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91-514: Trichomes likely serve a variety of purposes. In larvae, trichomes likely help with larval locomotion. By alternating between bands of trichomes and naked cuticle, larvae can tread across different surfaces. Additionally, trichomes may contribute to hydrophobicity and even stabilize adult flight. The shavenbaby locus is regulated by multiple signaling pathways, including the HOX factors, Wingless , EGF-R , Hedgehog , and Notch signaling. Additionally,

182-494: A body plan is seen in snakes . Where a typical vertebrate like a mouse has only around 60 vertebrae, snakes have between around 150 to 400, giving them extremely long spinal columns and enabling their sinuous locomotion . Snake embryos achieve this by accelerating their system for creating somites (body segments), which relies on an oscillator. The oscillator clock runs some four times faster in snake than in mouse embryos, initially creating very thin somites. These expand to adopt

273-605: 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

364-627: A binding site for the repressor Abrupt. These mutations contributed to a 46% decrease in total embryonic shavenbaby expression, and affected the pleiotropic expression in the pupal epidermis. The Z1.3 enhancer is a minimalized fragment of the Z enhancer, and drives expression in the embryonic quaternary cells, the larval pharynx and proventriculus, and the pupal epidermis. The Z1.3 enhancer contributed to an estimated 28% loss of total embryonic expression in Drosophila sechellia. However, unlike in E6,

455-511: A claim also made my Preger Ben-Noon et al. The E6 enhancer is expressed in the dorsal and quaternary cells of Drosophila embryos, larvae, and in the pupal epidermis. The E6 enhancer is one of the five enhancers that contributed to the loss of the larval dorsal trichomes in Drosophila sechellia . The molecular mechanism for this loss of expression was resolved by Preger Ben-Noon et al., where sechellia-E6 consecutively accumulated mutations in activator sites for Arrowhead and Pannier and gained

546-485: 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

637-713: A larva. The proposal implied (if it were correct) a shared phylogeny of tunicates and vertebrates, and that heterochrony was a principal mechanism of evolutionary change. Modern evolutionary developmental biology (evo-devo) studies the molecular genetics of development. It seeks to explain each step in the creation of an adult organism from an undifferentiated zygote in terms of the control of expression of one gene after another. Further, it relates such patterns of control of development to phylogeny . De Beer to some extent anticipated such late 20th-century science in his 1930 book Embryos and Ancestors , showing that evolution could occur by heterochrony, such as in paedomorphosis,

728-481: A number of behavior changes, as a result of increased brain plasticity and extended childhood. Progenesis (or paedogenesis) can be observed in the axolotl ( Ambystoma mexicanum ). Axolotls reach full sexual maturity while retaining their fins and gills (in other words, still in the juvenile form of their ancestors). They will remain in aquatic environments in this truncated developmental form, rather than moving onto land as other sexually mature salamander species. This

819-417: A particular process, such as the action of a single toolkit gene , relative to the ancestral condition or to other conspecifics, depending on whether inter- or intraspecific heterochrony is the focus. Identifying which of the six perturbations is occurring is critical in identifying the actual underlying mechanism driving peramorphosis or paedomorphosis. A dramatic illustration of how acceleration can change

910-413: A rate similar to that of humans, but growth stops soon after birth, whereas humans continue brain and head growth several years after birth. This particular type of heterochrony, hypermorphosis, involves a delay in the offset of a developmental process, or what is the same, the presence of an early developmental process in later stages of development. Humans have some 30 different neotenies in comparison to

1001-420: A segment (for example, legs, antennae, and wings in fruit flies), and Hox genes in vertebrates specify the types and shape of vertebrae that will form. In segmented animals, Hox proteins thus confer segmental or positional identity, but do not form the actual segments themselves. Studies on Hox genes in ciliated larvae have shown they are only expressed in future adult tissues. In larvae with gradual metamorphosis

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1092-566: A short and bulbous braincase. As an organism such as this aged, they would change greatly in their cranial morphology to develop a robust skull with larger, overlapping bones. Birds, however, retain this juvenile morphology. Evidence from molecular experiments suggests both fibroblast growth factor 8 (FGF8) and members of the WNT signalling pathway have facilitated paedomorphosis in birds. These signalling pathways are known to play roles in facial patterning in other vertebrate species. This retention of

1183-454: A significant effect on the enhancer expression pattern. Furthermore, the mutations affected multiple components of the pattern. This pleiotropic nature of the mutations was demonstrated when the emergence of novel salivary gland or mouth hook expression was linked with the nearly complete loss of the original embryonic expression pattern. Additionally, changes to the low-affinity Ultrabithorax binding sites resulted in pleiotropic effects modulating

1274-579: 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

1365-418: A slight decrease in the number of ventral trichomes. A closer look at the nuclei of these individual cells reveals both lower quanitifiable levels of the shavenbaby transcript and weaker nuclear microenvironment interactions between the ventral enhancers . Interestingly, transcript levels and the microenvironment can be stabilized by crossing flies carrying the deletion with flies carrying an artificial BAC of

1456-695: 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

1547-432: 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

1638-437: 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

1729-409: A typical vertebrate shape, elongating the body. Giraffes gain their long necks by a different heterochrony, extending the development of their cervical vertebrae; they retain the usual mammalian number of these vertebrae, seven. This number appears to be constrained by the use of neck somites to form the mammalian diaphragm muscle; the result is that the embryonic neck is divided into three modules,

1820-410: 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

1911-552: 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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2002-455: 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

2093-435: 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

2184-556: Is expressed in the dorsal and ventral regions, in the pharynix, esophagus, and proventriculus, and in the pupal epidermis. A closer look at the ventral nuclei reveals that the shavenbaby gene physically colocalizes with higher concentrations of the Ultrabithorax protein and its cofactor Homothorax. Additionally, the Drosophila line Df(svb)108 contains a deletion in the DG2, DG3, and Z enhancers. Heat shocking these lines does induce

2275-470: Is now used in a sense contrary to his coinage; Haeckel had assumed that embryonic development (ontogeny) of "higher" animals recapitulated their ancestral development ( phylogeny ), as when mammal embryos have structures on the neck that resemble fish gills at one stage. This, in his view, necessarily compressed the earlier developmental stages, representing the ancestors, into a shorter time, meaning accelerated development. The ideal for Haeckel would be when

2366-433: Is responsible for a wide variety of effects such as the lengthening of the fingers by adding extra phalanges in dolphins to form their flippers, sexual dimorphism , and the polymorphism seen between insect castes . Walter Garstang suggested the neotenous origin of the vertebrates from a tunicate larva, in opposition to Darwin's opinion that tunicates and vertebrates both evolved from animals whose adult form

2457-518: Is responsible for activating shavenbaby both directly and by driving expression of the factors SoxNeuro and Dichaete . Other transcription factors such as Ultrabithorax and its cofactor Homothorax also interact with the different shavenbaby enhancers to activate expression. During stage 12, the Hedgehog signaling pathway induces expression of the Wingless signal. The Wingless signaling pathway

2548-462: 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,

2639-435: Is responsible for repressing shavenbaby activity, and cells expressing Wingless have naked cuticle. Furthermore, mutations to the Wingless gene produce a lawn of trichomes in the naked region. Wingless signaling has been characterized to specifically integrate at the shavenbaby E3 enhancer, which also produces a lawn of expression in Wingless mutants. Wingless signaling is repressed by both SoxNeuro and Dichaete, products of

2730-428: 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

2821-592: Is the extinct Irish elk . From the fossil record, its antlers spanned up to 12 feet (3.7 m) wide, which is about a third larger than the antlers of its close relative, the moose . The Irish elk had larger antlers due to extended development during their period of growth. Another example of peramorphosis is seen in insular (island) rodents. Their characteristics include gigantism, wider cheek and teeth, reduced litter size, and longer lifespan. Their relatives that inhabit continental environments are much smaller. Insular rodents have evolved these features to accommodate

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2912-482: 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

3003-403: Is thought to be a form of hypomorphosis (earlier ending of development) that is both hormonally and genetically driven. The entire metamorphosis that would allow the salamander to transition into the adult form is essentially blocked by both of these drivers. Paedomorphosis by progenesis may play a critical role in avian cranial evolution . The skulls and beaks of living, adult birds retain

3094-565: 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

3185-467: 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

3276-412: 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

3367-461: The shavenbaby locus. The studies from Tsai et al. reveals microenvironments and potentially transvection to be potential mechanisms for how redundant enhancers canalize gene expression. The 7H enhancer drives expression in both the ventral and dorsal embryonic and larval epidermis, the larval pharynx, and the pupal epidermis. Deletion of the 7H enhancer results in a 38% decrease in total embryonic shavenbaby expression. 7H, DG3, and E3N are

3458-492: The timing , pattern intensity, and ectopic expression. The authors concluded that enhancers are densely encoded with regulatory information and enhancer mutations are usually pleiotropic. Other recent studies in the yellow spot enhancer and the Sonic Hedgehog ZRS enhancer also support this claim. These findings may even suggest that the underlying cis-regulatory logic of an enhancer may constrain its evolution,

3549-520: 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

3640-530: The "Hox Paradox", by suggesting that low-affinity binding sites would provide the specificity, and encoding clusters of the sites would account for the potential weak activation. Low-affinity transcription factor binding sites have also been observed in other enhancers. In a follow-up study, Fuqua et al. created a library of random mutants to the E3N enhancer to study the enhancer grammar and how enhancers can evolve. The study revealed that even single point mutations had

3731-520: 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

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3822-682: The EGFR signaling pathway. Developmental enhancers are DNA sequences which control the spatial-temporal patterning of genes during development to set up the bodyplan of an organism. Developmental enhancers are thought to be the main drivers of phenotypic evolution. There are currently seven putative developmental enhancers in the shavenbaby locus: DG2, DG3, Z1.3, A, E3, E6, and 7H . All of these enhancers are pleiotropic , expressing shavenbaby across different developmental stages. The enhancers are somewhat modular, where different patterning components are partitioned to different enhancers. However, many of

3913-431: The Hox genes are activated in tissues of 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

4004-476: The Hox genes can be made to 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

4095-428: The abundance of food and resources they have on their islands. These factors are part of a complex phenomenon termed Island syndrome or Foster's rule . The mole salamander , a close relative to the axolotl, displays both paedomorphosis and peramorphosis. The larva can develop in either direction. Population density, food, and the amount of water may have an effect on the expression of heterochrony. A study conducted on

4186-435: The anatomy of the juvenile theropod dinosaurs from which they evolved. Extant birds have large eyes and brains relative to the rest of the skull; a condition seen in adult birds that represents (broadly speaking) the juvenile stage of a dinosaur. A juvenile avian ancestor (as typified by Coelophysis ) would have a short face, large eyes, a thin palate, narrow jugal bone, tall and thin postorbitals, restricted adductors, and

4277-451: 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

4368-492: The body. A large difference between vertebrates and invertebrates is the location and layering of HOX genes. The fundamental mechanisms of development are strongly conserved among vertebrates from fish to mammals. Heterochrony In evolutionary developmental biology , heterochrony is any genetically controlled difference in the timing, rate, or duration of a developmental process in an organism compared to its ancestors or other organisms. This leads to changes in

4459-533: The cause of these trends. These ideas were reinforced by other studies, such as peramorphosis in the Puerto Rican tree frog . Another reason could be generation time , or the lifespan of the species in question. When a species has a relatively short lifespan, natural selection favors evolution of paedomorphosis (e.g. Axolotl: 7–10 years). Conversely, in long lifespans natural selection favors evolution of peramorphosis (e.g. Irish Elk: 20–22 years). Heterochrony

4550-412: 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

4641-417: The denticles. Hox gene Hox genes , a subset of homeobox genes , are a group of related genes that specify regions of the body plan of an embryo along the head-tail axis of animals. Hox proteins encode and specify the characteristics of 'position', ensuring that the correct structures form in the correct places of the body. For example, Hox genes in insects specify which appendages form on

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4732-421: The development of every part of an organism was thus accelerated, but he recognised that some organs could develop with displacements in position ( heterotopy , another concept he originated) or time (heterochrony), as exceptions to his rule. He thus intended the term to mean a change in the timing of the embryonic development of one organ with respect to the rest of the same animal, whereas it is now used, following

4823-411: The development of the organism into an adult, and has been described as "eternal childhood". In this form of heterochrony, the developmental stage of childhood is itself extended, and certain developmental processes that normally take place only during childhood (such as accelerated brain growth in humans ), is also extended throughout this period. Neoteny has been implicated as a developmental cause for

4914-495: 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

5005-438: 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

5096-552: 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

5187-476: 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

5278-474: The expression patterns overlap with each other making the enhancers seemingly redundant. Enhancer redundancy is a commonly observed phenomenon. Why would evolution evolve redundant enhancers ? The mystery of enhancer redundancy was partially resolved by studying the shavenbaby locus in 2010. Frankel et al. found that the redundant enhancers help maintain proper shavenbaby expression under different temperature stresses, canalizing its expression. This finding

5369-427: 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,

5460-443: 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

5551-584: 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

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5642-430: The identity of cells, locking them into a fate to produce a particular structure such as wings, halteres, antennae, abdomen, etc. All of the HOX factors are evolutionarily related, and bind to the same homeodomain sequence: TAAT. How enhancers encode the specific binding of certain HOX factors and prevent the ectopic binding of others is called the "Hox Paradox". The E3N study from Crocker et al., 2015 provided an answer to

5733-400: The juvenile ancestral state has driven other changes in the anatomy that result in a light, highly kinetic (moveable) skull composed of many small, non - overlapping bones. This is believed to have facilitated the evolution of cranial kinesis in birds which has played a critical role in their ecological success. Peramorphosis is delayed maturation with extended periods of growth. An example

5824-417: 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

5915-479: 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

6006-425: 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

6097-470: The middle one (C3 to C5) serving the diaphragm. The assumption is that disrupting this would kill the embryo rather than giving it more vertebrae. Heterochrony can be identified by comparing phylogenetically close species, for example a group of different bird species whose legs differ in their average length. These comparisons are complex because there are no universal ontogenetic timemarkers. The method of event pairing attempts to overcome this by comparing

6188-404: The mole salamander in 1987 found it evident that a higher percentage of individuals became paedomorphic when there was a low larval population density in a constant water level as opposed to a high larval population density in drying water. This had an implication that led to hypotheses that selective pressures imposed by the environment, such as predation and loss of resources, were instrumental to

6279-452: The mutations that affected the embryonic pattern of Z1.3 had no effect on its pleiotropic pupal epidermis expression. Preger Ben-Noon et al. further dissected the Z1.3 enhancer and were able to minimalize the pleiotropic activity into two separate enhancers: Z0.3 and Z1.3R . The DG3 enhancer is primarily expressed in the ventral embryonic epidermis along with E3N and 7H. In larvae, DG3

6370-428: The pendulum in favour of Darwin's original theory. If the larvaceans constitute a recent re-enactment of an ancient Garstang scenario, they should find closer kinship with some modern sea squirts than with others. Alas, this is not so. Several heterochronies have been described in humans, relative to the chimpanzee . In chimpanzee fetuses , brain and head growth starts at about the same developmental stage and grow at

6461-418: The primary ventral enhancers in the embryo. Shavenbaby activates over 150 different downstream targets to express actin-remodeling proteins to form the denticle. Some of these factors include forked, shavenoid, singed, wasp, yellow, and miniature . Activation of these target genes is also dependent on SoxNeuro, one of the regulators of shavenbaby. Together, SoxNeuro and Shavenbaby act cooperatively to shape

6552-548: The rate or timing of a descendant species relative to its ancestor. This can result in either paedomophosis (truncating the ancestral ontogeny), peramorphosis (extending past the ancestral ontogeny), or isomorphosis (reaching the same ancestral state via a different mechanism). There are three major mechanisms of heterochrony, each of which can change in either of two directions, giving six types of perturbations, which can be combined in various ways. These ultimately result in extended, shifted, or truncated development of

6643-588: The relative timing of two events at a time. This method detects event heterochronies, as opposed to allometric changes. It is cumbersome to use because the number of event pair characters increases with the square of the number of events compared. Event pairing can however be automated, for instance with the PARSIMOV script. A recent method, continuous analysis, rests on a simple standardization of ontogenetic time or sequences, on squared change parsimony and phylogenetic independent contrasts . Paedomorphosis can be

6734-443: The result of neoteny , the retention of juvenile traits into the adult form as a result of retardation of somatic development, or of progenesis, the acceleration of developmental processes such that the juvenile form becomes a sexually mature adult. This means that in progenesis, germ cell growth is accelerated relative to normal or in neoteny; while somatic cell growth is normal in progenesis, but retarded in neoteny. Neoteny retards

6825-424: The retention of juvenile features in the adult. De Beer argued that this enabled rapid evolutionary change, too brief to be recorded in the fossil record , and in effect explaining why apparent gaps were likely. Heterochrony can be divided into intraspecific and interspecific types. Intraspecific heterochrony means changes in the rate or timing of development within a species. For example, some individuals of

6916-448: The salamander species Ambystoma talpoideum delay the metamorphosis of the skull. Reilly and colleagues argue we can define these variant individuals as paedotypic (with truncated development relative to the ancestral condition), peratypic (with extended development relative to the ancestral condition), or isotypic (reaching the same ancestral shape, but via a different mechanism). Interspecific heterochrony means differences in

7007-467: 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

7098-627: 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

7189-537: 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

7280-448: 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

7371-508: 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

7462-484: The size, shape, characteristics and even presence of certain organs and features. It is contrasted with heterotopy , a change in spatial positioning of some process in the embryo, which can also create morphological innovation. Heterochrony can be divided into intraspecific heterochrony, variation within a species, and interspecific heterochrony, phylogenetic variation, i.e. variation of a descendant species with respect to an ancestral species. These changes all affect

7553-519: The start, end, rate or time span of a particular developmental process. The concept of heterochrony was introduced by Ernst Haeckel in 1875 and given its modern sense by Gavin de Beer in 1930. The concept of heterochrony was introduced by the German zoologist Ernst Haeckel in 1875, where he used it to define deviations from recapitulation theory , which held that " ontogeny recapitulates phylogeny ". As Stephen Jay Gould pointed out, Haeckel's term

7644-444: The transcription factors SoxNeuro, Pointed, and Dichaete regulate shavenbaby expression. During stage 12 of embryonic development, Engrailed is expressed in a subset of cells, which activates the hedgehog signaling pathway. The Hedgehog signal is received by cells expressing Patched, which induces expression of rhomboid ( rho ) with Serrate-Notch signaling, which activates the EGFR signaling pathway. The drosophila EGF receptor (DER)

7735-414: The ventral side of stages 15 and 16+ embryos and larvae. E3 is also expressed pleiotropically in the pharynx and esophagus or third-instar larvae. In adult Drosophila, E3 is expressed in the abdomen, head, legs, and wing. The E3 fragment has been tested as smaller fragments such as E3-14 and E3N . Unlike the other shavenbaby enhancers, E3 activity is maintained in Drosophila sechellia . E3N

7826-513: The work of the British evolutionary embryologist Gavin de Beer in 1930, to mean a change with respect to the development of the same organ in the animal's ancestors. In 1928, the English embryologist Walter Garstang showed that tunicate larvae shared structures such as the notochord with adult vertebrates , and suggested that the vertebrates arose by paedomorphosis (neoteny) from such

7917-421: Was also observed eight years later for redundant mammalian enhancers, suggesting that this observation is not limited to Drosophila . Redundant enhancers have also been observed to use different transcription factors , incorporating a diverse set of signaling inputs to canalize gene expression under different environmental stresses. The E3 enhancer is a 1,042 base-pair (bp) enhancer which drives shavenbaby on

8008-495: Was first described in Crocker et al., 2015, and was found to encode "homotypic clusters" of binding sites for the transcription factor : Ultrabithorax (Ubx). These binding sites, however, were non-canonical, and Ubx binds to E3N at a very low-affinity. Mutations to increase the affinity of these binding sites caused the ectopic binding of other Homeobox (HOX) factors, resulting in ectopic enhancer expression. HOX factors license

8099-409: 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

8190-450: 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

8281-447: Was similar to (frog) tadpoles and the 'tadpole larvae' of tunicates. According to Richard Dawkins , Garstang's opinion was also held by Alister Hardy , and is still held by some modern biologists. However, according to others, closer genetic investigation rather seems to support Darwin's old opinion: Garstang's theory is certainly an attractive one, and it was much in favour for many years ... Unfortunately, recent DNA evidence has swung

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