drosophila melanogaster carrying a specific mutant allele. It has also been induced in many crops and fish via irradiation of an egg cell to destroy
9592-404: The little fire ant Wasmannia auropunctata , Vollenhovia emeryi , Paratrechina longicornis , occasionally in Apis mellifera , the Hypseleotris carp gudgeons, the parasitoid Venturia canescens , and occasionally in fruit flies Drosophila melanogaster carrying a specific mutant allele. It has also been induced in many crops and fish via irradiation of an egg cell to destroy
9701-537: The marine worm Bonellia viridis has a similar reproductive strategy, although the larvae are planktonic. Examples of the major variant strategies are illustrated. Parasitism has an extremely wide taxonomic range, including animals, plants, fungi, protozoans, bacteria, and viruses. Parasitism is widespread in the animal kingdom, and has evolved independently from free-living forms hundreds of times. Many types of helminth including flukes and cestodes have complete life cycles involving two or more hosts. By far
9810-412: The maternal nuclear genome can be eliminated from the zygote, or the female can produce an egg with no nucleus , resulting in an embryo developing with only the genome of the male gamete. Other type of androgenesis is the male apomixis or paternal apomixis is a reproductive process in which a plant develops from a sperm cell (male gamete) without the participation of a female cell (ovum). In this process,
9919-415: The maternal nuclear genome. Obligate androgenesis is the process in which males are capable of producing both eggs and sperm, however, the eggs have no genetic contribution and the offspring come only from the sperm, which allows these individuals to self-fertilize and produce clonal offspring without the need for females. They are also capable of interbreeding with sexual and other androgenetic lineages in
10028-597: The maternal nuclear genome. Some species can alternate between sexual and asexual strategies, an ability known as heterogamy , depending on many conditions. Alternation is observed in several rotifer species (cyclical parthenogenesis e.g. in Brachionus species) and a few types of insects. One example of this is aphids which can engage in heterogony. In this system, females are born pregnant and produce only female offspring. This cycle allows them to reproduce very quickly. However, most species reproduce sexually once
10137-428: The more common apomixis, where development occurs without fertilization, but with genetic material only from the mother. There are also clonal species that reproduce through vegetative reproduction like Lomatia tasmanica and Pando , where the genetic material is exclusively male. Other species where androgenesis has been observed naturally are the stick insects Bacillus rossius and Bassillus Grandii ,
10246-452: The multiple Dicyemida species for habitat or other resources. Calotte shape determines where a dicyemid can comfortably live. In general, dicyemida with conical shaped calottes fit best within the folds of the kidneys, while those with rounded calottes (disk or cap shaped) are more easily able to attach to the smooth surfaces of the kidneys. This extreme segregation of habitats allows multiple species of dicyemids to comfortably exist within
10355-407: The next generation. Adelphoparasitism, (from Greek ἀδελφός ( adelphós ), brother ), also known as sibling-parasitism, occurs where the host species is closely related to the parasite, often in the same family or genus. In the citrus blackfly parasitoid, Encarsia perplexa , unmated females may lay haploid eggs in the fully developed larvae of their own species, producing male offspring, while
10464-619: The only difference between the two species that scientists were able to observe was between the shape of their calottes .The idea that D. misakiense and D. japonicum are two different species is still very controversial among scientific groups. Some scientists have speculated that when closely related species of dicyemids coexist in the same region, such as in the case of D. misakiense and D. japonicum , competition for habitat causes them to evolve to develop two distinct calotte shapes. Dicyemids exist in both asexual and sexual forms. The former predominate in juvenile and immature hosts, and
10573-557: The parent organism. Internal budding is a process of asexual reproduction, favoured by parasites such as Toxoplasma gondii . It involves an unusual process in which two ( endodyogeny ) or more ( endopolygeny ) daughter cells are produced inside a mother cell, which is then consumed by the offspring prior to their separation. Also, budding (external or internal) occurs in some worms like Taenia or Echinococcus ; these worms produce cysts and then produce (invaginated or evaginated) protoscolex with budding . Vegetative propagation
10682-417: The point where, while they are evidently able to infect all other organisms from bacteria and archaea to animals, plants and fungi, it is unclear whether they can themselves be described as living. They can be either RNA or DNA viruses consisting of a single or double strand of genetic material ( RNA or DNA , respectively), covered in a protein coat and sometimes a lipid envelope. They thus lack all
10791-535: The population movements of the host–parasite groupings. The microorganisms and viruses that can reproduce and complete their life cycle within the host are known as microparasites. Macroparasites are the multicellular organisms that reproduce and complete their life cycle outside of the host or on the host's body. Much of the thinking on types of parasitism has focused on terrestrial animal parasites of animals, such as helminths. Those in other environments and with other hosts often have analogous strategies. For example,
10900-446: The same cell membrane, sporogony results in sporozoites , and gametogony results in micro gametes . Some cells divide by budding (for example baker's yeast ), resulting in a "mother" and a "daughter" cell that is initially smaller than the parent. Budding is also known on a multicellular level; an animal example is the hydra , which reproduces by budding. The buds grow into fully matured individuals which eventually break away from
11009-506: The same host while not still competing for space or resources (by occupying different ecological niches ). While most dicyemid species have been found to prefer to live within specific cephalopods, no one species is unique in their preferences. In fact, It is also almost unheard of that a host infected with a dicyemid is only infected with one species. This means that if a select cephalopod is found to be infected with one species Dicyemid, their body will likely be found to contain organisms with
11118-740: The social pathway, they form a multi-cellular slug which then forms a fruiting body with asexually generated spores. In the sexual pathway, two cells fuse to form a giant cell that develops into a large cyst. When this macrocyst germinates, it releases hundreds of amoebic cells that are the product of meiotic recombination between the original two cells. The hyphae of the common mold ( Rhizopus ) are capable of producing both mitotic as well as meiotic spores. Many algae similarly switch between sexual and asexual reproduction. A number of plants use both sexual and asexual means to produce new plants, some species alter their primary modes of reproduction from sexual to asexual under varying environmental conditions. In
11227-466: The testes of over two-thirds of their crab hosts degenerate sufficiently for these male crabs to develop female secondary sex characteristics such as broader abdomens, smaller claws and egg-grasping appendages. Various species of helminth castrate their hosts (such as insects and snails). This may happen directly, whether mechanically by feeding on their gonads, or by secreting a chemical that destroys reproductive cells; or indirectly, whether by secreting
11336-403: The tropics, however effectively cheat by taking carbon from a fungus rather than exchanging it for minerals. They have much reduced roots, as they do not need to absorb water from the soil; their stems are slender with few vascular bundles , and their leaves are reduced to small scales, as they do not photosynthesize. Their seeds are small and numerous, so they appear to rely on being infected by
11445-506: The usual machinery of the cell such as enzymes , relying entirely on the host cell's ability to replicate DNA and synthesise proteins. Most viruses are bacteriophages , infecting bacteria. Parasitism is a major aspect of evolutionary ecology; for example, almost all free-living animals are host to at least one species of parasite. Vertebrates, the best-studied group, are hosts to between 75,000 and 300,000 species of helminths and an uncounted number of parasitic microorganisms. On average,
11554-589: The way that the CHV1 virus helps to control the damage that chestnut blight , Cryphonectria parasitica , does to American chestnut trees, and in the way that bacteriophages can limit bacterial infections. It is likely, though little researched, that most pathogenic microparasites have hyperparasites which may prove widely useful in both agriculture and medicine. Social parasites take advantage of interspecific interactions between members of eusocial animals such as ants , termites , and bumblebees . Examples include
11663-471: The world's most important food crops. Orobanche also threatens a wide range of other important crops, including peas , chickpeas , tomatoes , carrots , and varieties of cabbage . Yield loss from Orobanche can be total; despite extensive research, no method of control has been entirely successful. Many plants and fungi exchange carbon and nutrients in mutualistic mycorrhizal relationships. Some 400 species of myco-heterotrophic plants, mostly in
11772-494: The zygote is formed solely with genetic material from the father, resulting in offspring genetically identical to the male organism. This has been noted in many plants like Nicotiana , Capsicum frutescens , Cicer arietinum , Poa arachnifera , Solanum verrucosum , Phaeophyceae , Pripsacum dactyloides , Zea mays , and occurs as the regular reproductive method in Cupressus dupreziana . This contrasts with
11881-478: Was thought that the ZW chromosome system used by reptiles was incapable of producing viable WW offspring, but a (ZW) female boa constrictor was discovered to have produced viable female offspring with WW chromosomes. The female boa could have chosen any number of male partners (and had successfully in the past) but on this occasion she reproduced asexually, creating 22 female babies with WW sex-chromosomes. Polyembryony
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