Symbiosis ( Ancient Greek συμβίωσις symbíōsis : living with, companionship < σύν sýn : together; and βίωσις bíōsis : living) is any type of a close and long-term biological interaction , between two organisms of different species . The two organisms, termed symbionts , can be either in a mutualistic , a commensalistic , or a parasitic relationship. In 1879, Heinrich Anton de Bary defined symbiosis as "the living together of unlike organisms".
113-399: The term is sometimes more exclusively used in a restricted, mutualistic sense, where both symbionts contribute to each other's subsistence. Symbiosis can be obligatory , which means that one, or both of the symbionts depend on each other for survival, or facultative (optional), when they can also subsist independently. Symbiosis is also classified by physical attachment. Symbionts forming
226-408: A mouse , the plant Arabidopsis thaliana , the puffer fish , and the bacteria E. coli . In December 2013, scientists first sequenced the entire genome of a Neanderthal , an extinct species of humans . The genome was extracted from the toe bone of a 130,000-year-old Neanderthal found in a Siberian cave . New sequencing technologies, such as massive parallel sequencing have also opened up
339-430: A big potential to modify the genetic control in a host organism. The movement of TEs is a driving force of genome evolution in eukaryotes because their insertion can disrupt gene functions, homologous recombination between TEs can produce duplications, and TE can shuffle exons and regulatory sequences to new locations. Retrotransposons are found mostly in eukaryotes but not found in prokaryotes. Retrotransposons form
452-427: A closely related species, Pediculus schaeffi . Other species of lice infest most orders of mammals and all orders of birds. Lice differ from other hematophagic ectoparasites such as fleas in spending their entire lifecycle on a host. Head lice cannot fly, and their short, stumpy legs render them incapable of jumping, or even walking efficiently on flat surfaces. The non-disease-carrying head louse differs from
565-506: A defined structure that are able to change their location in the genome. TEs are categorized as either as a mechanism that replicates by copy-and-paste or as a mechanism that can be excised from the genome and inserted at a new location. In the human genome, there are three important classes of TEs that make up more than 45% of the human DNA; these classes are The long interspersed nuclear elements (LINEs), The interspersed nuclear elements (SINEs), and endogenous retroviruses. These elements have
678-510: A few days, having laid very few eggs. Similarly, death of a virgin female was reported after admitting a male to her confinement. The female laid only one egg after mating, and her entire body was tinged with red—a condition attributed to rupture of the alimentary canal during the sexual act. Old females frequently die following, if not during, copulation. During its lifespan of 4 weeks a female louse lays 50-150 eggs. Eggs hatch within 6–9 days, each nymphal stage last for 4–5 days and accordingly
791-402: A few shark species when exposed to cleaner fish. In this experiment, cleaner wrasse (Labroides dimidiatus) and various shark species were placed in a tank together and observed. The different shark species exhibited different responses and behaviors around the wrasse. For example, Atlantic and Pacific lemon sharks consistently react to the wrasse fish in a fascinating way. During the interaction,
904-403: A food resource. Head louse Pediculus capitis (De Geer, 1767) The head louse ( Pediculus humanus capitis ) is an obligate ectoparasite of humans . Head lice are wingless insects that spend their entire lives on the human scalp and feed exclusively on human blood . Humans are the only known hosts of this specific parasite, while chimpanzees and bonobos host
1017-618: A genome sequence and aids in navigating around the genome. The Human Genome Project was organized to map and to sequence the human genome . A fundamental step in the project was the release of a detailed genomic map by Jean Weissenbach and his team at the Genoscope in Paris. Reference genome sequences and maps continue to be updated, removing errors and clarifying regions of high allelic complexity. The decreasing cost of genomic mapping has permitted genealogical sites to offer it as
1130-534: A large portion of the genomes of many eukaryotes. A retrotransposon is a transposable element that transposes through an RNA intermediate. Retrotransposons are composed of DNA , but are transcribed into RNA for transposition, then the RNA transcript is copied back to DNA formation with the help of a specific enzyme called reverse transcriptase. A retrotransposon that carries reverse transcriptase in its sequence can trigger its own transposition but retrotransposons that lack
1243-405: A main driving role to generate genetic novelty and natural genome editing. Works of science fiction illustrate concerns about the availability of genome sequences. Michael Crichton's 1990 novel Jurassic Park and the subsequent film tell the story of a billionaire who creates a theme park of cloned dinosaurs on a remote island, with disastrous outcomes. A geneticist extracts dinosaur DNA from
SECTION 10
#17327758656311356-413: A major role in shaping the genome. Duplication may range from extension of short tandem repeats , to duplication of a cluster of genes, and all the way to duplication of entire chromosomes or even entire genomes . Such duplications are probably fundamental to the creation of genetic novelty. Horizontal gene transfer is invoked to explain how there is often an extreme similarity between small portions of
1469-467: A major theme of the book. The 1997 film Gattaca is set in a futurist society where genomes of children are engineered to contain the most ideal combination of their parents' traits, and metrics such as risk of heart disease and predicted life expectancy are documented for each person based on their genome. People conceived outside of the eugenics program, known as "In-Valids" suffer discrimination and are relegated to menial occupations. The protagonist of
1582-505: A medieval Latin word meaning sharing food, formed from com- (with) and mensa (table). Commensal relationships may involve one organism using another for transportation ( phoresy ) or for housing ( inquilinism ), or it may also involve one organism using something another created, after its death ( metabiosis ). Examples of metabiosis are hermit crabs using gastropod shells to protect their bodies, and spiders building their webs on plants . Mutualism or interspecies reciprocal altruism
1695-477: A new site. This cut-and-paste mechanism typically reinserts transposons near their original location (within 100 kb). DNA transposons are found in bacteria and make up 3% of the human genome and 12% of the genome of the roundworm C. elegans . Genome size is the total number of the DNA base pairs in one copy of a haploid genome. Genome size varies widely across species. Invertebrates have small genomes, this
1808-406: A pointed end of the abdomen and a well-developed genital apparatus visible inside the abdomen. Females are characterized by two gonopods in the shape of a W at the end of their abdomens. Like most insects, head lice are oviparous . Females lay about three or four eggs per day. Louse eggs (also known as nits ), are attached near the base of a host hair shaft. Eggs are usually laid on the base of
1921-501: A precise definition of "genome." It usually refers to the DNA (or sometimes RNA) molecules that carry the genetic information in an organism but sometimes it is difficult to decide which molecules to include in the definition; for example, bacteria usually have one or two large DNA molecules ( chromosomes ) that contain all of the essential genetic material but they also contain smaller extrachromosomal plasmid molecules that carry important genetic information. The definition of 'genome' that
2034-415: A reference, whereas analyses of coverage depth and mapping topology can provide details regarding structural variations such as chromosomal translocations and segmental duplications. DNA sequences that carry the instructions to make proteins are referred to as coding sequences. The proportion of the genome occupied by coding sequences varies widely. A larger genome does not necessarily contain more genes, and
2147-487: A reverse transcriptase must use reverse transcriptase synthesized by another retrotransposon. Retrotransposons can be transcribed into RNA, which are then duplicated at another site into the genome. Retrotransposons can be divided into long terminal repeats (LTRs) and non-long terminal repeats (Non-LTRs). Long terminal repeats (LTRs) are derived from ancient retroviral infections, so they encode proteins related to retroviral proteins including gag (structural proteins of
2260-651: A service, to the extent that one may submit one's genome to crowdsourced scientific endeavours such as DNA.LAND at the New York Genome Center , an example both of the economies of scale and of citizen science . Viral genomes can be composed of either RNA or DNA. The genomes of RNA viruses can be either single-stranded RNA or double-stranded RNA , and may contain one or more separate RNA molecules (segments: monopartit or multipartit genome). DNA viruses can have either single-stranded or double-stranded genomes. Most DNA virus genomes are composed of
2373-458: A single circular chromosome but extensively fragmented. For the head louse, and the body louse, they are on 20 minichromosomes, for the pubic louse 14 minichromosomes and the chimpanzee louse,18 minichromosomes. Human lice are divided into three deeply divergent mitochondrial clades known as A, B, and C. Three subclades have been identified, D (a sister clade of A), E (a sister clade of C), and F (a sister clade of B). Genome In
SECTION 20
#17327758656312486-456: A single body live in conjunctive symbiosis, while all other arrangements are called disjunctive symbiosis. When one organism lives on the surface of another, such as head lice on humans, it is called ectosymbiosis ; when one partner lives inside the tissues of another, such as Symbiodinium within coral , it is termed endosymbiosis . The definition of symbiosis was a matter of debate for 130 years. In 1877, Albert Bernhard Frank used
2599-404: A single, linear molecule of DNA, but some are made up of a circular DNA molecule. Prokaryotes and eukaryotes have DNA genomes. Archaea and most bacteria have a single circular chromosome , however, some bacterial species have linear or multiple chromosomes. If the DNA is replicated faster than the bacterial cells divide, multiple copies of the chromosome can be present in a single cell, and if
2712-518: A small mitochondrial genome . Algae and plants also contain chloroplasts with a chloroplast genome. The study of the genome is called genomics . The genomes of many organisms have been sequenced and various regions have been annotated. The Human Genome Project was started in October 1990, and then reported the sequence of the human genome in April 2003, although the initial "finished" sequence
2825-560: A species. Within a species, the vast majority of nucleotides are identical between individuals, but sequencing multiple individuals is necessary to understand the genetic diversity. In 1976, Walter Fiers at the University of Ghent (Belgium) was the first to establish the complete nucleotide sequence of a viral RNA-genome ( Bacteriophage MS2 ). The next year, Fred Sanger completed the first DNA-genome sequence: Phage Φ-X174 , of 5386 base pairs. The first bacterial genome to be sequenced
2938-491: A specific pollinator that is correspondingly adapted. The first flowering plants in the fossil record had relatively simple flowers. Adaptive speciation quickly gave rise to many diverse groups of plants, and, at the same time, corresponding speciation occurred in certain insect groups . Some groups of plants developed nectar and large sticky pollen, while insects evolved more specialized morphologies to access and collect these rich food sources. In some taxa of plants and insects,
3051-405: A supreme situation, the host species (fish or marine life) will display itself at a designated station deemed the "cleaning station". Cleaner fish play an essential role in the reduction of parasitism on marine animals. Some shark species participate in cleaning symbiosis, where cleaner fish remove ectoparasites from the body of the shark. A study by Raymond Keyes addresses the atypical behavior of
3164-457: Is a long-term relationship between individuals of different species where both individuals benefit. Mutualistic relationships may be either obligate for both species, obligate for one but facultative for the other, or facultative for both. Many herbivores have mutualistic gut flora to help them digest plant matter, which is more difficult to digest than animal prey. This gut flora comprises cellulose-digesting protozoans or bacteria living in
3277-485: Is also correlated to a small number of transposable elements. Fish and Amphibians have intermediate-size genomes, and birds have relatively small genomes but it has been suggested that birds lost a substantial portion of their genomes during the phase of transition to flight. Before this loss, DNA methylation allows the adequate expansion of the genome. In humans, the nuclear genome comprises approximately 3.1 billion nucleotides of DNA, divided into 24 linear molecules,
3390-416: Is an exploitative three-party interaction where one species, the mimic, has evolved to mimic another, the model, to deceive a third, the dupe. In terms of signalling theory , the mimic and model have evolved to send a signal; the dupe has evolved to receive it from the model. This is to the advantage of the mimic but to the detriment of both the model, whose protective signals are effectively weakened, and of
3503-432: Is another DIRS-like elements belong to Non-LTRs. Non-LTRs are widely spread in eukaryotic genomes. Long interspersed elements (LINEs) encode genes for reverse transcriptase and endonuclease, making them autonomous transposable elements. The human genome has around 500,000 LINEs, taking around 17% of the genome. Short interspersed elements (SINEs) are usually less than 500 base pairs and are non-autonomous, so they rely on
Symbiosis - Misplaced Pages Continue
3616-524: Is any symbiotic relationship in which one symbiont lives within the tissues of the other, either within the cells or extracellularly. Examples include diverse microbiomes : rhizobia , nitrogen-fixing bacteria that live in root nodules on legume roots; actinomycetes , nitrogen-fixing bacteria such as Frankia , which live in alder root nodules; single-celled algae inside reef-building corals ; and bacterial endosymbionts that provide essential nutrients to about 10%–15% of insects. In endosymbiosis,
3729-427: Is any symbiotic relationship in which the symbiont lives on the body surface of the host , including the inner surface of the digestive tract or the ducts of exocrine glands . Examples of this include ectoparasites such as lice ; commensal ectosymbionts such as the barnacles , which attach themselves to the jaw of baleen whales ; and mutualist ectosymbionts such as cleaner fish . Contrastingly, endosymbiosis
3842-444: Is carried in plasmids . For this, the word genome should not be used as a synonym of chromosome . Eukaryotic genomes are composed of one or more linear DNA chromosomes. The number of chromosomes varies widely from Jack jumper ants and an asexual nemotode , which each have only one pair, to a fern species that has 720 pairs. It is surprising the amount of DNA that eukaryotic genomes contain compared to other genomes. The amount
3955-408: Is commonly used in the scientific literature is usually restricted to the large chromosomal DNA molecules in bacteria. Eukaryotic genomes are even more difficult to define because almost all eukaryotic species contain nuclear chromosomes plus extra DNA molecules in the mitochondria . In addition, algae and plants have chloroplast DNA. Most textbooks make a distinction between the nuclear genome and
4068-431: Is damaged or killed by another through a chemical secretion. An example of competition is a sapling growing under the shadow of a mature tree. The mature tree can rob the sapling of necessary sunlight and, if the mature tree is very large, it can take up rainwater and deplete soil nutrients. Throughout the process, the mature tree is unaffected by the sapling. Indeed, if the sapling dies, the mature tree gains nutrients from
4181-564: Is even more than what is necessary for DNA protein-coding and noncoding genes due to the fact that eukaryotic genomes show as much as 64,000-fold variation in their sizes. However, this special characteristic is caused by the presence of repetitive DNA, and transposable elements (TEs). A typical human cell has two copies of each of 22 autosomes , one inherited from each parent, plus two sex chromosomes , making it diploid. Gametes , such as ova, sperm, spores, and pollen, are haploid, meaning they carry only one copy of each chromosome. In addition to
4294-492: Is facilitated by active DNA demethylation , a process that entails the DNA base excision repair pathway. This pathway is employed in the erasure of CpG methylation (5mC) in primordial germ cells. The erasure of 5mC occurs via its conversion to 5-hydroxymethylcytosine (5hmC) driven by high levels of the ten-eleven dioxygenase enzymes TET1 and TET2 . Genomes are more than the sum of an organism's genes and have traits that may be measured and studied without reference to
4407-406: Is known as co-development. In cases of co-development the symbionts send signals to their host which determine developmental processes. Co-development is commonly seen in both arthropods and vertebrates. One hypothesis for the origin of the nucleus in eukaryotes (plants, animals, fungi, and protists ) is that it developed from a symbiogenesis between bacteria and archaea. It is hypothesized that
4520-401: Is rather exceptional, eukaryotes generally have these features in their genes and their genomes contain variable amounts of repetitive DNA. In mammals and plants, the majority of the genome is composed of repetitive DNA. High-throughput technology makes sequencing to assemble new genomes accessible to everyone. Sequence polymorphisms are typically discovered by comparing resequenced isolates to
4633-536: Is subtle. The only visible differences between different instars and the adult, other than size, is the relative length of the abdomen, which increases with each molt, as well as the existence of reproductive organs in the adults. Aside from reproduction, nymph behavior is similar to the adult. Like adults, nymphs feed also only on human blood (hematophagia), and cannot survive long away from a host. Outside their hosts lice cannot survive more than 24 hrs. The time required for head lice to complete their nymph development to
Symbiosis - Misplaced Pages Continue
4746-589: Is the movement, spread or transport of seeds away from the parent plant. Plants have limited mobility and rely upon a variety of dispersal vectors to transport their propagules, including both abiotic vectors such as the wind and living ( biotic ) vectors like birds. In order to attract animals, these plants evolved a set of morphological characters such as fruit colour, mass, and persistence correlated to particular seed dispersal agents. For example, plants may evolve conspicuous fruit colours to attract avian frugivores, and birds may learn to associate such colours with
4859-572: Is the relationship between the siboglinid tube worms and symbiotic bacteria that live at hydrothermal vents and cold seeps . The worm has no digestive tract and is wholly reliant on its internal symbionts for nutrition. The bacteria oxidize either hydrogen sulfide or methane, which the host supplies to them. These worms were discovered in the late 1980s at the hydrothermal vents near the Galapagos Islands and have since been found at deep-sea hydrothermal vents and cold seeps in all of
4972-421: Is to reduce the number of genes in a genome to the bare minimum and still have the organism in question survive. There is experimental work being done on minimal genomes for single cell organisms as well as minimal genomes for multi-cellular organisms (see developmental biology ). The work is both in vivo and in silico . There are many enormous differences in size in genomes, specially mentioned before in
5085-527: The New World to the Old World , instead of the other way around. The sequencing of the genome of the body louse was first proposed in the mid-2000s and the annotated genome was published in 2010. An analysis of the body and head louse transcriptomes revealed these two organisms are extremely similar genetically. Unlike other bilateral animals , the 37 mitochondrial genes of human lice are not on
5198-498: The University of Hamburg , Germany. The website Oxford Dictionaries and the Online Etymology Dictionary suggest the name is a blend of the words gene and chromosome . However, see omics for a more thorough discussion. A few related -ome words already existed, such as biome and rhizome , forming a vocabulary into which genome fits systematically. It is very difficult to come up with
5311-420: The globe by combat , but by networking ." About 80% of vascular plants worldwide form symbiotic relationships with fungi, in particular in arbuscular mycorrhizas . Flowering plants and the animals that pollinate them have co-evolved. Many plants that are pollinated by insects (in entomophily ), bats , or birds (in ornithophily ) have highly specialized flowers modified to promote pollination by
5424-406: The tentacles of Ritteri sea anemones . The territorial fish protects the anemone from anemone-eating fish, and in turn, the anemone stinging tentacles protect the clownfish from its predators . A special mucus on the clownfish protects it from the stinging tentacles. A further example is the goby , a fish which sometimes lives together with a shrimp . The shrimp digs and cleans up a burrow in
5537-434: The 21st century, the latter has become the definition widely accepted by biologists. In 1949, Edward Haskell proposed an integrative approach with a classification of "co-actions", later adopted by biologists as "interactions". Relationships can be obligate, meaning that one or both of the symbionts entirely depend on each other for survival. For example, in lichens , which consist of fungal and photosynthetic symbionts,
5650-553: The United States alone. In the UK, it is estimated that two thirds of children will experience at least one case of head lice before leaving primary school. High levels of louse infestations have also been reported from all over the world, including Australia, Denmark, France, Ireland, Israel, and Sweden. Analysis of the DNA of lice found on Peruvian mummies may indicate that some diseases (such as typhus ) may have passed from
5763-426: The X and Y chromosomes of mammals, so the technical definition of the genome must include both copies of the sex chromosomes. For example, the standard reference genome of humans consists of one copy of each of the 22 autosomes plus one X chromosome and one Y chromosome. A genome sequence is the complete list of the nucleotides (A, C, G, and T for DNA genomes) that make up all the chromosomes of an individual or
SECTION 50
#17327758656315876-431: The average mammal species is host to 4 nematodes, 2 cestodes, and 2 trematodes. Mimicry is a form of symbiosis in which a species adopts distinct characteristics of another species to alter its relationship dynamic with the species being mimicked, to its own advantage. Among the many types of mimicry are Batesian and Müllerian, the first involving one-sided exploitation, the second providing mutual benefit. Batesian mimicry
5989-412: The blood of ancient mosquitoes and fills in the gaps with DNA from modern species to create several species of dinosaurs. A chaos theorist is asked to give his expert opinion on the safety of engineering an ecosystem with the dinosaurs, and he repeatedly warns that the outcomes of the project will be unpredictable and ultimately uncontrollable. These warnings about the perils of using genomic information are
6102-411: The broad definition, while simultaneously attempting to clarify its relevance to infestation: In the United States the term "nit" refers to any egg regardless of its viability. Because nits are simply egg casings that can contain a developing embryo or be empty shells, not all nits are infective. Head lice eggs (nits) are brown or white (empty shells) and attached to the hair In British and Irish slang
6215-549: The cells divide faster than the DNA can be replicated, multiple replication of the chromosome is initiated before the division occurs, allowing daughter cells to inherit complete genomes and already partially replicated chromosomes. Most prokaryotes have very little repetitive DNA in their genomes. However, some symbiotic bacteria (e.g. Serratia symbiotica ) have reduced genomes and a high fraction of pseudogenes: only ~40% of their DNA encodes proteins. Some bacteria have auxiliary genetic material, also part of their genome, which
6328-478: The chromosomes in the nucleus, organelles such as the chloroplasts and mitochondria have their own DNA. Mitochondria are sometimes said to have their own genome often referred to as the " mitochondrial genome ". The DNA found within the chloroplast may be referred to as the " plastome ". Like the bacteria they originated from, mitochondria and chloroplasts have a circular chromosome. Unlike prokaryotes where exon-intron organization of protein coding genes exists but
6441-471: The claws on their legs to move from hair to hair. Normally, head lice infest a new host only by close contact between individuals, making social contacts among children and parent-child interactions more likely routes of infestation than shared combs, hats, brushes, towels, clothing, beds, or closets. Head-to-head contact is by far the most common route of lice transmission. About 6–12 million people, mainly children, are treated annually for head lice in
6554-408: The competition can also be for other resources. Amensalism is a non-symbiotic, asymmetric interaction where one species is harmed or killed by the other, and one is unaffected by the other. There are two types of amensalism, competition and antagonism (or antibiosis). Competition is where a larger or stronger organism deprives a smaller or weaker one of a resource. Antagonism occurs when one organism
6667-466: The decaying sapling. An example of antagonism is Juglans nigra (black walnut), secreting juglone , a substance which destroys many herbaceous plants within its root zone. The term amensalism is often used to describe strongly asymmetrical competitive interactions, such as between the Spanish ibex and weevils of the genus Timarcha which feed upon the same type of shrub. Whilst the presence of
6780-550: The degree of separation is contentious as they can produce fertile offspring in a laboratory. A much more distantly related species of hair-clinging louse, the pubic or crab louse ( Pthirus pubis ), also infests humans. It is morphologically different from the other two species and is much closer in appearance to the lice which infest other primates. Louse infestation of the body is known as pediculosis , pediculosis capitis for head lice, pediculosis corporis for body lice, and phthiriasis for pubic lice. Like other insects of
6893-445: The details of any particular genes and their products. Researchers compare traits such as karyotype (chromosome number), genome size , gene order, codon usage bias , and GC-content to determine what mechanisms could have produced the great variety of genomes that exist today (for recent overviews, see Brown 2002; Saccone and Pesole 2003; Benfey and Protopapas 2004; Gibson and Muse 2004; Reese 2004; Gregory 2005). Duplications play
SECTION 60
#17327758656317006-617: The dupe, which is deprived of an edible prey. For example, a wasp is a strongly-defended model, which signals with its conspicuous black and yellow coloration that it is an unprofitable prey to predators such as birds which hunt by sight; many hoverflies are Batesian mimics of wasps, and any bird that avoids these hoverflies is a dupe. In contrast, Müllerian mimicry is mutually beneficial as all participants are both models and mimics. For example, different species of bumblebee mimic each other, with similar warning coloration in combinations of black, white, red, and yellow, and all of them benefit from
7119-848: The endosymbiont adapts to the host's lifestyle, the endosymbiont changes dramatically. There is a drastic reduction in its genome size, as many genes are lost during the process of metabolism , and DNA repair and recombination, while important genes participating in the DNA-to-RNA transcription , protein translation and DNA/RNA replication are retained. The decrease in genome size is due to loss of protein coding genes and not due to lessening of inter-genic regions or open reading frame (ORF) size. Species that are naturally evolving and contain reduced sizes of genes can be accounted for an increased number of noticeable differences between them, thereby leading to changes in their evolutionary rates. When endosymbiotic bacteria related with insects are passed on to
7232-556: The fact that mitochondria and chloroplasts divide independently of the cell, and that these organelles have their own genome. The biologist Lynn Margulis , famous for her work on endosymbiosis , contended that symbiosis is a major driving force behind evolution . She considered Darwin 's notion of evolution, driven by competition, to be incomplete and claimed that evolution is strongly based on co-operation , interaction , and mutual dependence among organisms. According to Margulis and her son Dorion Sagan , " Life did not take over
7345-518: The fields of molecular biology and genetics , a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses ). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as regulatory sequences (see non-coding DNA ), and often a substantial fraction of junk DNA with no evident function. Almost all eukaryotes have mitochondria and
7458-438: The first 10 hours of adult life. After 24 hours, adult lice copulate frequently, with mating occurring during any period of the night or day. Mating attachment frequently lasts more than an hour. Young males can successfully pair with older females, and vice versa. Experiments with P. h. humanus (body lice) emphasize the attendant hazards of lice copulation. A single young female confined with six or more males will die in
7571-408: The fungal partners cannot live on their own. The algal or cyanobacterial symbionts in lichens, such as Trentepohlia , can generally live independently, and their part of the relationship is therefore described as facultative (optional), or non-obligate. When one of the participants in a symbiotic relationship is capable of photosynthesis, as with lichens, it is called photosymbiosis. Ectosymbiosis
7684-470: The generation of phenotypic diversity and complex phenotypes able to colonise new environments. Evolution originated from changes in development where variations within species are selected for or against because of the symbionts involved. The hologenome theory relates to the holobiont and symbionts genome together as a whole. Microbes live everywhere in and on every multicellular organism. Many organisms rely on their symbionts in order to develop properly, this
7797-406: The genomes of two organisms that are otherwise very distantly related. Horizontal gene transfer seems to be common among many microbes . Also, eukaryotic cells seem to have experienced a transfer of some genetic material from their chloroplast and mitochondrial genomes to their nuclear chromosomes. Recent empirical data suggest an important role of viruses and sub-viral RNA-networks to represent
7910-420: The hair, 3–5 mm off the scalp surface. In warm climates, and especially the tropics, eggs may be laid 6 inches (15 cm) or more down the hair shaft. To attach an egg, the adult female secretes a glue from her reproductive organ. This glue quickly hardens into a "nit sheath" that covers the hair shaft and large parts of the egg except for the operculum, a cap through which the embryo breathes. The glue
8023-416: The herbivores' intestines. Coral reefs result from mutualism between coral organisms and various algae living inside them. Most land plants and land ecosystems rely on mutualism between the plants, which fix carbon from the air, and mycorrhyzal fungi, which help in extracting water and minerals from the ground. An example of mutualism is the relationship between the ocellaris clownfish that dwell among
8136-426: The host cell lacks some of the nutrients which the endosymbiont provides. As a result, the host favors endosymbiont's growth processes within itself by producing some specialized cells. These cells affect the genetic composition of the host in order to regulate the increasing population of the endosymbionts and ensure that these genetic changes are passed onto the offspring via vertical transmission ( heredity ). As
8249-510: The host's blood throughout the insect body. This results in death within a day or two. Whether the high mortality recorded under experimental conditions is representative of conditions in the wild is unclear. Head lice reproduce sexually , and copulation is necessary for the female to produce fertile eggs. Parthenogenesis , the production of viable offspring by virgin females, does not occur in Pediculus humanus . Pairing can begin within
8362-455: The human genome All the cells of an organism originate from a single cell, so they are expected to have identical genomes; however, in some cases, differences arise. Both the process of copying DNA during cell division and exposure to environmental mutagens can result in mutations in somatic cells. In some cases, such mutations lead to cancer because they cause cells to divide more quickly and invade surrounding tissues. In certain lymphocytes in
8475-425: The human genome and 9% of the fruit fly genome. Tandem repeats can be functional. For example, telomeres are composed of the tandem repeat TTAGGG in mammals, and they play an important role in protecting the ends of the chromosome. In other cases, expansions in the number of tandem repeats in exons or introns can cause disease . For example, the human gene huntingtin (Htt) typically contains 6–29 tandem repeats of
8588-517: The human immune system, V(D)J recombination generates different genomic sequences such that each cell produces a unique antibody or T cell receptors. During meiosis , diploid cells divide twice to produce haploid germ cells. During this process, recombination results in a reshuffling of the genetic material from homologous chromosomes so each gamete has a unique genome. Genome-wide reprogramming in mouse primordial germ cells involves epigenetic imprint erasure leading to totipotency . Reprogramming
8701-411: The imago lasts for 12–15 days. Nymph mortality in captivity is about 38%, especially within the first two days of life. In the wild, mortality may instead be highest in the third instar. Nymph hazards are numerous. Failure to completely hatch from the egg is invariably fatal. Death during molting can also occur, although it is reportedly uncommon. During feeding, the nymph gut can rupture, dispersing
8814-661: The insect's head except during feeding. Six legs project from the fused segments of the thorax . As is typical in the Anoplura, these legs are short and terminate with a single claw and opposing " thumb ". Between its claw and thumb, the louse grasps the hair of its host. With their short legs and large claws, lice are well adapted to clinging to the hair of their host. These adaptations leave them incapable of jumping, or even walking efficiently on flat surfaces. Lice can climb up strands of hair very quickly, allowing them to move quickly and reach another host. Seven segments of
8927-418: The louse abdomen are visible. The first six segments each have a pair of spiracles through which the insect breathes. The last segment contains the anus and (separately) the genitalia . In male lice, the front two legs are slightly larger than the other four. This specialized pair of legs are used for holding the female during copulation. Males are slightly smaller than females and are characterized by
9040-402: The louse nymph leaves behind its egg shell, still attached to the hair shaft. The empty egg shell remains in place until physically removed by abrasion or the host, or until it slowly disintegrates, which may take six or more months. Empty shells are matte, collapsed and white in color. The term nit may include any of the following: Of these three, only eggs containing viable embryos have
9153-522: The louse body to take on a reddish color. One pair of antennae , each with five segments, protrudes from the insect's head. Head lice also have one pair of eyes . Eyes are present in all species within the Pediculidae family, but are reduced or absent in most other members of the Anoplura suborder. Like other members of the Anoplura, head louse mouthparts are highly adapted for piercing the skin and sucking blood. These mouth parts are retracted into
9266-572: The most frequently infested group. All stages except eggs are blood-feeders and bite the skin four to five times daily to feed. They inject saliva which contains an anticoagulant and suck blood. The digested blood is excreted as dark red frass . Although any part of the scalp may be colonized, lice favor the nape of the neck and the area behind the ears, where the eggs are usually laid. Head lice are repelled by light and move towards shadows or dark-coloured objects in their vicinity. Lice have no wings or powerful legs for jumping, so they move using
9379-436: The mouth to examine the gills, specifically the buccopharyngeal area, which typically holds the most parasites. When the shark begins to close its mouth, the wrasse finishes its examination and goes elsewhere. Male bull sharks exhibit slightly different behavior at cleaning stations: as the shark swims into a colony of wrasse fish, it drastically slows its speed to allow the cleaners to do their job. After approximately one minute,
9492-476: The multicellular eukaryotic genomes. Much of this is due to the differing abundances of transposable elements, which evolve by creating new copies of themselves in the chromosomes. Eukaryote genomes often contain many thousands of copies of these elements, most of which have acquired mutations that make them defective. Here is a table of some significant or representative genomes. See #See also for lists of sequenced genomes. Initial sequencing and analysis of
9605-459: The non-essential genes of the intracellular bacteria. This can be due to lack of selection mechanisms prevailing in the relatively "rich" host environment. Competition can be defined as an interaction between organisms or species, in which the fitness of one is lowered by the presence of another. Limited supply of at least one resource (such as food , water , and territory ) used by both usually facilitates this type of interaction, although
9718-795: The nucleotides CAG (encoding a polyglutamine tract). An expansion to over 36 repeats results in Huntington's disease , a neurodegenerative disease. Twenty human disorders are known to result from similar tandem repeat expansions in various genes. The mechanism by which proteins with expanded polygulatamine tracts cause death of neurons is not fully understood. One possibility is that the proteins fail to fold properly and avoid degradation, instead accumulating in aggregates that also sequester important transcription factors, thereby altering gene expression. Tandem repeats are usually caused by slippage during replication, unequal crossing-over and gene conversion. Transposable elements (TEs) are sequences of DNA with
9831-508: The offspring strictly via vertical genetic transmission, intracellular bacteria go across many hurdles during the process, resulting in the decrease in effective population sizes, as compared to the free-living bacteria. The incapability of the endosymbiotic bacteria to reinstate their wild type phenotype via a recombination process is called Muller's ratchet phenomenon. Muller's ratchet phenomenon, together with less effective population sizes, leads to an accretion of deleterious mutations in
9944-447: The organelle (mitochondria and chloroplast) genomes so when they speak of, say, the human genome, they are only referring to the genetic material in the nucleus. This is the most common use of 'genome' in the scientific literature. Most eukaryotes are diploid , meaning that there are two of each chromosome in the nucleus but the 'genome' refers to only one copy of each chromosome. Some eukaryotes have distinctive sex chromosomes, such as
10057-466: The period from egg to adults lasts for 18–24 days. Adult lice live for an additional 3–4 weeks. The number of children per family, the sharing of beds and closets, hair washing habits, local customs and social contacts, healthcare in a particular area (e.g. school), and socioeconomic status were found to be significant factors in head louse infestation. Girls are two to four times more frequently infested than boys. Children between 4 and 14 years of age are
10170-430: The potential to infest or reinfest a host. However, a no nit policy is a common public health measure to prevent transmission of lice. Some authors have therefore restricted the definition of nit to describe only a hatched or nonviable egg: In many languages, the terms used for the hatched eggs, which were obvious for all to see, have subsequently become applied to the embryonated eggs that are difficult to detect. Thus,
10283-590: The proportion of non-repetitive DNA decreases along with increasing genome size in complex eukaryotes. Noncoding sequences include introns , sequences for non-coding RNAs, regulatory regions, and repetitive DNA. Noncoding sequences make up 98% of the human genome. There are two categories of repetitive DNA in the genome: tandem repeats and interspersed repeats. Short, non-coding sequences that are repeated head-to-tail are called tandem repeats . Microsatellites consisting of 2–5 basepair repeats, while minisatellite repeats are 30–35 bp. Tandem repeats make up about 4% of
10396-412: The prospect of personal genome sequencing as a diagnostic tool, as pioneered by Manteia Predictive Medicine . A major step toward that goal was the completion in 2007 of the full genome of James D. Watson , one of the co-discoverers of the structure of DNA. Whereas a genome sequence lists the order of every DNA base in a genome, a genome map identifies the landmarks. A genome map is less detailed than
10509-439: The proteins encoded by LINEs for transposition. The Alu element is the most common SINE found in primates. It is about 350 base pairs and occupies about 11% of the human genome with around 1,500,000 copies. DNA transposons encode a transposase enzyme between inverted terminal repeats. When expressed, the transposase recognizes the terminal inverted repeats that flank the transposon and catalyzes its excision and reinsertion in
10622-408: The related disease-carrying body louse ( Pediculus humanus humanus ) in preferring to attach eggs to scalp hair rather than to clothing. The two subspecies are morphologically almost identical, but do not normally interbreed. From genetic studies, they are thought to have diverged as subspecies about 30,000–110,000 years ago, when many humans began to wear a significant amount of clothing. However,
10735-405: The relationship has become dependent, where the plant species can only be pollinated by one species of insect. The acacia ant ( Pseudomyrmex ferruginea ) is an obligate plant ant that protects at least five species of "Acacia" ( Vachellia ) from preying insects and from other plants competing for sunlight, and the tree provides nourishment and shelter for the ant and its larvae. Seed dispersal
10848-597: The relationship. Cleaning symbiosis is an association between individuals of two species, where one (the cleaner) removes and eats parasites and other materials from the surface of the other (the client). It is putatively mutually beneficial, but biologists have long debated whether it is mutual selfishness, or simply exploitative. Cleaning symbiosis is well known among marine fish, where some small species of cleaner fish – notably wrasses , but also species in other genera – are specialized to feed almost exclusively by cleaning larger fish and other marine animals. In
10961-437: The sand in which both the shrimp and the goby fish live. The shrimp is almost blind, leaving it vulnerable to predators when outside its burrow. In case of danger, the goby touches the shrimp with its tail to warn it, and both quickly retreat into the burrow. Different species of gobies ( Elacatinus spp. ) also clean up ectoparasites in other fish, possibly another kind of mutualism. A spectacular example of obligate mutualism
11074-409: The shark remains passive and the wrasse swims to it. It begins to scan the shark's body, sometimes stopping to inspect specific areas. Commonly, the wrasse would inspect the gills, labial regions, and skin. When the wrasse makes its way to the mouth of the shark, the shark often ceases breathing for up to two and a half minutes so that the fish is able to scan the mouth. Then, the fish passes further into
11187-493: The shark returns to normal swimming speed. Symbiosis is increasingly recognized as an important selective force behind evolution; many species have a long history of interdependent co-evolution . Although symbiosis was once discounted as an anecdotal evolutionary phenomenon, evidence is now overwhelming that obligate or facultative associations among microorganisms and between microorganisms and multicellular hosts had crucial consequences in many landmark events in evolution and in
11300-423: The shortest 45 000 000 nucleotides in length and the longest 248 000 000 nucleotides, each contained in a different chromosome. There is no clear and consistent correlation between morphological complexity and genome size in either prokaryotes or lower eukaryotes . Genome size is largely a function of the expansion and contraction of repetitive DNA elements. Since genomes are very complex, one research strategy
11413-451: The suborder Anoplura , adult head lice are small (2.5–3 mm long), dorsoventrally flattened (see anatomical terms of location ), and wingless. The thoracic segments are fused, but otherwise distinct from the head and abdomen , the latter being composed of seven visible segments . Head lice are grey in general, but their precise color varies according to the environment in which they were raised. After feeding, consumed blood causes
11526-450: The symbiosis originated when ancient archaea, similar to modern methanogenic archaea, invaded and lived within bacteria similar to modern myxobacteria, eventually forming the early nucleus. This theory is analogous to the accepted theory for the origin of eukaryotic mitochondria and chloroplasts, which are thought to have developed from a similar endosymbiotic relationship between proto-eukaryotes and aerobic bacteria. Evidence for this includes
11639-562: The term symbiosis to describe the mutualistic relationship in lichens . In 1878, the German mycologist Heinrich Anton de Bary defined it as "the living together of unlike organisms". The definition has varied among scientists, with some advocating that it should only refer to persistent mutualisms , while others thought it should apply to all persistent biological interactions (in other words, to mutualism, commensalism , and parasitism , but excluding brief interactions such as predation ). In
11752-504: The term "nit" in English is often used for both. However, in recent years, my colleagues and I have felt the need for some simple means of distinguishing between the two without laborious qualification. We have, therefore, come to reserve the term "nit" for the hatched and empty egg shell and refer to the developing embryonated egg as an "egg". The empty eggshell, termed a nit... ...nits (dead eggs or empty egg cases)... Others have retained
11865-458: The term "nit" is often used, across different age groups, to refer to the head lice themselves. Head lice, like other insects of the order Phthiraptera , are hemimetabolous . Newly hatched nymphs will moult three times before reaching the sexually mature adult stage. Thus, mobile head lice populations may contain eggs, nits, three nymphal instars , and the adults (male and female) ( imago ). Metamorphosis during head louse development
11978-541: The virus), pol (reverse transcriptase and integrase), pro (protease), and in some cases env (envelope) genes. These genes are flanked by long repeats at both 5' and 3' ends. It has been reported that LTRs consist of the largest fraction in most plant genome and might account for the huge variation in genome size. Non-long terminal repeats (Non-LTRs) are classified as long interspersed nuclear elements (LINEs), short interspersed nuclear elements (SINEs), and Penelope-like elements (PLEs). In Dictyostelium discoideum , there
12091-473: The weevil has almost no influence on food availability, the presence of ibex has an enormous detrimental effect on weevil numbers, as they consume significant quantities of plant matter and incidentally ingest the weevils upon it. Commensalism describes a relationship between two living organisms where one benefits and the other is not significantly harmed or helped. It is derived from the English word commensal , used of human social interaction . It derives from
12204-431: The world's oceans. Mutualism improves both organism's competitive ability and will outcompete organisms of the same species that lack the symbiont. A facultative symbiosis is seen in encrusting bryozoans and hermit crabs . The bryozoan colony ( Acanthodesia commensale ) develops a cirumrotatory growth and offers the crab ( Pseudopagurus granulimanus ) a helicospiral-tubular extension of its living chamber that initially
12317-429: Was completed in 1996, again by The Institute for Genomic Research. The development of new technologies has made genome sequencing dramatically cheaper and easier, and the number of complete genome sequences is growing rapidly. The US National Institutes of Health maintains one of several comprehensive databases of genomic information. Among the thousands of completed genome sequencing projects include those for rice ,
12430-477: Was missing 8% of the genome consisting mostly of repetitive sequences. With advancements in technology that could handle sequencing of the many repetitive sequences found in human DNA that were not fully uncovered by the original Human Genome Project study, scientists reported the first end-to-end human genome sequence in March 2022. The term genome was created in 1920 by Hans Winkler , professor of botany at
12543-405: Was previously thought to be chitin -based, but more recent studies have shown it to be made of proteins similar to hair keratin . Each egg is oval-shaped and about 0.8 mm in length. They are bright, transparent, and tan to coffee-colored so long as they contain an embryo, but appear white after hatching. Head lice hatch typically six to nine days after oviposition . After hatching,
12656-438: Was situated within a gastropod shell. In a parasitic relationship, the parasite benefits while the host is harmed. Parasitism takes many forms, from endoparasites that live within the host's body to ectoparasites and parasitic castrators that live on its surface and micropredators like mosquitoes that visit intermittently. Parasitism is an extremely successful mode of life; about 40% of all animal species are parasites, and
12769-400: Was that of Haemophilus influenzae , completed by a team at The Institute for Genomic Research in 1995. A few months later, the first eukaryotic genome was completed, with sequences of the 16 chromosomes of budding yeast Saccharomyces cerevisiae published as the result of a European-led effort begun in the mid-1980s. The first genome sequence for an archaeon , Methanococcus jannaschii ,
#630369