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9 letter words whose second letter is E

In conjunction to the ‘Archipterygium hypothesis’ (, right side), ‘Fin fold hypothesis’ (, centre) and new ideas related with gene patterning, we will examine the tail bud as a structure from which potentially the developmental mechanism for the appendage development was co-opted. This idea builds up on previously suggested similarities between the tail and limb buds (; ) and the hypothesis of Axis paramorphism (, ).

Aegophony (n.) Same as Egophony

The structural origin of the vertebrates’ paired limbs is still an unsolved problem. Historically, two hypotheses have been raised to explain the origin of vertebrate limbs: the Archipterygium Hypothesis and the Fin Fold Hypothesis. Current knowledge provides support for both ideas. In the recent years, it has been also suggested that (1) all appendages correspond to body axis duplications and (2) they are originated by the ventralization of the developmental program present in the median fins. The tail bud is also a relevant structure in the attempt to understand the origin of the vertebrates’ limbs. Due to their similarities in gene expression and general organization, both structures should be studied more closely to understand their potential evolutionary link. Interestingly, in non-vertebrate chordates such as , it is possible to find a tail fin that during development expresses several genes that are conserved with other vertebrates’ limbs and tails. This shared gene expression could be considered as an evidence of potential co-option of the same genetic tool kit from the tail to the extremities. This observation is congruent with the hypothesis of Axis paramorphism, which previously suggested similarities between the tail and limb buds.

Fins are usually the most distinctive anatomical features of a fish

(archipterygium hypothesis).

Traditionally there have been suggested two alternatives as potential ancestral structures to vertebrates’ extremities: continuous lateral fin folds (centre; Lateral Fin Fold Hypothesis) or branchial gills (right; Archipterygium Hypothesis). We propose that the same genes involved on tail fin development could later have been co-opted in other fins (left; Tail Bud Hypothesis). It is also possible to postulate another scenario where the genetic tool kit moves from the branchial gills to the ribbon fin or from the tail fin into the branchial gills.

In the middle of 19th century, , ) proposed that the limbs might be derived from the gill arches based on observations in Chondrichthyes’ fins and in the archipterygial fins found in the Australian lungfish (Krefft, 1870) (). He proposed that the archipterygial axis present in the fins corresponds to the extended gill radial and its gill arch would give rise to the pectoral girdle. It has also been reported that during the breeding season the male pelvic fin of (Fitzinger, 1837), also a lungfish, becomes a gill-like organ (reviewed by ). Despite this being a seasonal change, it may reveal a developmental relationship between those structures. Many genes are expressed in both gill arches and limbs in tetrapods. ) published an extensive review of the genes expressed in limb and branchial arch in the mouse. There are a numerous genes expressed in both organs belonging to different signaling pathways such as Fgf (, , , and ), Shh (, , , and ), Wnt ( and ) and Bmp (, and ). In addition, there is a long list of shared transcription factors, which include: and . Other examples are , and , which are expressed in the ectoderm of the gill arch and in the apical ectodermal ridge (AER) of the limb (). The gene () is expressed in the mesenchyme of both structures. (), on the other hand, is expressed in the ectoderm of the branchial arch and in both the mesenchyme and ectoderm of the limb (). Additional interesting cases are and , these genes are expressed in the mesenchyme of the branchial arch and in the AER of the limb (). Studies in the little skate (Mitchill, 1825) () verified that gene expression patterns typical of the limb are found in the gill arches. is expressed in the epithelium covering the gill arch and its receptor is expressed in the underlying mesenchyme. Meanwhile, is expressed in the posterior region of the epithelium and has a regulatory feedback with . These patterns bear similarity with the expression of these genes during limb development. Furthermore, the exogenous application of retinoic acid (RA) or generates mirror duplication on the gill arch skeleton as it happens in the extremities. On the other hand, the gill arch of the ray has a ridge of pseudostratified epithelium, which closely resembles the AER of limb buds. Shared gene expression between gill arches and developing extremities in different vertebrates supports the anatomical based hypothesis of Gegenbaur. However, a more systematic survey across different developmental pathways and vertebrate species is required.

Karl Gegenbaur noted that the most reliable clue to evolutionary ..

The Conservatives selected merchant banker Neil Balfour and the Liberals ..

The structural origin of the vertebrates’ paired limbs is still an unsolved problem. In the 19th century, morphologists proposed two explanations for the origin of the limbs/fins: The ‘Archipterygium Hypothesis’ and the ‘Fin Fold Hypothesis’ (both reviewed by ). Later, it was suggested that the extremities were related with side folds in the Cambrian vertebrates and . This has been rejected due to lack of evidence of a skeletal and muscular support, which are the distinctive features of true limbs ().

Traditionally there have been suggested two alternatives as potential ancestral structures to vertebrates’ extremities: continuous lateral fin folds (centre; Lateral Fin Fold Hypothesis) or branchial gills (right; Archipterygium Hypothesis). We propose that the same genes involved on tail fin development could later have been co-opted in other fins (left; Tail Bud Hypothesis). It is also possible to postulate another scenario where the genetic tool kit moves from the branchial gills to the ribbon fin or from the tail fin into the branchial gills.

Note taking. Smashing Pumpkins. Mathematics. Physics. ノート。数学。物理学。
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cva exam 3 Flashcards | Quizlet

The structural origin of the vertebrates’ paired limbs is still an unsolved problem. Historically, two hypotheses have been raised to explain the origin of vertebrate limbs: the Archipterygium Hypothesis and the Fin Fold Hypothesis. Current knowledge provides support for both ideas. In the recent years, it has been also suggested that (1) all appendages correspond to body axis duplications and (2) they are originated by the ventralization of the developmental program present in the median fins. The tail bud is also a relevant structure in the attempt to understand the origin of the vertebrates’ limbs. Due to their similarities in gene expression and general organization, both structures should be studied more closely to understand their potential evolutionary link. Interestingly, in non-vertebrate chordates such as , it is possible to find a tail fin that during development expresses several genes that are conserved with other vertebrates’ limbs and tails. This shared gene expression could be considered as an evidence of potential co-option of the same genetic tool kit from the tail to the extremities. This observation is congruent with the hypothesis of Axis paramorphism, which previously suggested similarities between the tail and limb buds.

really a modified biserial archipterygium, ..

The idea that limb and tail buds present similar development was first mentioned by ) and later suggested again by other authors (). At the same time it also matches the idea of Axis paramorphism as long as the tail is considered as an appendage itself. Histologically the VER and the AER correspond to an ectodermal epithelial tissue that covers proliferative mesenchyme. In both cases the epithelium/mesenchyme interaction is important for the proliferation of mesodermal cells (). In zebrafish there is a ‘tail organizer’, in the sense of been the source of signaling pathway components such as Wnt, Bmp and Nodal (), in a similar way that the ZPA is a source of Shh in the tetrapod limb bud (). Additionally, in mouse, a mutation in the gene produces the phenotype called in which the VER and the AER are very thin and there is an abnormal development of both limbs and tail (; ). Concerning the development of limbs, the Shh pathway has been studied extensively and it is associated with the antero-posterior polarization processes (). The presence of Shh was detected in the posterior regions of tetrapod limbs (), teleost fish (), the little skate () and sharks ((Müller and Henle, 1838) and ) (; ) (). The caudal fin of zebrafish expresses transcripts of several genes (e.g. and ) present in the Shh signaling pathway (; ). In mouse there is expression in the caudal region, however it is in the future spinal cord area (). As this expression occurs later in development (day 9.5) it is probably not related with the formation of the tail itself. Elements from the Bmp pathway are expressed recurrently in both structures (). For example, is expressed in the chicken AER (), the mouse limb bud () and the zebrafish pectoral fin (). It is also present in the mouse VER from the earliest stages until the growth of the tail finishes. Another gene from this pathway is , in mouse it is present in the AER (), but not in the VER (). In addition, many BMPs have been detected in the caudal fin primordium of zebrafish (). A final example is Bmp11, which is present in the tail bud and also in the limb bud of (). Several proteins of the Wnt pathway are found in vertebrate limbs and tails (). Wnt3a is expressed in mice limbs (), as well as the most caudal portion of the tail bud (). Mice carrying null alleles for have truncated tail bud development, but there was no major effect on the extremities (). It could suggest the expression of other genes with redundant functions or the fact that is actually involved in other developmental processes on the limb. For chicken has been reported in the AER (). In zebrafish, Wnt3a is expressed in the AER () and morpholino knockdowns of and completely blocked the formation of the tail (). Consistent with this phenotype, expression is detected at the tip of the tail in zebrafish (). Moreover, the exogenous application of Wnt8c on the flank of chicken embryos induces the formation of an ectopic limb (). and are also expressed in the chicken AER (). The first one has a role related with the growth of the underlying mesenchyme (). The same gene is expressed in the pectoral fins of medaka, (Temminck and Schlegel, 1846) (). In mouse, is involved in the proliferation of branchial arches, facial protrusions, limb bud, VER (), fingers and genitals. In the mutant many of these tissues, including the tail and limbs, present a truncation in their growth (). The expression of this gene in the branchial arches could also be considered as an argument in favor of the Archipterygium Hypothesis. In addition, has a pattern of expression in the tail that is very similar to the one observed for (). On the other hand, during the regeneration process of the tadpole tail, it is possible to detect the expression of and (). Another example is , which is expressed in the tail bud of zebrafish (), chicken () and (), as well as in the limbs of chicken () and mouse (). Finally, the effector of the Wnt pathway, , is expressed in the mouse limbs and tail (). A very important gene family for limb development corresponds to the Fgf genes; interestingly very few of these genes are expressed in the tail (). On mouse AER the genes and are expressed, but only the latter is present in the VER (). In zebrafish, is expressed in the pectoral fin, tail and gill arches (). Another gene in this family, , is expressed in the mesenchyme of the pectoral fin () and in the tail bud of zebrafish (). No orthologues were found for this gene in tetrapods, but it is present in Chondrichthyes (). Functionally in mice, the maintenance of the AER depends only on Fgf10 () and there is no presence of this transcript in the VER (). The mutant mouse for this gene lacks lungs and anterior and posterior limbs (). Along the same line, during the regeneration process of the tadpole tail there is expression of and (). The Sprouty family of proteins is antagonist of receptor tyrosine kinases, including FGF receptors. and are expressed in the mouse extremities and in the VER (; ) (). In addition, is expressed in the zebrafish pectoral fin (). Also, there are a number of common transcription factors between the two structures (). Several genes of the Msx family ( and) are expressed in pectoral fins and the fin fold, including the caudal fin of zebrafish (). In mice, , functionally related to in zebrafish (), is expressed in the VER () and AER (). Other example is , which is expressed in the mouse limb and the zebrafish fin (), as well as in the tails of both organisms (). Another transcription factor that is found in a wide variety of appendages is (). The Tbx transcription factors are also important in limb and tail development (). In chicken, is expressed in the AER and in the tail bud, among other structures (). In the Japanese newt, (Boie, 1826), is expressed in the tail and the limb (). The Hox genes are usually related with segmental differentiation, but they also present shared expression between tail and limbs (). In Mexican axolotls, (Shaw and Nodder, 1798), there is expression of and (short transcript) in the tip of the tail as well as in the hindlimb and in lower levels of the forelimbs (). In mice, () and () are expressed in the limb and tail bud. In zebrafish, the genes (), (), and () are expressed in the pectoral fin and the tail bud. Finally, in there is also expression of in the tail fin (). ) proposed a possible relationship between the adult caudal fin of fishes and the autopod of tetrapods. The author suggests that the genes could be responsible for the axis bending which causes the heterocercal condition in fishes in the same way as they are responsible for the proximodistal finger specification of the tetrapod limb. In this scenario, genes would have been recruited secondarily for limb development. All these similarities between the genetic mechanism involved in limb and tail formation are also congruent with the Axis paramorphism idea (, ). On this conceptual framework, both structures could be considered as repetitions of the main body axis. Note that the tail is also a structure that presents sexual dimorphism. It has been documented on the tail length of birds () and snakes (); number of vertebrae in salamanders (); and colours on birds () and fish (). While it is often possible to identify mutations with a limb phenotype having no consequence in the tail or vice versa, this could be explained by the existence of functional redundancies in one of the tissues.

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