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94-469: Ascaris is a nematode genus of parasitic worms known as the "small intestinal roundworms". One species, Ascaris lumbricoides , affects humans and causes the disease ascariasis . Another species, Ascaris suum , typically infects pigs . Other ascarid genera infect other animals, such as Parascaris equorum , the equine roundworm, and Toxocara and Toxascaris , which infect dogs and cats. Their eggs are deposited in feces and soil. Plants with

188-605: A last common ancestor that lived over 550 million years ago, the chicken and fly version of the same Hox gene are similar enough to target the same downstream genes in flies. Drosophila melanogaster is an important model for understanding body plan generation and evolution. The general principles of Hox gene function and logic elucidated in flies will apply to all bilaterian organisms, including humans. Drosophila , like all insects, has eight Hox genes. These are clustered into two complexes, both of which are located on chromosome 3. The Antennapedia complex (not to be confused with

282-453: A characteristically bent or fan-shaped tail. During copulation , one or more chitinized spicules move out of the cloaca and are inserted into the genital pore of the female. Amoeboid sperm crawl along the spicule into the female worm. Nematode sperm is thought to be the only eukaryotic cell without the globular protein G-actin . Eggs may be embryonated or unembryonated when passed by

376-458: A collaborative wiki called 959 Nematode Genomes is underway to improve the systematics of this phylum. An analysis of the mitochondrial DNA suggests that the following groupings are valid In 2022 a new classification of the entire phylum Nematoda was presented by M. Hodda. It was based on current molecular, developmental and morphological evidence. Under this classification, the following classes and subclasses are presented: Nematode eggs from

470-531: A gene cluster. The Hox genes are named for the homeotic phenotypes that result when their function is disrupted, wherein one segment develops with the identity of another (e.g. legs where antennae should be). Hox genes in different phyla have been given different names, which has led to confusion about nomenclature. The complement of Hox genes in Drosophila is made up of two clusters, the Antennapedia complex and

564-483: A genetically determined correlation between DNA repair capacity and lifespan. In female C. elegans , germline processes that control DNA repair and formation of chromosomal crossovers during meiosis were shown to progressively deteriorate with age. Different free-living species feed on materials as varied as bacteria , algae , fungi , small animals, fecal matter, dead organisms, and living tissues. Free-living marine nematodes are important and abundant members of

658-534: A glandular and muscular ejaculatory duct associated with the vas deferens and cloaca . In females, the ovaries each open into an oviduct (in hermaphrodites, the eggs enter a spermatheca first) and then a glandular uterus . The uteri both open into a common vulva/vagina, usually located in the middle of the morphologically ventral surface. Reproduction is usually sexual, though hermaphrodites are capable of self-fertilization. Males are usually smaller than females or hermaphrodites (often much smaller) and often have

752-445: A large ventral, a smaller dorsal and two pairs of sublateral cords extend posteriorly. Each nerve lies within a cord of connective tissue lying beneath the cuticle and between the muscle cells. The ventral nerve is the largest, and has a double structure forward of the excretory pore . The dorsal nerve is responsible for motor control, while the lateral nerves are sensory, and the ventral combines both functions. The nervous system

846-502: A pore on the underside of the animal, close to the pharynx. In most other nematodes, these specialized cells have been replaced by an organ consisting of two parallel ducts connected by a single transverse duct. This transverse duct opens into a common canal that runs to the excretory pore. At the anterior end of the animal a dense, circular nerve ring which serves as the brain surrounds the pharynx. From this ring six labial papillary nerve cords extend anteriorly, while six nerve cords;

940-410: A range of reproductive modes, including sexual reproduction , facultative sexuality (in which most, but not all, generations reproduce asexually), and both meiotic and mitotic parthenogenesis . The genus Mesorhabditis exhibits an unusual form of parthenogenesis, in which sperm-producing males copulate with females, but the sperm do not fuse with the ovum. Contact with the sperm is essential for

1034-454: A series of inhibitors to target digestive and immune-related host proteases , which include pepsin , trypsin , chymotrypsin / elastase , cathepsins , and metallocarboxypeptidases (MCPs). Ascaris species inhibit MCPs by releasing an enzyme known as Ascaris carboxypeptidase inhibitor (ACI). This enzyme binds to the active site of MCP and blocks the cleavage of its own proteins by the host MCP. Similarly, they inhibit trypsin by releasing

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1128-579: A single Hox gene cluster, which was duplicated (twice) early in vertebrate evolution by whole genome duplications to give four Hox gene clusters: Hoxa, Hoxb, Hoxc and Hoxd. It is currently unclear whether these duplications occurred before or after the divergence of lampreys and hagfish from other vertebrates. Most tetrapods have four HOX clusters, while most teleost fish , including zebrafish and medaka , have seven or eight Hox gene clusters because of an additional genome duplication which occurred in their evolutionary history. In zebrafish, one of

1222-692: A specific set of gap or pair-rule genes. In flies, stripe 2 in the embryo is activated by the maternal proteins Bicoid and Hunchback, but repressed by the gap proteins Giant and Kruppel. Thus, stripe 2 will only form wherever there is Bicoid and Hunchback, but not where there is Giant and Kruppel. MicroRNA strands located in Hox clusters have been shown to inhibit more anterior hox genes ("posterior prevalence phenomenon"), possibly to better fine tune its expression pattern. Non-coding RNA (ncRNA) has been shown to be abundant in Hox clusters. In humans, 231 ncRNA may be present. One of these, HOTAIR , silences in trans (it

1316-432: A subset of the homeobox transcription factor genes. In many animals, the organization of the Hox genes in the chromosome is the same as the order of their expression along the anterior-posterior axis of the developing animal, and are thus said to display colinearity. Production of Hox gene products at wrong location in the body is associated with metaplasia and predisposes to oncological disease, e.g. Barrett's esophagus

1410-426: A synonym. However, in 1910, Grobben proposed the phylum Aschelminthes and the nematodes were included as class Nematoda along with class Rotifera, class Gastrotricha, class Kinorhyncha, class Priapulida, and class Nematomorpha (The phylum was later revived and modified by Libbie Henrietta Hyman in 1951 as Pseudoceolomata, but remained similar). In 1932, Potts elevated the class Nematoda to the level of phylum, leaving

1504-437: A transcription factor cascade: maternal factors activate gap or pair-rule genes; gap and pair-rule genes activate Hox genes; then, finally, Hox genes activate realisator genes that cause the segments in the developing embryo to differentiate. Regulation is achieved via protein concentration gradients, called morphogenic fields . For example, high concentrations of one maternal protein and low concentrations of others will turn on

1598-500: Is Bursaphelenchus xylophilus , the pine wood nematode, present in Asia and America and recently discovered in Europe. This nematode is transmitted from tree to tree by sawyer beetles ( Monochamus ). Hox gene Studies on Hox genes in ciliated larvae have shown they are only expressed in future adult tissues. In larvae with gradual metamorphosis the Hox genes are activated in tissues of

1692-410: Is a transcription factor . Each Hox gene contains a well-conserved DNA sequence known as the homeobox, of which the term "Hox" was originally a contraction. However, in current usage the term Hox is no longer equivalent to homeobox, because Hox genes are not the only genes to possess a homeobox sequence; for instance, humans have over 200 homeobox genes, of which 39 are Hox genes. Hox genes are thus

1786-444: Is a lethal parasite of gastropods such as slugs and snails . Some members of the genus Steinernema such as Steinernema carpocapsae and Steinernema riobrave are generalist parasites of webworms , cutworms , armyworms , girdlers , some weevils , wood-borers and corn earworm moths . These organisms are grown commercially as biological pest control agents which can be used as an alternative to pesticides ; their use

1880-533: Is also the only place in the nematode body that contains cilia , which are all nonmotile and with a sensory function. The bodies of nematodes are covered in numerous sensory bristles and papillae that together provide a sense of touch. Behind the sensory bristles on the head lie two small pits, or ' amphids '. These are well supplied with nerve cells and are probably chemoreception organs. A few aquatic nematodes possess what appear to be pigmented eye-spots, but whether or not these are actually sensory in nature

1974-552: Is conferred by a part of the protein referred to as the homeodomain . The homeodomain is a 60- amino-acid -long DNA-binding domain (encoded by its corresponding 180- base-pair DNA sequence, the homeobox). This amino acid sequence folds into a "helix-turn-helix" (i.e. homeodomain fold ) motif that is stabilized by a third helix. The consensus polypeptide chain is shown below: Hox proteins often act in partnership with co-factors, such as PBC and Meis proteins encoded by very different types of homeobox gene. Homeobox genes, and thus

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2068-451: Is conserved in nearly all sites recognized by homeodomains, and probably distinguishes such locations as DNA binding sites. The base pairs following this initial sequence are used to distinguish between homeodomain proteins, all of which have similar recognition sites. For instance, the nucleotide following the TAAT sequence is recognized by the amino acid at position 9 of the homeodomain protein. In

2162-560: Is considered very safe. Plant-parasitic nematodes include several groups causing severe crop losses, taking 10% of crops worldwide every year. The most common genera are Aphelenchoides ( foliar nematodes ), Ditylenchus , Globodera (potato cyst nematodes), Heterodera (soybean cyst nematodes), Longidorus , Meloidogyne ( root-knot nematodes ), Nacobbus , Pratylenchus (lesion nematodes), Trichodorus , and Xiphinema (dagger nematodes). Several phytoparasitic nematode species cause histological damages to roots, including

2256-435: Is ectopically expressed throughout the embryo, all segments anterior of A4 are transformed to an A4-like abdominal identity. The abd-A gene also affects the pattern of cuticle generation in the ectoderm , and pattern of muscle generation in the mesoderm . Gene abd-B is transcribed in two different forms, a regulatory protein, and a morphogenic protein. Regulatory abd-B suppress embryonic ventral epidermal structures in

2350-431: Is long, cylindrical, and fusiform (pointed at both the ends). The body wall is composed of cuticle , epidermis and musculature. There is a pseudocoelom . Respiration is by simple diffusion. The nervous system consists of a nerve ring and many longitudinal nerve cords. Reproduction is exclusively sexual, and males are usually shorter than females. As part of the parasite defense strategy, Ascaris roundworms secrete

2444-460: Is often found at the tip of the tail. The epidermis is either a syncytium or a single layer of cells, and is covered by a thick collagenous cuticle . The cuticle is often of a complex structure and may have two or three distinct layers. Underneath the epidermis lies a layer of longitudinal muscle cells. The relatively rigid cuticle works with the muscles to create a hydroskeleton, as nematodes lack circumferential muscles. Projections run from

2538-447: Is often ornamented with ridges, rings, bristles, or other distinctive structures. The head of a nematode is relatively distinct. Whereas the rest of the body is bilaterally symmetrical, the head is radially symmetrical, with sensory bristles and, in many cases, solid 'head-shields' radiating outwards around the mouth. The mouth has either three or six lips, which often bear a series of teeth on their inner edges. An adhesive 'caudal gland'

2632-402: Is present, with the pharynx connecting directly to a muscleless intestine that forms the main length of the gut. This produces further enzymes and also absorbs nutrients through its single-cell-thick lining. The last portion of the intestine is lined by a cuticle, forming a rectum , which expels waste through the anus just below and in front of the tip of the tail. The movement of food through

2726-462: Is responsible for cephalic and thoracic development in Drosophila embryo and adult. The second thoracic segment, or T2, develops a pair of legs and a pair of wings. The Antp gene specifies this identity by promoting leg formation and allowing (but not directly activating) wing formation. A dominant Antp mutation, caused by a chromosomal inversion , causes Antp to be expressed in the antennal imaginal disc, so that, instead of forming an antenna,

2820-428: Is responsible for the formation of the maxillary and mandibular segments in the larval head. The mutant phenotypes of Dfd are similar to those of labial. Loss of function of Dfd in the embryo results in a failure of head involution (see labial gene), with a loss of larval head structures. Mutations in the adult have either deletions of parts of the head or transformations of head to thoracic identity. The Scr gene

2914-482: Is the result of altered Hox coding and is a precursor to esophageal cancer . The products of Hox genes are Hox proteins. Hox proteins are a subset of transcription factors, which are proteins that are capable of binding to specific nucleotide sequences on DNA called enhancers through which they either activate or repress hundreds of other genes. The same Hox protein can act as a repressor at one gene and an activator at another. The ability of Hox proteins to bind DNA

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3008-561: Is transcribed from the HOXC cluster and inhibits late HOXD genes) by binding to Polycomb-group proteins (PRC2). The chromatin structure is essential for transcription but it also requires the cluster to loop out of the chromosome territory . In higher animals including humans, retinoic acid regulates differential expression of Hox genes along the anteroposterior axis. Genes in the 3' ends of Hox clusters are induced by retinoic acid resulting in expression domains that extend more anteriorly in

3102-455: Is unclear. Most nematode species are dioecious , with separate male and female individuals, though some, such as Caenorhabditis elegans , are androdioecious , consisting of hermaphrodites and rare males. Both sexes possess one or two tubular gonads . In males, the sperm are produced at the end of the gonad and migrate along its length as they mature. The testis opens into a relatively wide seminal vesicle and then during intercourse into

3196-519: The Antp gene) consists of five genes: labial ( lab ), proboscipedia ( pb ), deformed ( Dfd ), sex combs reduced ( Scr ), and Antennapedia ( Antp ). The Bithorax complex, named after the Ultrabithorax gene, consists of the remaining three genes: Ultrabithorax ( Ubx ), abdominal-A ( abd-A ) and abdominal-B ( abd-B ). The lab gene is the most anteriorly expressed gene. It is expressed in the head, primarily in

3290-533: The Moltrasio Formation , and the Rhynie chert , where the earliest known fossils are known from. Nematodes are very small, slender worms: typically about 5 to 100 μm thick, and 0.1 to 2.5 mm long. The smallest nematodes are microscopic, while free-living species can reach as much as 5 cm (2 in), and some parasitic species are larger still, reaching over 1 m (3 ft) in length. The body

3384-833: The Monhysterida representing a very ancient minor group of nematodes. Among the Secernentea, the Diplogasteria may need to be united with the Rhabditia , while the Tylenchia might be paraphyletic with the Rhabditia. The understanding of roundworm systematics and phylogeny as of 2002 is summarised below: Phylum Nematoda Later work has suggested the presence of 12 clades. The Secernentea—a group that includes virtually all major animal and plant 'nematode' parasites—apparently arose from within

3478-535: The Vermes . The name of the group Nematoda, informally called "nematodes", came from Nematoidea , originally defined by Karl Rudolphi (1808), from Ancient Greek νῆμα ( nêma, nêmatos , 'thread') and -ειδἠς ( -eidēs , 'species'). It was treated as family Nematodes by Burmeister (1837). At its origin, the "Nematoidea" erroneously included Nematodes and Nematomorpha , attributed by von Siebold (1843). Along with Acanthocephala , Trematoda , and Cestoidea , it formed

3572-454: The clades Ascaridina, Spirurina, and Trichocephalida have been discovered in coprolites from the Oligocene -aged Tremembé Formation, which represented a palaeolake in present-day São Paulo with a diverse fossil assemblage of birds, fish, and arthropods that lent itself to fostering high nematode diversity. Nematodes have also been found in various lagerstätten , such as Burmese amber ,

3666-412: The intercalary segment (an appendageless segment between the antenna and mandible), and also in the midgut. Loss of function of lab results in the failure of the Drosophila embryo to internalize the mouth and head structures that initially develop on the outside of its body (a process called head involution). Failure of head involution disrupts or deletes the salivary glands and pharynx. The lab gene

3760-651: The meiobenthos . They play an important role in the decomposition process, aid in recycling of nutrients in marine environments, and are sensitive to changes in the environment caused by pollution. One roundworm of note, C. elegans , lives in the soil and has found much use as a model organism . C. elegans has had its entire genome sequenced, the developmental fate of every cell determined, and every neuron mapped. Nematodes that commonly parasitise humans include ascarids ( Ascaris ), filarias , hookworms , pinworms ( Enterobius ), and whipworms ( Trichuris trichiura ). The species Trichinella spiralis , commonly known as

3854-413: The uterus until they hatch. The juvenile nematodes then ingest the parent nematode. This process is significantly promoted in environments with a low food supply. The nematode model species C. elegans , C. briggsae , and Pristionchus pacificus , among other species, exhibit androdioecy , which is otherwise very rare among animals. The single genus Meloidogyne (root-knot nematodes) exhibits

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3948-520: The "Cbx" enhancer mutation, it represses wing genes, and the wings develop as halteres, resulting in a four-haltered fly. In Drosophila , abd-A is expressed along most of the abdomen, from abdominal segments 1 (A1) to A8. Expression of abd-A is necessary to specify the identity of most of the abdominal segments. A major function of abd-A in insects is to repress limb formation. In abd-A loss-of-function mutants, abdominal segments A2 through A8 are transformed into an identity more like A1. When abd-A

4042-559: The Adenophorea were not in a uniform group. Initial studies of incomplete DNA sequences suggested the existence of five clades : The Secernentea seem to be a natural group of close relatives, while the "Adenophorea" appear to be a paraphyletic assemblage of roundworms that retain a good number of ancestral traits . The old Enoplia do not seem to be monophyletic, either, but do contain two distinct lineages. The old group " Chromadoria " seems to be another paraphyletic assemblage, with

4136-455: The Adenophorea. In 2019, a study identified one conserved signature indel (CSI) found exclusively in members of the phylum Nematoda through comparative genetic analyses. The CSI consists of a single amino acid insertion within a conserved region of a Na(+)/H(+) exchange regulatory factor protein NRFL-1 and is a molecular marker that distinguishes the phylum from other species. A major effort by

4230-511: The Bithorax complex, which together were historically referred to as the HOM-C (for Homeotic Complex). Although historically HOM-C genes have referred to Drosophila homologues, while Hox genes referred to vertebrate homologues, this distinction is no longer made, and both HOM-C and Hox genes are called Hox genes. Mice and humans have 39 Hox genes in four clusters: The ancestors of vertebrates had

4324-594: The Introverta—depending on the validity of the former—are often ranked as a superphylum . For an up-to-date view (as of 2022), see Phylogenomic Analysis of the Phylum Nematoda: Conflicts and Congruences With Morphology, 18S rRNA, and Mitogenomes . Due to the lack of knowledge regarding many nematodes, their systematics is contentious. An early and influential classification was proposed by Chitwood and Chitwood —later revised by Chitwood —who divided

4418-675: The Nematoda has always been considered to be well resolved. Morphological characters and molecular phylogenies agree with placement of the roundworms as a sister taxon to the parasitic Nematomorpha ; together, they make up the Nematoida . Along with the Scalidophora (formerly Cephalorhyncha), the Nematoida form the clade Cycloneuralia , but much disagreement occurs both between and among the available morphological and molecular data. The Cycloneuralia or

4512-630: The ancestral protostome Hox genotype, which shows that the reduction has occurred within the nematode phylum. Nematode species can be difficult to distinguish from one another. Consequently, estimates of the number of nematode species are uncertain. A 2013 survey of animal biodiversity published in the mega journal Zootaxa puts this figure at over 25,000. Estimates of the total number of extant species are subject to even greater variation. A widely referenced article published in 1993 estimated there may be over 1 million species of nematode. A subsequent publication challenged this claim, estimating

4606-408: The animal can thrust into its prey. In some species, the style is hollow and can be used to suck liquids from plants or animals. The oral cavity opens into a muscular, sucking pharynx , also lined with cuticle. Digestive glands are found in this region of the gut, producing enzymes that start to break down the food. In stylet-bearing species, these may even be injected into the prey. No stomach

4700-465: The bee less active, thus less effective in pollen collection. Depending on its species, a nematode may be beneficial or detrimental to plant health. From agricultural and horticulture perspectives, the two categories of nematodes are the predatory ones, which kill garden pests; and the pest nematodes, which attack plants, or act as vectors spreading plant viruses between crop plants. Predatory nematodes include Phasmarhabditis hermaphrodita which

4794-451: The body compared to 5' Hox genes that are not induced by retinoic acid resulting in expression domains that remain more posterior. Quantitative PCR has shown several trends regarding colinearity: the system is in equilibrium and the total number of transcripts depends on the number of genes present according to a linear relationship. In some organisms, especially vertebrates, the various Hox genes are situated very close to one another on

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4888-402: The cause of soil-transmitted helminthiases . They are taxonomically classified along with arthropods , tardigrades and other moulting animals in the clade Ecdysozoa . Unlike the vaguely similar flatworms , nematodes have a tubular digestive system , with openings at both ends. Like tardigrades, they have a reduced number of Hox genes , but their sister phylum Nematomorpha has kept

4982-412: The chromosome in groups or clusters. The order of the genes on the chromosome is the same as the expression of the genes in the developing embryo, with the first gene being expressed in the anterior end of the developing organism. The reason for this colinearity is not yet completely understood, but could be related to the activation of Hox genes in a temporal sequence by gradual unpacking of chromatin along

5076-495: The different species and plotted the protein sequence types onto the phylogenetic tree of the species. The approach identified the proteins that best represent ancestral forms ( Hox7 and Antp ) and the proteins that represent new, derived versions (or were lost in an ancestor and are now missing in numerous species). Hox genes act at many levels within developmental gene hierarchies: at the "executive" level they regulate genes that in turn regulate large networks of other genes (like

5170-495: The digestive system is the result of the body movements of the worm. The intestine has valves or sphincters at either end to help control food movement through the body. Nitrogenous waste is excreted in the form of ammonia through the body wall, and is not associated with any specific organs. However, the structures for excreting salt to maintain osmoregulation are typically more complex. In many marine nematodes, one or two unicellular ' renette glands ' excrete salt through

5264-438: The disc makes a leg, resulting in a leg coming out of the fly's head. The third thoracic segment, or T3, bears a pair of legs and a pair of halteres (highly reduced wings that function in balancing during flight). Ubx patterns T3 largely by repressing genes involved in wing formation. The wing blade is composed of two layers of cells that adhere tightly to one another, and are supplied with nutrient by several wing veins. One of

5358-409: The eggs on them infect any organism that consumes them. A. lumbricoides is the largest intestinal roundworm and is the most common helminth infection of humans worldwide. Infestation can cause morbidity by compromising nutritional status, affecting cognitive processes, inducing tissue reactions such as granuloma to larval stages, and by causing intestinal obstruction , which can be fatal. The body

5452-445: The eggshell is complicated and includes several layers; a detailed anatomical and terminological framework has been proposed for these layers in 2023. Nematodes as a whole possess a wide range of modes of reproduction. Some nematodes, such as Heterorhabditis spp., undergo a process called endotokia matricida : intrauterine birth causing maternal death. Some nematodes are hermaphroditic , and keep their self-fertilized eggs inside

5546-549: The eight Hox gene clusters (a Hoxd cluster) has lost all protein-coding genes, and just a single microRNA gene marks the location of the original cluster. In some teleost fish, such as salmon , an even more recent genome duplication occurred, doubling the seven or eight Hox gene clusters to give at least 13 clusters Another teleost, the freshwater butterflyfish , has instead seen a significant loss in HOX gene clusters, with only 5 clusters present. Vertebrate bodies are not segmented in

5640-476: The eighth and ninth segments of the Drosophila abdomen. Both the regulatory protein and the morphogenic protein are involved in the development of the tail segment. Proteins with a high degree of sequence similarity are also generally assumed to exhibit a high degree of functional similarity, i.e. Hox proteins with identical homeodomains are assumed to have identical DNA-binding properties (unless additional sequences are known to influence DNA-binding). To identify

5734-426: The female, meaning their fertilized eggs may not yet be developed. A few species are known to be ovoviviparous . The eggs are protected by an outer shell, secreted by the uterus. In free-living roundworms, the eggs hatch into larvae , which appear essentially identical to the adults, except for an underdeveloped reproductive system; in parasitic roundworms, the lifecycle is often much more complicated. The structure of

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5828-479: The figure to be at least 40,000 species. Although the highest estimates (up to 100 million species) have since been deprecated, estimates supported by rarefaction curves , together with the use of DNA barcoding and the increasing acknowledgment of widespread cryptic species among nematodes, have placed the figure closer to 1 million species. Nematodes have successfully adapted to nearly every ecosystem : from marine (salt) to fresh water, soils, from

5922-400: The first used term in case of synonyms. The phylogenetic relationships of the nematodes and their close relatives among the protostomian Metazoa are unresolved. Traditionally, they were held to be a lineage of their own, but in the 1990s, they were proposed to form the group Ecdysozoa together with moulting animals, such as arthropods . The identity of the closest living relatives of

6016-490: The formation of visible galls (e.g. by root-knot nematodes), which are useful characters for their diagnostic in the field. Some nematode species transmit plant viruses through their feeding activity on roots. One of them is Xiphinema index , vector of grapevine fanleaf virus , an important disease of grapes, another one is Xiphinema diversicaudatum , vector of arabis mosaic virus . Other nematodes attack bark and forest trees. The most important representative of this group

6110-427: The gene pathway that forms an appendage). They also directly regulate what are called realisator genes or effector genes that act at the bottom of such hierarchies to ultimately form the tissues, structures, and organs of each segment. Segmentation involves such processes as morphogenesis (differentiation of precursor cells into their terminal specialized cells), the tight association of groups of cells with similar fates,

6204-467: The genera occur as parasites of vertebrates ; about 35 nematode species occur in humans. The word nematode comes from the Modern Latin compound of nemat- 'thread' (from Greek nema , genitive nematos 'thread', from the stem nein 'to spin'; cf. needle ) + -odes 'like, of the nature of' (cf. -oid ). In 1758, Linnaeus described some nematode genera (e.g., Ascaris ), then included in

6298-443: The genes have been separated by chromosomal rearrangements. Comparing homeodomain sequences between Hox proteins often reveals greater similarity between species than within a species; this observation led to the conclusion that Hox gene clusters evolved early in animal evolution from a single Hox gene via tandem duplication and subsequent divergence, and that a prototypic Hox gene cluster containing at least seven different Hox genes

6392-579: The homeodomain protein motif, are found in most eukaryotes . The Hox genes, being a subset of homeobox genes, arose more recently in evolution within the animal kingdom or Metazoa . Within the animal kingdom, Hox genes are present across the bilateria (animals with a clear head-to-tail axis), and have also been found in Cnidaria such as sea anemones . This implies that Hox genes arose over 550 million years ago. In bilateria, Hox genes are often arranged in gene clusters, although there are many exceptions where

6486-450: The infected ants for berries, and eat them. Parasite eggs passed in the bird's feces are subsequently collected by foraging C. atratus and are fed to their larvae , thus completing the lifecycle of M. neotropicum . Similarly, multiple varieties of nematodes have been found in the abdominal cavities of the primitively social sweat bee, Lasioglossum zephyrus . Inside the female body, the nematode hinders ovarian development and renders

6580-418: The inner surface of muscle cells towards the nerve cords ; this is a unique arrangement in the animal kingdom, in which nerve cells normally extend fibers into the muscles rather than vice versa . The oral cavity is lined with cuticles, which are often strengthened with structures, such as ridges, especially in carnivorous species, which may bear several teeth. The mouth often includes a sharp stylet , which

6674-418: The larval body, generally in the trunk region, that will be maintained through metamorphosis. In larvae with complete metamorphosis the Hox genes are mainly expressed in juvenile rudiments and are absent in the transient larval tissues. The larvae of the hemichordate species Schizocardium californicum and the pilidium larva of Nemertea do not express Hox genes. An analogy for the Hox genes can be made to

6768-417: The many genes that Ubx represses is blistered, which activates proteins involved in cell-cell adhesion, and spalt, which patterns the placement of wing veins. In Ubx loss-of-function mutants, Ubx no longer represses wing genes, and the halteres develop as a second pair of wings, resulting in the famous four-winged flies. When Ubx is misexpressed in the second thoracic segment, such as occurs in flies with

6862-475: The maternal protein Bicoid, this position is occupied by lysine , which recognizes and binds to the nucleotide guanine . In Antennapedia, this position is occupied by glutamine , which recognizes and binds to adenine . If the lysine in Bicoid is replaced by glutamine, the resulting protein will recognize Antennapedia-binding enhancer sites. However, all homeodomain-containing transcription factors bind essentially

6956-425: The maxilla and mandible of the head (activates reaper) positions (represses decapentaplegic) distal limb that will form digit, carpal and tarsal bones (activates EphA7) monocytes (white blood cells), with cell cycle arrest (activates Cdkn1a) The DNA sequence bound by the homeodomain protein contains the nucleotide sequence TAAT, with the 5' terminal T being the most important for binding. This sequence

7050-437: The name the same. Despite Potts' classification being equivalent to Cobbs', both names have been used (and are still used today) and Nematode became a popular term in zoological science. Since Cobb was the first to include nematodes in a particular phylum separated from Nematomorpha, some researchers consider the valid taxon name to be Nemates or Nemata, rather than Nematoda, because of the zoological rule that gives priority to

7144-460: The nemas and gordiids was realized by Vejdovsky when he named a group to contain the horsehair worms the order Nematomorpha. In 1919, Nathan Cobb proposed that nematodes should be recognized alone as a phylum. He argued they should be called "nema" in English rather than "nematodes" and defined the taxon Nemates (later emended as Nemata, Latin plural of nema ), listing Nematoidea sensu restricto as

7238-411: The obsolete group Entozoa , created by Rudolphi (1808). They were also classed along with Acanthocephala in the obsolete phylum Nemathelminthes by Gegenbaur (1859). In 1861, K. M. Diesing treated the group as order Nematoda. In 1877, the taxon Nematoidea, including the family Gordiidae (horsehair worms), was promoted to the rank of phylum by Ray Lankester . The first clear distinction between

7332-491: The ovum to begin dividing, but because no fusion of the cells occurs, the male contributes no genetic material to the offspring, which are essentially clones of the female. The nematode Caenorhabditis elegans is often used as a model organism for studying aging at the molecular level. For example, in C. elegans aging negatively impacts DNA repair , and mutants of C. elegans that are long-lived were shown to have increased DNA repair capability. These findings suggest

7426-422: The phylum into two classes— Aphasmidia and Phasmidia . These were later renamed Adenophorea (gland bearers) and Secernentea (secretors), respectively. The Secernentea share several characteristics, including the presence of phasmids , a pair of sensory organs located in the lateral posterior region, and this was used as the basis for this division. This scheme was adhered to in many later classifications, though

7520-500: The polar regions to the tropics, as well as the highest to the lowest of elevations. They are ubiquitous in freshwater, marine, and terrestrial environments, where they often outnumber other animals in both individual and species counts, and are found in locations as diverse as mountains, deserts, and oceanic trenches . They are found in every part of the Earth's lithosphere , even at great depths, 0.9–3.6 km (3,000–12,000 ft) below

7614-569: The protein Ascaris Trypsin Inhibitor ( pdb 1ATA ). Ascaris has been present in humans for at least several thousand years, as evidenced by Ascaris eggs found in paleofeces and in the intestines of mummified humans . A. lumbricoides was originally called Lumbricus teres and was first described in detail by Edward Tyson in 1683. The genus Ascaris was originally described as the genus for Ascaris lumbricoides by Carl Linnaeus in 1758. The morphologically similar Ascaris suum

7708-447: The role of a play director who calls which scene the actors should carry out next. If the play director calls the scenes in the wrong order, the overall play will be presented in the wrong order. Similarly, mutations in the Hox genes can result in body parts and limbs in the wrong place along the body. Like a play director, the Hox genes do not act in the play or participate in limb formation themselves. The protein product of each Hox gene

7802-467: The same DNA sequence. The sequence bound by the homeodomain of a Hox protein is only six nucleotides long, and such a short sequence would be found at random many times throughout the genome, far more than the number of actual functional sites. Especially for Hox proteins, which produce such dramatic changes in morphology when misexpressed, this raises the question of how each transcription factor can produce such specific and different outcomes if they all bind

7896-627: The same sequence. One mechanism that introduces greater DNA sequence specificity to Hox proteins is to bind protein cofactors. Two such Hox cofactors are Extradenticle (Exd) and Homothorax (Hth). Exd and Hth bind to Hox proteins and appear to induce conformational changes in the Hox protein that increase its specificity. Just as Hox genes regulate realisator genes, they are in turn regulated themselves by other genes. In Drosophila and some insects (but not most animals), Hox genes are regulated by gap genes and pair-rule genes , which are in their turn regulated by maternally-supplied mRNA . This results in

7990-537: The same way as insects; they are on average much more complex, leading to more infrastructure in their body plan compared to insects. HOX genes control the regulation and development of many key structures in the body, such as somites , which form the vertebrae and ribs, the dermis of the dorsal skin, the skeletal muscles of the back, and the skeletal muscles of the body wall and limbs. HOX genes help differentiate somite cells into more specific identities and direct them to develop differently depending on where they are in

8084-448: The sculpting of structures and segment boundaries via programmed cell death, and the movement of cells from where they are first born to where they will ultimately function, so it is not surprising that the target genes of Hox genes promote cell division, cell adhesion, apoptosis , and cell migration. (represses distal-less) (represses distal-less) required for normal visceral morphology (activates decapentaplegic) boundary between

8178-508: The set of proteins between two different species that are most likely to be most similar in function, classification schemes are used. For Hox proteins, three different classification schemes exist: phylogenetic inference based, synteny-based, and sequence similarity-based. The three classification schemes provide conflicting information for Hox proteins expressed in the middle of the body axis ( Hox6-8 and Antp, Ubx and abd-A ). A combined approach used phylogenetic inference-based information of

8272-837: The surface of the Earth in gold mines in South Africa. They represent 90% of all animals on the ocean floor . In total, 4.4 × 10 nematodes inhabit the Earth's topsoil, or approximately 60 billion for each human, with the highest densities observed in tundra and boreal forests. Their numerical dominance, often exceeding a million individuals per square meter and accounting for about 80% of all individual animals on Earth, their diversity of lifecycles, and their presence at various trophic levels point to an important role in many ecosystems. They have been shown to play crucial roles in polar ecosystems. The roughly 2,271  genera are placed in 256  families . The many parasitic forms include pathogens in most plants and animals. A third of

8366-414: The trichina worm, occurs in rats, pigs, bears, and humans, and is responsible for the disease trichinosis . Baylisascaris usually infests wild animals, but can be deadly to humans, as well. Dirofilaria immitis is known for causing heartworm disease by inhabiting the hearts, arteries, and lungs of dogs and some cats. Haemonchus contortus is one of the most abundant infectious agents in sheep around

8460-452: The wasp, and their offspring await the birth of the next generation of wasps as the fig ripens. A parasitic tetradonematid nematode discovered in 2005, Myrmeconema neotropicum , induces fruit mimicry in the tropical ant Cephalotes atratus . Infected ants develop bright red gasters (abdomens), tend to be more sluggish, and walk with their gasters in a conspicuous elevated position. These changes likely cause frugivorous birds to confuse

8554-432: The world, causing great economic damage to sheep. In contrast, entomopathogenic nematodes parasitize insects and are mostly considered beneficial by humans, but some attack beneficial insects. One form of nematode is entirely dependent upon fig wasps , which are the sole source of fig fertilization. They prey upon the wasps, riding them from the ripe fig of the wasp's birth to the fig flower of its death, where they kill

8648-516: Was described from pigs by Johann August Ephraim Goeze in 1782. Nematode (see text ) The nematodes ( / ˈ n ɛ m ə t oʊ d z / NEM -ə-tohdz or NEEM - ; Ancient Greek : Νηματώδη ; Latin : Nematoda ), roundworms or eelworms constitute the phylum Nematoda . They are a diverse animal phylum inhabiting a broad range of environments. Most species are free-living, feeding on microorganisms , but many species are parasitic . The parasitic worms ( helminths ) are

8742-409: Was initially so named because it disrupted the labial appendage; however, the lab gene is not expressed in the labial segment, and the labial appendage phenotype is likely a result of the broad disorganization resulting from the failure of head involution. The pb gene is responsible for the formation of the labial and maxillary palps. Some evidence shows pb interacts with Scr . The Dfd gene

8836-450: Was present in the common ancestor of all bilaterian animals. In most bilaterian animals , Hox genes are expressed in staggered domains along the head-to-tail axis of the embryo, suggesting that their role in specifying position is a shared, ancient feature. The functional conservation of Hox proteins can be demonstrated by the fact that a fly can function to a large degree with a chicken Hox protein in place of its own. So, despite having

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