122-424: Ranavirus is a genus of viruses in the family Iridoviridae . There are six other genera of viruses within the family Iridoviridae , but Ranavirus is the only one that includes viruses that are infectious to amphibians and reptiles. Additionally, it is one of the three genera within this family which infect teleost fishes , along with Lymphocystivirus and Megalocytivirus . The Ranaviruses, like
244-455: A mutation (the deletion of two nucleotides ) that inactives it. These changes are explained by the fact that its prey does not need to be subdued. Several groups of predatory fish have the ability to detect, track, and sometimes, as in the electric ray , to incapacitate their prey by sensing and generating electric fields . The electric organ is derived from modified nerve or muscle tissue. Physiological adaptations to predation include
366-425: A virion , consists of nucleic acid surrounded by a protective coat of protein called a capsid . These are formed from protein subunits called capsomeres . Viruses can have a lipid "envelope" derived from the host cell membrane . The capsid is made from proteins encoded by the viral genome and its shape serves as the basis for morphological distinction. Virally-coded protein subunits will self-assemble to form
488-606: A basic optical microscope. In 2013, the Pandoravirus genus was discovered in Chile and Australia, and has genomes about twice as large as Megavirus and Mimivirus. All giant viruses have dsDNA genomes and they are classified into several families: Mimiviridae , Pithoviridae, Pandoraviridae , Phycodnaviridae , and the Mollivirus genus. Some viruses that infect Archaea have complex structures unrelated to any other form of virus, with
610-418: A better choice. If it chooses pursuit, its physical capabilities determine the mode of pursuit (e.g., ambush or chase). Having captured the prey, it may also need to expend energy handling it (e.g., killing it, removing any shell or spines, and ingesting it). Predators have a choice of search modes ranging from sit-and-wait to active or widely foraging . The sit-and-wait method is most suitable if
732-851: A broad range of taxa including arthropods. They are common among insects, including mantids, dragonflies , lacewings and scorpionflies . In some species such as the alderfly , only the larvae are predatory (the adults do not eat). Spiders are predatory, as well as other terrestrial invertebrates such as scorpions ; centipedes ; some mites , snails and slugs ; nematodes ; and planarian worms . In marine environments, most cnidarians (e.g., jellyfish , hydroids ), ctenophora (comb jellies), echinoderms (e.g., sea stars , sea urchins , sand dollars , and sea cucumbers ) and flatworms are predatory. Among crustaceans , lobsters , crabs , shrimps and barnacles are predators, and in turn crustaceans are preyed on by nearly all cephalopods (including octopuses , squid and cuttlefish ). Seed predation
854-418: A capsid diameter of 400 nm. Protein filaments measuring 100 nm project from the surface. The capsid appears hexagonal under an electron microscope, therefore the capsid is probably icosahedral. In 2011, researchers discovered the largest then known virus in samples of water collected from the ocean floor off the coast of Las Cruces, Chile. Provisionally named Megavirus chilensis , it can be seen with
976-581: A capsid, in general requiring the presence of the virus genome. Complex viruses code for proteins that assist in the construction of their capsid. Proteins associated with nucleic acid are known as nucleoproteins , and the association of viral capsid proteins with viral nucleic acid is called a nucleocapsid. The capsid and entire virus structure can be mechanically (physically) probed through atomic force microscopy . In general, there are five main morphological virus types: The poxviruses are large, complex viruses that have an unusual morphology. The viral genome
1098-596: A cell, viruses exist in the form of independent viral particles, or virions , consisting of (i) genetic material , i.e., long molecules of DNA or RNA that encode the structure of the proteins by which the virus acts; (ii) a protein coat, the capsid , which surrounds and protects the genetic material; and in some cases (iii) an outside envelope of lipids . The shapes of these virus particles range from simple helical and icosahedral forms to more complex structures. Most virus species have virions too small to be seen with an optical microscope and are one-hundredth
1220-639: A cellular structure, which is often seen as the basic unit of life. Viruses do not have their own metabolism and require a host cell to make new products. They therefore cannot naturally reproduce outside a host cell —although some bacteria such as rickettsia and chlamydia are considered living organisms despite the same limitation. Accepted forms of life use cell division to reproduce, whereas viruses spontaneously assemble within cells. They differ from autonomous growth of crystals as they inherit genetic mutations while being subject to natural selection. Virus self-assembly within host cells has implications for
1342-492: A contributing factor in the global decline of amphibian populations. The impact of Ranaviruses on amphibian populations has been compared to the chytrid fungus Batrachochytrium dendrobatidis , the causative agent of chytridiomycosis . In the UK, the severity of disease outbreaks is thought to have increased due to climate change. Rana is derived from the Latin for "frog", reflecting
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#17328528418261464-493: A different DNA (or RNA) molecule. This can occur when viruses infect cells simultaneously and studies of viral evolution have shown that recombination has been rampant in the species studied. Recombination is common to both RNA and DNA viruses. Coronaviruses have a single-strand positive-sense RNA genome. Replication of the genome is catalyzed by an RNA-dependent RNA polymerase . The mechanism of recombination used by coronaviruses likely involves template switching by
1586-648: A few hours post infection. There are several hypotheses for seasonal outbreak patterns observed for Ranavirosis mortality events. Ranaviruses grow in vitro between 8-30 °C, however for most isolates, warmer temperature result in faster viral replication. A combination of this optimal growth temperature along with shifts in larval amphibian susceptibility result in seasonal outbreak events most often observed during warm summer months. Amphibian mortality events are often observed as larval amphibians reach late Gosner stages approaching metamorphosis . As larval amphibians reach metamorphic stages of development, their immune system
1708-559: A few species, or broad for viruses capable of infecting many. Viral infections in animals provoke an immune response that usually eliminates the infecting virus. Immune responses can also be produced by vaccines , which confer an artificially acquired immunity to the specific viral infection. Some viruses, including those that cause HIV/AIDS , HPV infection , and viral hepatitis , evade these immune responses and result in chronic infections. Several classes of antiviral drugs have been developed. The English word "virus" comes from
1830-549: A fluid, by Wendell Meredith Stanley , and the invention of the electron microscope in 1931 allowed their complex structures to be visualised. Scientific opinions differ on whether viruses are a form of life or organic structures that interact with living organisms. They have been described as "organisms at the edge of life", since they resemble organisms in that they possess genes , evolve by natural selection , and reproduce by creating multiple copies of themselves through self-assembly. Although they have genes, they do not have
1952-582: A food trap, mechanical stimulation, and electrical impulses to eventually catch and consume its prey. Some carnivorous fungi catch nematodes using either active traps in the form of constricting rings, or passive traps with adhesive structures. Many species of protozoa ( eukaryotes ) and bacteria ( prokaryotes ) prey on other microorganisms; the feeding mode is evidently ancient, and evolved many times in both groups. Among freshwater and marine zooplankton , whether single-celled or multi-cellular, predatory grazing on phytoplankton and smaller zooplankton
2074-445: A genome size of only two kilobases; the largest—the pandoraviruses —have genome sizes of around two megabases which code for about 2500 proteins. Virus genes rarely have introns and often are arranged in the genome so that they overlap . In general, RNA viruses have smaller genome sizes than DNA viruses because of a higher error-rate when replicating, and have a maximum upper size limit. Beyond this, errors when replicating render
2196-506: A host organism. Amphibian mass mortality events due to Ranavirus have been reported in Asia, Europe, North America, and South America. Ranaviruses have been isolated from wild populations of amphibians in Australia, but have not been associated with mass mortality on that continent. Synthesis of viral proteins begins within hours of viral entry with necrosis or apoptosis occurring as early as
2318-471: A huge gulp of water and filtering it through their feathery baleen plates. Pursuit predators may be social , like the lion and wolf that hunt in groups, or solitary. Once the predator has captured the prey, it has to handle it: very carefully if the prey is dangerous to eat, such as if it possesses sharp or poisonous spines, as in many prey fish. Some catfish such as the Ictaluridae have spines on
2440-725: A kill, and the coyote can be either solitary or social. Other solitary predators include the northern pike, wolf spiders and all the thousands of species of solitary wasps among arthropods, and many microorganisms and zooplankton . Under the pressure of natural selection , predators have evolved a variety of physical adaptations for detecting, catching, killing, and digesting prey. These include speed, agility, stealth, sharp senses, claws, teeth, filters, and suitable digestive systems. For detecting prey , predators have well-developed vision , smell , or hearing . Predators as diverse as owls and jumping spiders have forward-facing eyes, providing accurate binocular vision over
2562-506: A ladder. The virus particles of some virus families, such as those belonging to the Hepadnaviridae , contain a genome that is partially double-stranded and partially single-stranded. For most viruses with RNA genomes and some with single-stranded DNA (ssDNA) genomes, the single strands are said to be either positive-sense (called the 'plus-strand') or negative-sense (called the 'minus-strand'), depending on if they are complementary to
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#17328528418262684-451: A large single linear dsDNA genome of roughly 105 kbp which codes for around 100 gene products. The main structural component of the protein capsid is the major capsid protein (MCP). Ranaviral replication is well studied using Frog virus 3 (FV3). Replication of FV3 occurs between 12 and 32 degrees Celsius. Ranaviruses enter the host cell by receptor-mediated endocytosis . Viral particles are uncoated and subsequently move into
2806-436: A life form, because they carry genetic material, reproduce, and evolve through natural selection , although they lack some key characteristics, such as cell structure, that are generally considered necessary criteria for defining life. Because they possess some but not all such qualities, viruses have been described as "organisms at the edge of life" and as replicators . Viruses spread in many ways. One transmission pathway
2928-472: A limited range of hosts and many are species-specific. Some, such as smallpox virus for example, can infect only one species—in this case humans, and are said to have a narrow host range . Other viruses, such as rabies virus, can infect different species of mammals and are said to have a broad range. The viruses that infect plants are harmless to animals, and most viruses that infect other animals are harmless to humans. The host range of some bacteriophages
3050-539: A long distance, sometimes for hours at a time. The method is used by human hunter-gatherers and by canids such as African wild dogs and domestic hounds. The African wild dog is an extreme persistence predator, tiring out individual prey by following them for many miles at relatively low speed. A specialised form of pursuit predation is the lunge feeding of baleen whales . These very large marine predators feed on plankton , especially krill , diving and actively swimming into concentrations of plankton, and then taking
3172-445: A patch and decide whether to spend time searching for prey in it. This may involve some knowledge of the preferences of the prey; for example, ladybirds can choose a patch of vegetation suitable for their aphid prey. To capture prey, predators have a spectrum of pursuit modes that range from overt chase ( pursuit predation ) to a sudden strike on nearby prey ( ambush predation ). Another strategy in between ambush and pursuit
3294-425: A powerful selective effect on prey, and the prey develop antipredator adaptations such as warning coloration , alarm calls and other signals , camouflage , mimicry of well-defended species, and defensive spines and chemicals. Sometimes predator and prey find themselves in an evolutionary arms race , a cycle of adaptations and counter-adaptations. Predation has been a major driver of evolution since at least
3416-785: A predator, while small prey might prove hard to find and in any case provide less of a reward. This has led to a correlation between the size of predators and their prey. Size may also act as a refuge for large prey. For example, adult elephants are relatively safe from predation by lions, but juveniles are vulnerable. Members of the cat family such as the snow leopard (treeless highlands), tiger (grassy plains, reed swamps), ocelot (forest), fishing cat (waterside thickets), and lion (open plains) are camouflaged with coloration and disruptive patterns suiting their habitats. In aggressive mimicry , certain predators, including insects and fishes, make use of coloration and behaviour to attract prey. Female Photuris fireflies , for example, copy
3538-421: A preferred target is scarce. When prey have a clumped (uneven) distribution, the optimal strategy for the predator is predicted to be more specialized as the prey are more conspicuous and can be found more quickly; this appears to be correct for predators of immobile prey, but is doubtful with mobile prey. In size-selective predation, predators select prey of a certain size. Large prey may prove troublesome for
3660-408: A prime target for natural selection. Segmented genomes confer evolutionary advantages; different strains of a virus with a segmented genome can shuffle and combine genes and produce progeny viruses (or offspring) that have unique characteristics. This is called reassortment or 'viral sex'. Genetic recombination is a process by which a strand of DNA (or RNA) is broken and then joined to the end of
3782-820: A relatively narrow field of view, whereas prey animals often have less acute all-round vision. Animals such as foxes can smell their prey even when it is concealed under 2 feet (60 cm) of snow or earth. Many predators have acute hearing, and some such as echolocating bats hunt exclusively by active or passive use of sound. Predators including big cats , birds of prey , and ants share powerful jaws, sharp teeth, or claws which they use to seize and kill their prey. Some predators such as snakes and fish-eating birds like herons and cormorants swallow their prey whole; some snakes can unhinge their jaws to allow them to swallow large prey, while fish-eating birds have long spear-like beaks that they use to stab and grip fast-moving and slippery prey. Fish and other predators have developed
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3904-478: A significant amount of energy, to locate each food patch. For example, the black-browed albatross regularly makes foraging flights to a range of around 700 kilometres (430 miles), up to a maximum foraging range of 3,000 kilometres (1,860 miles) for breeding birds gathering food for their young. With static prey, some predators can learn suitable patch locations and return to them at intervals to feed. The optimal foraging strategy for search has been modelled using
4026-412: A single viral particle that is released from the cell and is capable of infecting other cells of the same type. Viruses are found wherever there is life and have probably existed since living cells first evolved . The origin of viruses is unclear because they do not form fossils, so molecular techniques are used to infer how they arose. In addition, viral genetic material occasionally integrates into
4148-406: A small animal, gulping the prey in an extremely rapid movement when it is within range. Many smaller predators such as the box jellyfish use venom to subdue their prey, and venom can also aid in digestion (as is the case for rattlesnakes and some spiders ). The marbled sea snake that has adapted to egg predation has atrophied venom glands, and the gene for its three finger toxin contains
4270-456: A small part of the total diversity of viruses has been studied. As of 2022, 6 realms, 10 kingdoms, 17 phyla, 2 subphyla, 40 classes, 72 orders, 8 suborders, 264 families, 182 subfamilies , 2,818 genera, 84 subgenera , and 11,273 species of viruses have been defined by the ICTV. The general taxonomic structure of taxon ranges and the suffixes used in taxonomic names are shown hereafter. As of 2022,
4392-523: A smaller area. For example, when mixed flocks of birds forage, the birds in front flush out insects that are caught by the birds behind. Spinner dolphins form a circle around a school of fish and move inwards, concentrating the fish by a factor of 200. By hunting socially chimpanzees can catch colobus monkeys that would readily escape an individual hunter, while cooperating Harris hawks can trap rabbits. Predators of different species sometimes cooperate to catch prey. In coral reefs , when fish such as
4514-440: A variety of defences including the ability to hear the echolocation calls. Many pursuit predators that run on land, such as wolves, have evolved long limbs in response to the increased speed of their prey. Their adaptations have been characterized as an evolutionary arms race , an example of the coevolution of two species. In a gene centered view of evolution , the genes of predator and prey can be thought of as competing for
4636-605: A wide diversity of sizes and shapes, called ' morphologies '. In general, viruses are much smaller than bacteria and more than a thousand bacteriophage viruses would fit inside an Escherichia coli bacterium's cell. Many viruses that have been studied are spherical and have a diameter between 20 and 300 nanometres . Some filoviruses , which are filaments, have a total length of up to 1400 nm; their diameters are only about 80 nm. Most viruses cannot be seen with an optical microscope , so scanning and transmission electron microscopes are used to visualise them. To increase
4758-553: A wide variety of unusual shapes, ranging from spindle-shaped structures to viruses that resemble hooked rods, teardrops or even bottles. Other archaeal viruses resemble the tailed bacteriophages, and can have multiple tail structures. An enormous variety of genomic structures can be seen among viral species ; as a group, they contain more structural genomic diversity than plants, animals, archaea, or bacteria. There are millions of different types of viruses, although fewer than 7,000 types have been described in detail. As of January 2021,
4880-477: Is ballistic interception , where a predator observes and predicts a prey's motion and then launches its attack accordingly. Ambush or sit-and-wait predators are carnivorous animals that capture prey by stealth or surprise. In animals, ambush predation is characterized by the predator's scanning the environment from a concealed position until a prey is spotted, and then rapidly executing a fixed surprise attack. Vertebrate ambush predators include frogs, fish such as
5002-416: Is a feature of many bacterial and some animal viruses. Some viruses undergo a lysogenic cycle where the viral genome is incorporated by genetic recombination into a specific place in the host's chromosome. The viral genome is then known as a " provirus " or, in the case of bacteriophages a " prophage ". Whenever the host divides, the viral genome is also replicated. The viral genome is mostly silent within
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5124-405: Is a major change in the genome of the virus. This can be a result of recombination or reassortment . The Influenza A virus is highly prone to reassortment; occasionally this has resulted in novel strains which have caused pandemics . RNA viruses often exist as quasispecies or swarms of viruses of the same species but with slightly different genome nucleoside sequences. Such quasispecies are
5246-437: 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 . Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity. Since Dmitri Ivanovsky 's 1892 article describing a non-bacterial pathogen infecting tobacco plants and
5368-400: Is associated with proteins within a central disc structure known as a nucleoid . The nucleoid is surrounded by a membrane and two lateral bodies of unknown function. The virus has an outer envelope with a thick layer of protein studded over its surface. The whole virion is slightly pleomorphic , ranging from ovoid to brick-shaped. Mimivirus is one of the largest characterised viruses, with
5490-439: Is caused by cessation of its normal activities because of suppression by virus-specific proteins, not all of which are components of the virus particle. The distinction between cytopathic and harmless is gradual. Some viruses, such as Epstein–Barr virus , can cause cells to proliferate without causing malignancy, while others, such as papillomaviruses , are established causes of cancer. Some viruses cause no apparent changes to
5612-494: Is common, and found in many species of nanoflagellates , dinoflagellates , ciliates , rotifers , a diverse range of meroplankton animal larvae, and two groups of crustaceans, namely copepods and cladocerans . To feed, a predator must search for, pursue and kill its prey. These actions form a foraging cycle. The predator must decide where to look for prey based on its geographical distribution; and once it has located prey, it must assess whether to pursue it or to wait for
5734-441: Is controversy over whether the bornavirus , previously thought to cause neurological diseases in horses, could be responsible for psychiatric illnesses in humans. 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
5856-512: Is correct. It seems unlikely that all currently known viruses have a common ancestor, and viruses have probably arisen numerous times in the past by one or more mechanisms. The first evidence of the existence of viruses came from experiments with filters that had pores small enough to retain bacteria. In 1892, Dmitri Ivanovsky used one of these filters to show that sap from a diseased tobacco plant remained infectious to healthy tobacco plants despite having been filtered. Martinus Beijerinck called
5978-743: Is evidence that ranavirus infections target macrophages as a mechanism for gaining entry to cells. Andrias davidianus ranavirus, isolated from the Chinese giant salamander, encodes a protein (Rad2 homolog) that has a key role in the repair of DNA by homologous recombination and in DNA double-strand break repair . Transmission of ranaviruses is thought to occur by multiple routes, including contaminated soil, direct contact, waterborne exposure, and ingestion of infected tissues during predation , necrophagy or cannibalism . Ranaviruses are relatively stable in aquatic environments, persisting several weeks or longer outside
6100-498: Is first recorded in 1728, long before the discovery of viruses by Dmitri Ivanovsky in 1892. The English plural is viruses (sometimes also vira ), whereas the Latin word is a mass noun , which has no classically attested plural ( vīra is used in Neo-Latin ). The adjective viral dates to 1948. The term virion (plural virions ), which dates from 1959, is also used to refer to
6222-607: Is identical in sequence to the viral mRNA and is thus a coding strand, while negative-sense viral ssDNA is complementary to the viral mRNA and is thus a template strand. Several types of ssDNA and ssRNA viruses have genomes that are ambisense in that transcription can occur off both strands in a double-stranded replicative intermediate. Examples include geminiviruses , which are ssDNA plant viruses and arenaviruses , which are ssRNA viruses of animals. Genome size varies greatly between species. The smallest—the ssDNA circoviruses, family Circoviridae —code for only two proteins and have
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#17328528418266344-430: Is likely that sympatric fish and reptile species may serve as reservoirs for virus as well. Interclass transmission has been proven through the use of mesocosm studies. Gross lesions associated with Ranavirus infection include erythema, generalized swelling, hemorrhage, limb swelling, and swollen and friable livers. [REDACTED] Data related to List of viruses at Wikispecies Viruses A virus
6466-431: Is limited to a single strain of bacteria and they can be used to trace the source of outbreaks of infections by a method called phage typing . The complete set of viruses in an organism or habitat is called the virome ; for example, all human viruses constitute the human virome . A novel virus is one that has not previously been recorded. It can be a virus that is isolated from its natural reservoir or isolated as
6588-451: Is not understood well, however in realistic environmental conditions the T90 value of an FV3-like virus is 1 day. The duration of persistence is likely affected by temperature and microbial conditions. It is unlikely that ranaviruses persist in the environment outside of host species between outbreak events. Researchers have explored several pathogen reservoirs for the virus which might explain how
6710-488: Is reorganized prior to the development of adult tissues. During this time period, amphibians are stressed, and their immune systems are down regulated. This decrease in immune function and warmer environmental temperatures allows for greater viral replication and cellular damage to occur. Across 64 mortality events in the United States 54% were found to occur between June-August. The environmental persistence of Ranaviruses
6832-514: Is restricted to mammals, birds, and insects but is found in almost all terrestrial ecosystems. Egg predation includes both specialist egg predators such as some colubrid snakes and generalists such as foxes and badgers that opportunistically take eggs when they find them. Some plants, like the pitcher plant , the Venus fly trap and the sundew , are carnivorous and consume insects . Methods of predation by plants varies greatly but often involves
6954-430: Is size. Prey that is too small may not be worth the trouble for the amount of energy it provides. Too large, and it may be too difficult to capture. For example, a mantid captures prey with its forelegs and they are optimized for grabbing prey of a certain size. Mantids are reluctant to attack prey that is far from that size. There is a positive correlation between the size of a predator and its prey. A predator may assess
7076-475: Is through disease-bearing organisms known as vectors : for example, viruses are often transmitted from plant to plant by insects that feed on plant sap , such as aphids ; and viruses in animals can be carried by blood-sucking insects. Many viruses spread in the air by coughing and sneezing, including influenza viruses , SARS-CoV-2 , chickenpox , smallpox , and measles . Norovirus and rotavirus , common causes of viral gastroenteritis , are transmitted by
7198-474: The CD4 molecule—a chemokine receptor —which is most commonly found on the surface of CD4+ T-Cells . This mechanism has evolved to favour those viruses that infect only cells in which they are capable of replication. Attachment to the receptor can induce the viral envelope protein to undergo changes that result in the fusion of viral and cellular membranes, or changes of non-enveloped virus surface proteins that allow
7320-451: The Cambrian period. At the most basic level, predators kill and eat other organisms. However, the concept of predation is broad, defined differently in different contexts, and includes a wide variety of feeding methods; moreover, some relationships that result in the prey's death are not necessarily called predation. A parasitoid , such as an ichneumon wasp , lays its eggs in or on its host;
7442-540: The International Committee on Taxonomy of Viruses (ICTV) was formed. The system proposed by Lwoff, Horne and Tournier was initially not accepted by the ICTV because the small genome size of viruses and their high rate of mutation made it difficult to determine their ancestry beyond order. As such, the Baltimore classification system has come to be used to supplement the more traditional hierarchy. Starting in 2018,
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#17328528418267564-628: The Latin vīrus , which refers to poison and other noxious liquids. Vīrus comes from the same Indo-European root as Sanskrit viṣa , Avestan vīša , and Ancient Greek ἰός ( iós ), which all mean "poison". The first attested use of "virus" in English appeared in 1398 in John Trevisa 's translation of Bartholomeus Anglicus 's De Proprietatibus Rerum . Virulent , from Latin virulentus ('poisonous'), dates to c. 1400 . A meaning of 'agent that causes infectious disease'
7686-484: The NCBI Virus genome database has more than 193,000 complete genome sequences, but there are doubtlessly many more to be discovered. A virus has either a DNA or an RNA genome and is called a DNA virus or an RNA virus , respectively. Most viruses have RNA genomes. Plant viruses tend to have single-stranded RNA genomes and bacteriophages tend to have double-stranded DNA genomes. Viral genomes are circular, as in
7808-465: The angel shark , the northern pike and the eastern frogfish . Among the many invertebrate ambush predators are trapdoor spiders and Australian Crab spiders on land and mantis shrimps in the sea. Ambush predators often construct a burrow in which to hide, improving concealment at the cost of reducing their field of vision. Some ambush predators also use lures to attract prey within striking range. The capturing movement has to be rapid to trap
7930-451: The cell nucleus , where viral DNA replication begins via a virally encoded DNA polymerase . Viral DNA then abandons the cell nucleus and begins the second stage of DNA replication in the cytoplasm, ultimately forming DNA concatemers . The viral DNA is then packaged via a headful mechanism into infectious virions. The ranavirus genome, like other iridoviral genomes is circularly permuted and exhibits terminally redundant DNA . There
8052-539: The common cold , influenza , chickenpox , and cold sores . Many serious diseases such as rabies , Ebola virus disease , AIDS (HIV) , avian influenza , and SARS are caused by viruses. The relative ability of viruses to cause disease is described in terms of virulence . Other diseases are under investigation to discover if they have a virus as the causative agent, such as the possible connection between human herpesvirus 6 (HHV6) and neurological diseases such as multiple sclerosis and chronic fatigue syndrome . There
8174-455: The common garter snake has developed a resistance to the toxin in the skin of the rough-skinned newt . Predators affect their ecosystems not only directly by eating their own prey, but by indirect means such as reducing predation by other species, or altering the foraging behaviour of a herbivore, as with the biodiversity effect of wolves on riverside vegetation or sea otters on kelp forests. This may explain population dynamics effects such as
8296-422: The faecal–oral route , passed by hand-to-mouth contact or in food or water. The infectious dose of norovirus required to produce infection in humans is fewer than 100 particles. HIV is one of several viruses transmitted through sexual contact and by exposure to infected blood. The variety of host cells that a virus can infect is called its host range : this is narrow for viruses specialized to infect only
8418-430: The germline of the host organisms, by which they can be passed on vertically to the offspring of the host for many generations. This provides an invaluable source of information for paleovirologists to trace back ancient viruses that existed as far back as millions of years ago. There are three main hypotheses that aim to explain the origins of viruses: In the past, there were problems with all of these hypotheses:
8540-475: The grouper and coral trout spot prey that is inaccessible to them, they signal to giant moray eels , Napoleon wrasses or octopuses . These predators are able to access small crevices and flush out the prey. Killer whales have been known to help whalers hunt baleen whales . Social hunting allows predators to tackle a wider range of prey, but at the risk of competition for the captured food. Solitary predators have more chance of eating what they catch, at
8662-470: The host ) and parasitoidism (which always does, eventually). It is distinct from scavenging on dead prey, though many predators also scavenge ; it overlaps with herbivory , as seed predators and destructive frugivores are predators. Predators may actively search for or pursue prey or wait for it, often concealed. When prey is detected, the predator assesses whether to attack it. This may involve ambush or pursuit predation , sometimes after stalking
8784-461: The marginal value theorem . Search patterns often appear random. One such is the Lévy walk , that tends to involve clusters of short steps with occasional long steps. It is a good fit to the behaviour of a wide variety of organisms including bacteria, honeybees, sharks and human hunter-gatherers. Having found prey, a predator must decide whether to pursue it or keep searching. The decision depends on
8906-408: The polyomaviruses , or linear, as in the adenoviruses . The type of nucleic acid is irrelevant to the shape of the genome. Among RNA viruses and certain DNA viruses, the genome is often divided into separate parts, in which case it is called segmented. For RNA viruses, each segment often codes for only one protein and they are usually found together in one capsid. All segments are not required to be in
9028-447: The three domains . This discovery has led modern virologists to reconsider and re-evaluate these three classical hypotheses. The evidence for an ancestral world of RNA cells and computer analysis of viral and host DNA sequences give a better understanding of the evolutionary relationships between different viruses and may help identify the ancestors of modern viruses. To date, such analyses have not proved which of these hypotheses
9150-527: The ICTV began to acknowledge deeper evolutionary relationships between viruses that have been discovered over time and adopted a 15-rank classification system ranging from realm to species. Additionally, some species within the same genus are grouped into a genogroup . The ICTV developed the current classification system and wrote guidelines that put a greater weight on certain virus properties to maintain family uniformity. A unified taxonomy (a universal system for classifying viruses) has been established. Only
9272-550: The Megalocytiviruses, are an emerging group of closely related dsDNA viruses which cause systemic infections in a wide variety of wild and cultured fresh and saltwater fishes. As with Megalocytiviruses, Ranavirus outbreaks are therefore of considerable economic importance in aquaculture , as epizootics can result in moderate fish loss or mass mortality events of cultured fishes. Unlike Megalocytiviruses, however, Ranavirus infections in amphibians have been implicated as
9394-709: The ability of predatory bacteria to digest the complex peptidoglycan polymer from the cell walls of the bacteria that they prey upon. Carnivorous vertebrates of all five major classes (fishes, amphibians, reptiles, birds, and mammals) have lower relative rates of sugar to amino acid transport than either herbivores or omnivores, presumably because they acquire plenty of amino acids from the animal proteins in their diet. To counter predation, prey have evolved defences for use at each stage of an attack. They can try to avoid detection, such as by using camouflage and mimicry . They can detect predators and warn others of their presence. If detected, they can try to avoid being
9516-728: The ability to crush or open the armoured shells of molluscs. Many predators are powerfully built and can catch and kill animals larger than themselves; this applies as much to small predators such as ants and shrews as to big and visibly muscular carnivores like the cougar and lion . Predators are often highly specialized in their diet and hunting behaviour; for example, the Eurasian lynx only hunts small ungulates . Others such as leopards are more opportunistic generalists, preying on at least 100 species. The specialists may be highly adapted to capturing their preferred prey, whereas generalists may be better able to switch to other prey when
9638-584: The assault. When animals eat seeds ( seed predation or granivory ) or eggs ( egg predation ), they are consuming entire living organisms, which by definition makes them predators. Scavengers , organisms that only eat organisms found already dead, are not predators, but many predators such as the jackal and the hyena scavenge when the opportunity arises. Among invertebrates, social wasps such as yellowjackets are both hunters and scavengers of other insects. While examples of predators among mammals and birds are well known, predators can be found in
9760-684: The back (dorsal) and belly (pectoral) which lock in the erect position; as the catfish thrashes about when captured, these could pierce the predator's mouth, possibly fatally. Some fish-eating birds like the osprey avoid the danger of spines by tearing up their prey before eating it. In social predation, a group of predators cooperates to kill prey. This makes it possible to kill creatures larger than those they could overpower singly; for example, hyenas , and wolves collaborate to catch and kill herbivores as large as buffalo, and lions even hunt elephants. It can also make prey more readily available through strategies like flushing of prey and herding it into
9882-484: The basis of similarities. In 1962, André Lwoff , Robert Horne , and Paul Tournier were the first to develop a means of virus classification, based on the Linnaean hierarchical system. This system based classification on phylum , class , order , family , genus , and species . Viruses were grouped according to their shared properties (not those of their hosts) and the type of nucleic acid forming their genomes. In 1966,
10004-410: The contrast between viruses and the background, electron-dense "stains" are used. These are solutions of salts of heavy metals, such as tungsten , that scatter the electrons from regions covered with the stain. When virions are coated with stain (positive staining), fine detail is obscured. Negative staining overcomes this problem by staining the background only. A complete virus particle, known as
10126-416: The costs and benefits involved. A bird foraging for insects spends a lot of time searching but capturing and eating them is quick and easy, so the efficient strategy for the bird is to eat every palatable insect it finds. By contrast, a predator such as a lion or falcon finds its prey easily but capturing it requires a lot of effort. In that case, the predator is more selective. One of the factors to consider
10248-418: The discovery of the tobacco mosaic virus by Martinus Beijerinck in 1898, more than 11,000 of the millions of virus species have been described in detail. The study of viruses is known as virology , a subspeciality of microbiology . When infected, a host cell is often forced to rapidly produce thousands of copies of the original virus. When not inside an infected cell or in the process of infecting
10370-993: The eggs hatch into larvae, which eat the host, and it inevitably dies. Zoologists generally call this a form of parasitism , though conventionally parasites are thought not to kill their hosts. A predator can be defined to differ from a parasitoid in that it has many prey, captured over its lifetime, where a parasitoid's larva has just one, or at least has its food supply provisioned for it on just one occasion. There are other difficult and borderline cases. Micropredators are small animals that, like predators, feed entirely on other organisms; they include fleas and mosquitoes that consume blood from living animals, and aphids that consume sap from living plants. However, since they typically do not kill their hosts, they are now often thought of as parasites. Animals that graze on phytoplankton or mats of microbes are predators, as they consume and kill their food organisms, while herbivores that browse leaves are not, as their food plants usually survive
10492-404: The evolution of mimicry. Avoidance is not necessarily an evolutionary response as it is generally learned from bad experiences with prey. However, when the prey is capable of killing the predator (as can a coral snake with its venom), there is no opportunity for learning and avoidance must be inherited. Predators can also respond to dangerous prey with counter-adaptations. In western North America,
10614-431: The extreme of the ssRNA virus case. Viruses undergo genetic change by several mechanisms. These include a process called antigenic drift where individual bases in the DNA or RNA mutate to other bases. Most of these point mutations are "silent"—they do not change the protein that the gene encodes—but others can confer evolutionary advantages such as resistance to antiviral drugs . Antigenic shift occurs when there
10736-446: The filtered, infectious substance a "virus" and this discovery is considered to be the beginning of virology. The subsequent discovery and partial characterization of bacteriophages by Frederick Twort and Félix d'Herelle further catalyzed the field, and by the early 20th century many viruses had been discovered. In 1926, Thomas Milton Rivers defined viruses as obligate parasites. Viruses were demonstrated to be particles, rather than
10858-736: The first isolation of a Ranavirus in 1960s from the Northern leopard frog ( Lithobates pipiens ). The ranaviruses appear to have evolved from a fish virus which subsequently infected amphibians and reptiles. The genus contains the following species: The family Iridoviridae is divided into seven genera which include Chloriridovirus , Iridovirus , Lymphocystivirus , Megalocytivirus , and Ranavirus . The genus Ranavirus contains three viruses known to infect amphibians ( Ambystoma tigrinum virus (ATV), Bohle iridovirus (BIV), and frog virus 3 ). Ranaviruses are large icosahedral DNA viruses measuring approximately 150 nm in diameter with
10980-399: The form of single-stranded nucleoprotein complexes, through pores called plasmodesmata . Bacteria, like plants, have strong cell walls that a virus must breach to infect the cell. Given that bacterial cell walls are much thinner than plant cell walls due to their much smaller size, some viruses have evolved mechanisms that inject their genome into the bacterial cell across the cell wall, while
11102-406: The host. At some point, the provirus or prophage may give rise to the active virus, which may lyse the host cells. Enveloped viruses (e.g., HIV) typically are released from the host cell by budding . During this process, the virus acquires its envelope, which is a modified piece of the host's plasma or other, internal membrane. The genetic material within virus particles, and the method by which
11224-517: The infected cell. Cells in which the virus is latent and inactive show few signs of infection and often function normally. This causes persistent infections and the virus is often dormant for many months or years. This is often the case with herpes viruses . Viruses are by far the most abundant biological entities on Earth and they outnumber all the others put together. They infect all types of cellular life including animals, plants, bacteria and fungi . Different types of viruses can infect only
11346-431: The larvae of coccinellid beetles (ladybirds) , alternate between actively searching and scanning the environment. Prey distributions are often clumped, and predators respond by looking for patches where prey is dense and then searching within patches. Where food is found in patches, such as rare shoals of fish in a nearly empty ocean, the search stage requires the predator to travel for a substantial time, and to expend
11468-429: The light signals of other species, thereby attracting male fireflies, which they capture and eat. Flower mantises are ambush predators; camouflaged as flowers, such as orchids , they attract prey and seize it when it is close enough. Frogfishes are extremely well camouflaged, and actively lure their prey to approach using an esca , a bait on the end of a rod-like appendage on the head, which they wave gently to mimic
11590-416: The material is replicated, varies considerably between different types of viruses. The range of structural and biochemical effects that viruses have on the host cell is extensive. These are called ' cytopathic effects '. Most virus infections eventually result in the death of the host cell. The causes of death include cell lysis, alterations to the cell's surface membrane and apoptosis . Often cell death
11712-461: The original virus. Their life cycle differs greatly between species, but there are six basic stages in their life cycle: Attachment is a specific binding between viral capsid proteins and specific receptors on the host cellular surface. This specificity determines the host range and type of host cell of a virus. For example, HIV infects a limited range of human leucocytes . This is because its surface protein, gp120 , specifically interacts with
11834-434: The other hand, the fitness cost of a given lost dinner is unpredictable, as the predator may quickly find better prey. In addition, most predators are generalists, which reduces the impact of a given prey adaption on a predator. Since specialization is caused by predator-prey coevolution, the rarity of specialists may imply that predator-prey arms races are rare. It is difficult to determine whether given adaptations are truly
11956-416: The polymerase during genome replication. This process appears to be an adaptation for coping with genome damage. Viral populations do not grow through cell division, because they are acellular. Instead, they use the machinery and metabolism of a host cell to produce multiple copies of themselves, and they assemble in the cell. When infected, the host cell is forced to rapidly produce thousands of copies of
12078-538: The prey an opportunity to escape. Some frogs wait until snakes have begun their strike before jumping, reducing the time available to the snake to recalibrate its attack, and maximising the angular adjustment that the snake would need to make to intercept the frog in real time. Ballistic predators include insects such as dragonflies, and vertebrates such as archerfish (attacking with a jet of water), chameleons (attacking with their tongues), and some colubrid snakes . In pursuit predation, predators chase fleeing prey. If
12200-549: The prey are dense and mobile, and the predator has low energy requirements. Wide foraging expends more energy, and is used when prey is sedentary or sparsely distributed. There is a continuum of search modes with intervals between periods of movement ranging from seconds to months. Sharks, sunfish , Insectivorous birds and shrews are almost always moving while web-building spiders, aquatic invertebrates, praying mantises and kestrels rarely move. In between, plovers and other shorebirds , freshwater fish including crappies , and
12322-413: The prey flees in a straight line, capture depends only on the predator's being faster than the prey. If the prey manoeuvres by turning as it flees, the predator must react in real time to calculate and follow a new intercept path, such as by parallel navigation , as it closes on the prey. Many pursuit predators use camouflage to approach the prey as close as possible unobserved ( stalking ) before starting
12444-428: The prey's body. However, the "life-dinner" principle of Dawkins and Krebs predicts that this arms race is asymmetric: if a predator fails to catch its prey, it loses its dinner, while if it succeeds, the prey loses its life. The metaphor of an arms race implies ever-escalating advances in attack and defence. However, these adaptations come with a cost; for instance, longer legs have an increased risk of breaking, while
12566-433: The prey, given that the attack is not modifiable once launched. Ballistic interception is the strategy where a predator observes the movement of a prey, predicts its motion, works out an interception path, and then attacks the prey on that path. This differs from ambush predation in that the predator adjusts its attack according to how the prey is moving. Ballistic interception involves a brief period for planning, giving
12688-497: The prey. If the attack is successful, the predator kills the prey, removes any inedible parts like the shell or spines, and eats it. Predators are adapted and often highly specialized for hunting, with acute senses such as vision , hearing , or smell . Many predatory animals , both vertebrate and invertebrate , have sharp claws or jaws to grip, kill, and cut up their prey. Other adaptations include stealth and aggressive mimicry that improve hunting efficiency. Predation has
12810-504: The price of increased expenditure of energy to catch it, and increased risk that the prey will escape. Ambush predators are often solitary to reduce the risk of becoming prey themselves. Of 245 terrestrial members of the Carnivora (the group that includes the cats, dogs, and bears), 177 are solitary; and 35 of the 37 wild cats are solitary, including the cougar and cheetah. However, the solitary cougar does allow other cougars to share in
12932-414: The pursuit. Pursuit predators include terrestrial mammals such as humans, African wild dogs, spotted hyenas and wolves; marine predators such as dolphins, orcas and many predatory fishes, such as tuna; predatory birds (raptors) such as falcons; and insects such as dragonflies . An extreme form of pursuit is endurance or persistence hunting , in which the predator tires out the prey by following it over
13054-960: The ranks of subrealm, subkingdom, and subclass are unused, whereas all other ranks are in use. The Nobel Prize-winning biologist David Baltimore devised the Baltimore classification system. The ICTV classification system is used in conjunction with the Baltimore classification system in modern virus classification. The Baltimore classification of viruses is based on the mechanism of mRNA production. Viruses must generate mRNAs from their genomes to produce proteins and replicate themselves, but different mechanisms are used to achieve this in each virus family. Viral genomes may be single-stranded (ss) or double-stranded (ds), RNA or DNA, and may or may not use reverse transcriptase (RT). In addition, ssRNA viruses may be either sense (+) or antisense (−). This classification places viruses into seven groups: Examples of common human diseases caused by viruses include
13176-422: The regressive hypothesis did not explain why even the smallest of cellular parasites do not resemble viruses in any way. The escape hypothesis did not explain the complex capsids and other structures on virus particles. The virus-first hypothesis contravened the definition of viruses in that they require host cells. Viruses are now recognised as ancient and as having origins that pre-date the divergence of life into
13298-479: The result of spread to an animal or human host where the virus had not been identified before. It can be an emergent virus , one that represents a new virus, but it can also be an extant virus that has not been previously identified . The SARS-CoV-2 coronavirus that caused the COVID-19 pandemic is an example of a novel virus. Classification seeks to describe the diversity of viruses by naming and grouping them on
13420-470: The result of coevolution, where a prey adaptation gives rise to a predator adaptation that is countered by further adaptation in the prey. An alternative explanation is escalation , where predators are adapting to competitors, their own predators or dangerous prey. Apparent adaptations to predation may also have arisen for other reasons and then been co-opted for attack or defence. In some of the insects preyed on by bats, hearing evolved before bats appeared and
13542-451: The same virion for the virus to be infectious, as demonstrated by brome mosaic virus and several other plant viruses. A viral genome, irrespective of nucleic acid type, is almost always either single-stranded (ss) or double-stranded (ds). Single-stranded genomes consist of an unpaired nucleic acid, analogous to one-half of a ladder split down the middle. Double-stranded genomes consist of two complementary paired nucleic acids, analogous to
13664-462: The size of most bacteria. The origins of viruses in the evolutionary history of life are still unclear. Some viruses may have evolved from plasmids , which are pieces of DNA that can move between cells. Other viruses may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer , which increases genetic diversity in a way analogous to sexual reproduction . Viruses are considered by some biologists to be
13786-417: The specialized tongue of the chameleon, with its ability to act like a projectile, is useless for lapping water, so the chameleon must drink dew off vegetation. The "life-dinner" principle has been criticized on multiple grounds. The extent of the asymmetry in natural selection depends in part on the heritability of the adaptive traits. Also, if a predator loses enough dinners, it too will lose its life. On
13908-399: The structure-mediated self-assembly of the virus particles, some modification of the proteins often occurs. In viruses such as HIV, this modification (sometimes called maturation) occurs after the virus has been released from the host cell. Release – Viruses can be released from the host cell by lysis , a process that kills the cell by bursting its membrane and cell wall if present: this
14030-457: The study of the origin of life , as it lends further credence to the hypothesis that life could have started as self-assembling organic molecules . The virocell model first proposed by Patrick Forterre considers the infected cell to be the "living form" of viruses and that virus particles (virions) are analogous to spores . Although the living versus non-living debate continues, the virocell model has gained some acceptance. Viruses display
14152-684: The target of an attack, for example, by signalling that they are toxic or unpalatable , by signalling that a chase would be unprofitable, or by forming groups. If they become a target, they can try to fend off the attack with defences such as armour, quills , unpalatability, or mobbing; and they can often escape an attack in progress by startling the predator, playing dead , shedding body parts such as tails, or simply fleeing. Predators and prey are natural enemies, and many of their adaptations seem designed to counter each other. For example, bats have sophisticated echolocation systems to detect insects and other prey, and insects have developed
14274-446: The viral messenger RNA (mRNA). Positive-sense viral RNA is in the same sense as viral mRNA and thus at least a part of it can be immediately translated by the host cell. Negative-sense viral RNA is complementary to mRNA and thus must be converted to positive-sense RNA by an RNA-dependent RNA polymerase before translation. DNA nomenclature for viruses with genomic ssDNA is similar to RNA nomenclature, in that positive-strand viral ssDNA
14396-824: The viral capsid remains outside. Uncoating is a process in which the viral capsid is removed: This may be by degradation by viral enzymes or host enzymes or by simple dissociation; the end-result is the releasing of the viral genomic nucleic acid. Replication of viruses involves primarily multiplication of the genome. Replication involves the synthesis of viral messenger RNA (mRNA) from "early" genes (with exceptions for positive-sense RNA viruses), viral protein synthesis , possible assembly of viral proteins, then viral genome replication mediated by early or regulatory protein expression. This may be followed, for complex viruses with larger genomes, by one or more further rounds of mRNA synthesis: "late" gene expression is, in general, of structural or virion proteins. Assembly – Following
14518-403: The virus can persist within an amphibian community. In some amphibian populations, sub-clinically infected individuals may serve as reservoirs for the pathogen. These sub-clinically infected individuals are responsible for reintroduction of the virus to the larval population. With ranaviruses being capable of infected multiple taxa, and with there being differences in susceptibility between taxa, it
14640-503: The virus to enter. Penetration or viral entry follows attachment: Virions enter the host cell through receptor-mediated endocytosis or membrane fusion . The infection of plant and fungal cells is different from that of animal cells. Plants have a rigid cell wall made of cellulose , and fungi one of chitin, so most viruses can get inside these cells only after trauma to the cell wall. Nearly all plant viruses (such as tobacco mosaic virus) can also move directly from cell to cell, in
14762-460: The virus useless or uncompetitive. To compensate, RNA viruses often have segmented genomes—the genome is split into smaller molecules—thus reducing the chance that an error in a single-component genome will incapacitate the entire genome. In contrast, DNA viruses generally have larger genomes because of the high fidelity of their replication enzymes. Single-strand DNA viruses are an exception to this rule, as mutation rates for these genomes can approach
14884-401: Was used to hear signals used for territorial defence and mating. Their hearing evolved in response to bat predation, but the only clear example of reciprocal adaptation in bats is stealth echolocation. A more symmetric arms race may occur when the prey are dangerous, having spines, quills, toxins or venom that can harm the predator. The predator can respond with avoidance, which in turn drives
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