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1927 - Brachyury (T) locus was introduced to the world in a report describing the effects of a mutation at this locus on both embryonic viability in homozygotes and the development of the tail in heterozygotes
'''1927''' - Brachyury (T) locus was introduced to the world in a report describing the effects of a mutation at this locus on both embryonic viability in homozygotes and the development of the tail in heterozygotes

Revision as of 13:35, 21 October 2016

2016 Student Projects 
Signalling: 1 Wnt | 2 Notch | 3 FGF Receptor | 4 Hedgehog | 5 T-box | 6 TGF-Beta
2016 Group Project Topic - Signaling in Development

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T-box genes and their signalling pathway


The T-Box genes encode for T-box proteins, a family of transcription factors with more than 20 members identified in humans so far, and homologues in many other organisms. These T-box proteins are termed transcription factors because of their ability to regulate the expression and subsequent activity of other genes by binding to DNA. T-box proteins are characterised by a DNA-binding motif known as the T-box that binds DNA. Many researchers have identified important roles of the T-box genes in the development of the heart and limbs. T-box genes have also found to regulate patterning and cell fate, cell survival, and/or proliferation. And so, mutations in these genes lead to human disorders including Di-George Syndrome and Holt-Oram syndrome. [1]

Mark's comments - what the signalling pathway does - clearly identify exactly what this page will talk about - eg. this pathway in reference to this system/embryonic development etc - limitations of this page - what we won't discuss

PMID 9504043

Origins of the T-box name

The founding member of the T-box family is brachyura which comes from the greek and means short tail [1]. This gene was discovered following experimental studies with a short tailed mouse that harboured a mutation which affected tail length and embryonic development (Dobrovolskaïa-Zavadskaïa, 1927). Brakhus means short in greek and oura meaning tail.

Nadezhda Alexandrovna Dobrovolskaya-Zavadskaya first described the brachyury mutation in 1927 as a mutation that affected tail length and sacral vertebrae in heterozygous mice [2].

The brachyury gene (which is also known as T) soon was studied in great detail due to its important role in the development of the notochord and posterior mesoderm. Mutations in T are shown to cause embryonic lethality in homozygote mice and short tails in heterozygote mice [1]. Now according to human and mouse genomes, the gene brachyura has the symbol T and gene name T. However the gene is described as brachyury.

Ancient origins and evolution of the T-box gene family

T-box gene family is ancient in origin and it is thought to be found in all metazoans [3]. Due to the increasing availability of sequenced genomes from a diverse group of animal taxa we now know that the origin of the T-box family has been pushed back to unicellular organisms and fungi, in which one or two T-box genes, including T, have been identified. [4]

With the analysis of the genomes of bilaterian organisms and representatives of the different phyla, four basal metazoan phyla indicate that T is the most ancient member of the T-box family and that the family has expanded throughout metazoan evolution. [1] In Metazoan evolution it seems that genes were added progressively one by one by gene or genome duplication and some of these genes were lost and gained in specific lineages [1]. In present day or extant vertebrates we now know that the T-box family has radiated throughout the vast vertebrate linages and can be grouped into five subfamilies T, Tbx1, Tbx2, Tbx6 and Tbr1 [1]. In the common ancestor of vertebrates and sponges, four of these five subfamilies were already present [4].

Brachyury expression in 7.5dpc CD1 mouse embryos.jpg

Brachyury expression in 7.5dpc CD1 mouse embryos Image from here

The discovery of T-box genes

The story of the T-box genes began in Paris at the Pasteur laboratory in the 1920s with the Russian scientist Nadine Dobrovolskaïa-Zavadskaïa, who embarked on a pioneering screen for X-ray-induced developmental mouse mutants. Her isolation of a mouse strain with a short tail, caused by a semidominant heterozygous mutation in a locus she called T, represented one of the first successful mammalian genetic screens, and provided one of the earliest links between gene activity and cell behaviour during embryogenesis[5]. The mid-gestational death of homozygous T embryos, with perturbed development of the posterior mesoderm and notochord, demonstrated an essential requirement for T during gastrulation, and led to the earliest insights into the inductive influences of notochord on neural tube and somite development. Over 60 years later T, now also known as brachyury, meaning ‘short tail’ in Greek, was cloned in one of the earliest positional cloning efforts in the mouse[6]. At the time, lack of homology in the T-gene product to any previously characterized protein gave no clues as to its biochemical role until, in 1993, it was revealed to be a novel sequence-specific DNA-binding protein[7]. Crystallographic determination of the structure of the DNA-binding domain, now called the Tbox, revealed a new way through which proteins recognize DNA[8].

TO DO: Produce a timeline for the history of the TBox gene - type TBOx into PubMed and sort the findings by age - use this to see when the first journal articles where released related to TBox and limb development, how this research has improved and developed, where it is heading in the future


1927 - Brachyury (T) locus was introduced to the world in a report describing the effects of a mutation at this locus on both embryonic viability in homozygotes and the development of the tail in heterozygotes

Over the following decades, the embryological defects caused by the T mutation were studied.

1990 - The T gene itself was cloned

1992 - The discovery of sequence homology between the mouse T gene and a newly cloned Drosophila gene called "omb"

1994 - Bollag and his colleagues showed the existence of a family of T-related genes in the mouse genome, which was christened the T-box gene family.

1998- Alison Isaac and the team confirmed that in chicken embryo, Tbx-2 & Tbx-3 are related to both forelimb and hindlimb development, and Tbx-4 & Tbx-5 have limited expression domains in leg and wing respectively.

Comments - timeline put references - eg. the papers from 1990 etc so that they are useful for other students - this is part of the criteria for the project

Features of the T-box family

The defining feature of the T-box gene family is a conserved domain that was first uncovered in the sequence of the mouse T locus, or Brachyury gene. [9] This homology domain encodes a polypeptide region that has been named the T-box. [3] The following table outlines the functions of some important T-box genes, including their location and associated human diseases.

Typical tbx protein structure.png

Summary of the main T-box genes

T-box gene Main expression sites during embryogenesis Function Abnormalities
Tbx1 Pharyngeal endoderm, mesoderm core of the first pharyngeal arch, head mesoderm ventral to hindbrain, sclerotome Pharyngeal arch arteries development, governs the transition between stem cell quiescence and proliferation in hair follicles, associated with developmental abnormalities with the ear, facial and cardiac outflow DiGeorge syndrome, Velocardiofacial syndrome, Conotruncal anomaly face syndrome, Tetralogy of Fallot
Tbx2 Allantois, non-chanmber myocardium, optic and otic vesicles, naso-facial mesenchyme, limbs, lungs, genitalia Potent immortalizing gene that acts by downregulating CDKN2A, regulates Anf expression in chamber myocardium development None identified
Tbx3 Non-chamber myocardium (sinoatrial region, AV canal and interventricular ring), expressed with TBX2, TBX3, and TBX5 in the embryonic neural retina Provides positional information important for topographic mapping in differentiation of distinct cell types across the laminar axis of the retina, development of functional ectopic pacemakers, stimulates Nanog, Tbx3 specifies digit III and the combination of Tbx2 and Tbx3 specifies digit IV, acting together with the interdigital BMP signaling cascade. The fetal lung, kidney, heart, liver, and spleen in humans expresses this gene Ulnar-mammary syndrome
Tbx4 Hindlimb, mandibular and lung mesenchyme, atrium and body wall Developmental pathways of the lower limbs and the pelvis in humans Ischiocoxopodopatellar syndrome, Small patella syndrome
Tbx5 Cardiac cresent, heart tube, sinus venous, common atrium, left ventricle (LV) and right ventricle (RV) forelimb, eye Promotes cardiomyocyte differentiation, interaction with GATA4 cause of human cardiac septal defects Holt-Oram syndrome
Tbx18 Splanchnic mesoderm, septum traversum, epicardium Maintain the separation of anterior and posterior somite compartments, specification of ureteral mesenchyme and SMC differentiation in the ureter Congenital anomalies of kidney and urinary tract 2
Tbx20 Allantois, lateral plate mesoderm, cardiac crescent, heart tube, hindbrain, eye Cardiac development and yolk sac vascular remodeling Atrial septal defect 4

Table adapted from: Table 1. Embryonic expression and mutant phenotypes of mouse cardiac T-box genes [10]

Functions of T-box in development

T-box genes are a growing family of transcription factors that are expressed in diverse patterns throughout vertebrate development. They have emerged as key players in embryonic patterning, tissue differentiation and morphogenesis, particularly in vertebrates and some specific examples have been described in the following sections.

Cardiac development

The heart is one of the first organs to develop and function in vertebrate embryos. Its formation is a complex process and requires contributions from multiple transcription factors, including GATA-4, eHAND, dHAND, Irx4, and TBX genes [11].

In the developing heart, Tbx5 expression can be first detected at stage 12 along the entire rostrocaudal length of the fused heart tube[12]. Although it is expressed very early in cardiac embryogenesis, Tbx5 is not essential for cardiac crescent formation or for development of the early heart tube. It is at the stage where growth of the posterior segment (atria and left ventricle) of the heart which requires Tbx5, and does not occur in embryos that lack a Tbx5. In contrast, RV and outflow tract development appears to be Tbx5 independent[11]. It is interesting to note that the role of TBX5 in the growth and maturation of posterior heart is evolutionarily conserved from amphibia (Xenopus) to mammals[13].

The signalling mechanism of TBX5 involves interaction with NKX2-5 which synergistically promotes cardiomyocyte differentiation. Both these molecules bind directly to the promoter of the gene encoding cardiac-specific natriuretic peptide precursor type A (NPPA) alongside each other, and the 2 transcription factors lead to synergistic activation[14]. Moreover, GATA4 a transcription factor essential for heart formation has been shown to interact with TBX5. A mutation of GATA4 can result in human congenital heart defects as this transcription factor is essential for functional separation of the four cardiac chambers[15]. Therefore Tbx5 transcription factor is involved in directing gene expression in morphogenetic processes associated with specific chamber formation and any mutations can cause heart septal defects as seen in Holt–Oram syndrome[16] (See section below on Abnormalities for further info).

Furthermore, in the secondary heart field another Tbx gene, Tbx1 has been found to function in both growth and differentiation. Fibroblast growth factors (Fgfs) 8 and 10 are key downstream effectors of Tbx1 that are expressed in secondary heart field mesoderm and associated endoderm, as well as weakly in the outflow tract. Tbx1 -> Fgf8/Fgf10 pathway in driving proliferation in secondary heart field mesoderm, contributing to OFT growth[17]. BMP proteins are also induced in secondary heart field cells proximal to the inflow and outflow poles of the heart[18]. BMPs can induce cardiomyogenic differentiation in collaboration with Fgf8 and can moderate the proliferation of secondary heart field mesoderm. Therefore, a delicate balance between the levels of Fgf and BMP factors appears essential for secondary heart field development[19].

PMID 11702954 PMID 11572777 PMID 15580613 PMID 16258075 PMID 17460765

Limb Development

Limb formation occurs as a result of interplay between fibroblast growth factor (FGF) and Wnt signaling. What initiates these signaling cascades and thus limb bud outgrowth at defined locations involve four members of the T-box family of transcription factors (Tbx2-Tbx5) as well as other molecules which are expressed in developing limb buds. Limb bud outgrowth is initiated and maintained by establishing a positive feedback loop of FGF signaling comprised of Fgf10 expressed in the lateral plate mesoderm (LPM), inducing the expression of Fgf8 in the overlying, distal ectoderm. Initial expression of Fgf10 in the forelimb- and hindlimb-forming LPM is controlled by Tbx transcription factors, Tbx5 in the forelimb and Tbx4 in the hindlimb, and deletion of either Tbx5 or Tbx4 causes outgrowth defects of limb buds.

The expression of Tbx4 and Tbx5, is primarily restricted to the developing hindlimbs and forelimbs, respectively. While the expression of Tbx2 and Tbx3 is present in both limbs[20].

Using the chick model, Tbx genes have been proven to specify posterior digit identity through Shh and BMP signaling. [21]. In particular, Tbx2 acts upstream of Shh and BMP2, and Tbx3 regulates BMP2. Conversely, Shh and BMP4 upregulate the posterior expression of Tbx2 and Tbx3. These lines of evidence suggest that the feedback and feedforward regulation between Tbx2/3 and the Shh and BMP signaling cascades is pivotal for the specification of posterior digit identities. Furthermore, RA and Shh both induced Tbx2[22].

Tbx4 and Tbx5 genes are also implicated in the development of the limbs. They are first expressed in lateral plate mesoderm within clearly defined territories at the time the prospective limb fields are being specified by Homeobox (Hox) genes[23]. Hox genes may therefore be responsible for regulating expression of these Tbox genes within the limb fields. Fgf-10 expression is also initiated in lateral plate mesoderm around this time, and FGF10 is a good candidate for the mesodermal factor that initiates limb outgrowth and signals the adjacent ectoderm to express FGF8 [24][25].

PMID 14723846 PMID 9609833 PMID 9655805 PMID 9550719 PMID 8798150 PMID 11782414 PMID 12490567 PMID 22872086 PMID 12736212 PMID 26212321 PMID 21932311

Limb induction-initiation signal 01.jpg

Close up of Tbx5 role in the Initiation of Limb Bud Formation[26]

http://www.ncbi.nlm.nih.gov/books/NBK10003/ http://dev.biologists.org/content/130/3/623/F1 From the limb development lecture, two major signalling pathways that involve Tbx: 1) FGF signaling (FGF10 and FGF8) 2) Wnt signaling pathway

Respiratory Development

In a chicken model, all Tbx2 subfamily genes, Tbx2, Tbx3, Tbx4 and Tbx5 are expressed in the developing lung buds and trachea between stages 15–21 [27]. In the mouse however, Tbx1 is expressed in lung epithelium at E12.5, Tbx2 and Tbx3 are expressed in lung mesenchyme at E11.5, and Tbx4 and Tbx5 are expressed in both lung and trachea mesenchyme at E12.5 and later[28].

In lung branching morphogenesis, Tbx4 and Tbx5 genetically interact with one another. In an article by Ripla et al. (2012), they demonstrated that both Tbx4 and Tbx5 are expressed throughout the mesenchyme of the developing lung and trachea[29]. They showed that a loss of Tbx5 leads to a unilateral loss of lung bud specification and absence of tracheal specification in organ culture. Concordant with this defect, the expression of mesenchymal markers Wnt2 and Fgf10, as well as Fgf10 target genes Bmp4 and Spry2, in the epithelium is downregulated. This suggests that Fgf10 signaling pathway is activated downstream of Tbx4 and Tbx5 in the developing lung and that Fgf10 genetically interacts with Tbx4 and Tbx5. This is consistent with finding from Sakiyama et al. (2003) that found out that the Tbx4-Fgf10 system controls lung bud formation during chicken embryonic development. Tbx4 was found to trigger Fgf10 expression in the lung primordium mesoderm which then acquires an inductive capability for the initial budding morphogenesis of primary lung buds[30]. Of significance, lung-specific Tbx4 heterozygous;Tbx5 conditional null mice died soon after birth due to respiratory distress. These pups have small lungs and show severe disruptions in tracheal/bronchial cartilage rings, highlighting the important role of Tbx4 and Tbx5 in respiratory development.

Tbx in lung and trachea development.png

Tbx in lung and trachea development[29]

Other developmental events

TBX signalling also involved in palate development. In humans, TBX1 mutation is responsible for the major phenotypes of 22q11.2 deletion syndrome (Velo-cardio-facial/DiGeorge syndrome, discussed in Abnormalities section) as well as non-syndromic submucous cleft palate, suggesting that Tbx1 is a regulator of palatogenesis. A study by Funato et al. (2012) revealed that Tbx1 regulates oral epithelial adhesion and palatal development and showed that Tbx1 -/- mice exhibit various forms of cleft palate phenotypes, including submucosal cleft palate and soft palate cleft.

Dorsoventral Patterning of the Mouse Coat by Tbx15 PMID 14737183 Palate Development PMID 22371266

Tbx6 interacting with Ripply for the formation of somite boundaries (in zebrafish) PMID 25725067 Tbx6 is required for the expression of mesp-b and ripply1 in the paraxial mesoderm during somite formation, and for the specification of the central Pax3+/Pax7+ dermomyotome. Mesp-b is necessary and sufficient for central dermomyotome formation, it inhibits myogenic differentiation and promotes dermomyotome development. Ripply1 function is required for maturation and fast muscle fiber differentiation.

Results show that Tbx6 protein has to be removed for the expression of pax3/7 and myoD in the lateral paraxial mesoderm, indicating that Tbx6 and/or Tbx6-dependent genes inhibit maturation of myogenic cells. Downstream of Tbx6, Mesp-ba promotes dermomyotome development.


A number of human disorders have been linked to mutations in T-box genes, confirming their medical importance. They include Holt– Oram syndrome/TBX5, Ulnar-Mammary syndrome/TBX3, and more recently DiGeorge syndrome/TBX1, ACTH deficiency/TBX19 and cleft palate with ankyloglossia/TBX22. [31] [32] [33]

Z5039628 (talk) 19:37, 1 September 2016 (AEST)


The TBX1 gene can be mapped on chromosome 21 within the DiGeorge syndrome region. Studies using mice, have shown that the TBX1 gene is haploinsufficient.[34] This means that while although mice are diploid organisms, only one functional copy of the gene exists, while the other copy has undergone a mutation, making it inactivated. This single gene is unable to produce sufficient amounts of the protein needed for TBX1 to carry out its function- the development of pouches and pharyngeal arches in these mice. In this study of mice, a deficiency of the TBx1 gene resulted in heart defects, hence suggesting the close association between the TBx1 gene and cardiovascular development.

In this study, a mouse model was used where the relevant region on chromosome 16 of the mice species, corresponding to chromosome 22q11.2, or del22q11, in humans, was deleted. [35] These 'mutated' mice exhibited a range of cardiovascular abnormalities and defects, as well as behavioural changes. They also revealed incomplete or mutated pharangeal arch and pouch development. Tbx 1 (7-9) was identified as the gene responsible for these cardiovascular malformations. The normal development of the cochlear and vestibular organs was also observed in these mice. Thus, Tbx1 was also identified as crucial for the development of otic epithelial cells, which then later contribute to the development of these inner ear organs. These defects are also exhibited in the birth defects in humans, hence making this mouse model highly effective and suitable. [34]


a-c: ums patient and d-f: mother of patient, normal [36]

Mutations of the TBX3 gene leads to ulnar-mammary syndrome, caused by a reduce in the levels of the functional proteins needed for normal development of limbs, mammary glands and other structures. Like DiGeorge Syndrome, this syndrome is a result of the haploinsufficiency of TBX3. This disorder is expressed in abnormalities of the limbs, teeth, genitals and mammary glands. [37] Again, animal models of mice have shown abnormalities in mammary glands, limbs and genitalia, often dying before birth. These abnormalities are often characterised by short, stunted growth of the hindlimbs of mice, as well as missing elements to the forelimb. Images of these can be seen on the right.

On the other hand, when this gene is abundant and over-expressed, cancers in the breast, liver and skin have been seen to develop, as high levels of this gene assist in the development of tumours. Lung cancers, breast cancers, ovarian cancers, bladder cancers and liver tumours have been shown to have high levels of the TBX3 gene. [37]


shortened thumb (Fig. 1A). radial flexion (Fig. 1B). enlarged heart (Fig. 1C). [38]

Mutations of the TBX5 gene has been shown to cause defects in cardiac septation and the production of isomers in humans affected with holt-oran syndrome. [39] The TBX5 gene is mutated in affected individuals, on chromosome 12q24. [40] Holt-Oram syndrome is an autosomal-dominant disorder, and in humans, is expressed by deformities of the upper limb, the shoulder girdle as well as defects in the septa of the heart. Animal models such as chicks have been used, where the overexpression of this gene in chicken embryos has resulted in incomplete growth of the myocardium and the trabeculae and septa of the heart. In humans, the overexpression and mutation of this gene has also been shown to inhibit normal cardiac development. This is because TBX5 is responsible for the normal proliferation of cells during cardiac development. [41]Thumb anomaly is another expression of holt-oran syndrome, where the thumb may be completely absent, or may develop as another finger-like digit, not different from the other digits of the finger. Other indicators of this disease are heart and skeletal lesions, patent ductus arteriosus (PDA), malformation of the ventricles in the heart, mitral valve prolapse and superior vena cava anomaly. [40] These thumb abnormalities can be seen in the images on the left.


The TBx19 gene initiates the transcription process of the Proopiomelanocortin (POMC) gene. [34] This gene contains the instructions for the synthesis of the proopiomelanocortin protein. This protein is further transformed into smaller peptides which bind to proteins in the body, initiating various signalling pathways throughout the body. When this gene is mutated, congenital isolated adrenocorticotropic hormone deficiency develops, caused by the reduction of the secretion of Isolated adrenocorticotropic hormone (ACTH) from the anterior pituitary gland. This is because ACTH-producing cells of the pituitary gland are damaged and cannot secrete sufficient amounts of adrenocorticotropic hormone. Cite error: Invalid <ref> tag; invalid names, e.g. too many This results in severe hypoglycaemia and seizures in neonates. Cholestatic Liver disease also arises from this mutation, blocking or reducing the normal flow of bile through the liver. Cite error: Invalid <ref> tag; invalid names, e.g. too many This disease has also been seen to cause a range of other clinical symptoms, including urinary incontinence, gait disturbance and dementia in older patients. Ventricular enlargement in the brain, loss of appetite and vomiting are other symptoms associated with isolated ACTH deficiency. The main treatment for this disease is hormone replacement therapy. [34]


Studies on mice with cleft palate have shown that the mutations of the gene encoding TBX22, causing the gene to no longer function. Tbx22 is involved in the development of the intramembranous bone formation of the posterior hard palate, rather than palate closure, and hence a mutation of this gene results in cleft palate. These mutations of the TBX22 gene included frame shifts of the gene, splices, nonsense and missense changes to the gene sequence. TBX22 is regulated by the Mn1 transcription factor, working together to achieve normal palate development. Mutations of Tbx1 and Tbx10 are also responsible for cleft palate. The mutation of this gene also causes ankyloglossia, the development of a short and thick lingual frenulum under the tongue, limiting tongue movement. This can be corrected by surgery. Choanal atresia, a blockage of the nasal airway, was also seen in affected mice. Other developmental mutations can be seen in the incomplete formation of the vomer bone in the skull. [42] [43] [44]

Still get image - Marianne

Z5039628 (talk) 23:16, 8 September 2016 (AEST)

Animal models

The gene brachyury is important in all bilateral organisms (vertebrates- chordates and invertebrates such as mollusca). The brachyury gene is believed to have a conserved role in defining the midline of a bilateral organism [45] and are also fundamental in the establishment of the anterior-posterior axis [46]

It is thought to play a role in the development of organisms in the Phylum Cnidaria, appears to be in defining the blastopore during early development[47]. It is also important during gastrulation where it defines the mesoderm [48] and experiments using tissue culture have demonstrated that the gene brachyury is also important in controlling the velocity of cells as they leave the primitive streak. [49]

Another important role that Brachyury has also been shown to have is to help establish the cervical vertebral blueprint during (human) fetal development. In mammals the number of cervical vertebrae is highly conserved; however a spontaneous vertebral and spinal dysplasia (VSD) mutation in this gene has been associated with the development of six or fewer cervical vertebrae instead of the usual seven. [50]

The T-box gene family have been identified in organisms ranging from hydra to humans and due to extensive research by many investigators , we now know that T-box is important in metazoan development including transcriptional activity, genetic targets, developmental regulatory functions, and associated disease mechanisms (reviewed in Papaioannou, 2001).

Organisms used in animal models for T-Box

Animal model studies have been performed in a number of organisms including: Drosophila (the fruit fly), Xenopus (a genus of aquatic clawed frog native to sub-Saharan Africa) zebrafish, avians, and mice. These different animals are used to examine T-box gene regulation in the developmental and disease process (reviewed in Naiche et al., 2005).

T-box genes are involved in the development and patterning of many organ systems and embryonic structures including the heart, limb, eye, central axis, and face. In addition, T-box genes are subject to regulation by or induce the expression of developmentally important signaling molecules, such as retinoic acid (RA), bone morphogenetic proteins (BMPs), fibroblast growth factors (FGFs), and Wnts, in different organ systems (reviewed in Naiche et al., 2005). T-box proteins can act as transcriptional activators or repressors with a variety of cofactors to regulate expression of genes involved in cell lineage determination, differentiation, and maturation (reviewed in Tada and Smith, 2001). Overall, T-box genes are integrated into regulatory networks that control patterning, growth, and maturation of many cell types and tissues in the developing embryo.

File:Evolution of T box gene Family.jpg

This figure is modified from Papaioannou (2014) [1] and shows the subfamilies or classes of genes that have been identified in the different animal groups that are indicated in green in the boxes. What can be demonstrated is that there has been a remarkable conservation of transcription factors between lineages that have been evolving independently since the last common ancestor to metazoans, and many of the T-box gene families have their origin at the base of the tree. However, diversification at the onset of metazoan evolution is evident. T, which is the most ancient T-box gene, is represented in unicellular organisms, as is Tbx7/8, a class not present in Bilateria. Sponges have a diverse set of T-box genes including several that have not been retained in Bilateria, whereas Tbx6 subfamily genes apparently arose in Bilateria. Note that this diagram represents one possible order of divergence of phyla.

Marsupial forelimb development

This study [51] published in 2012 is the first which describes the T Box gene expression in a marsupial the Tammar wallaby (Macropus eugenii) and how these genes are also responsible for limb and digit formation. Marsupials mammals differ from placental mammals because at birth, neonates are born highly altricial and not very developed. They lack fur, their eyes and ears are not developed and most of the skeleton is still cartilaginous. Another interesting feature that is observed in all marsupial neonates is that the forelimbs are more "developed" than the hindlimb. This is believed to be an adaptation to aid the tiny neonate after birth to climb from the urogenital opening to the pouch or the mammary area, and thus the neonate can attach to the teat where it completes its development- using its forelimbs to help move towards the pouch.

This image is from Chew et al. 2012 and demonstrates the development of tammar fetal limbs at selected stages before birth. (A) day 19, (B) day 20, (C) day 22, (D) day 24 and (E) day 25 (one day before birth). High magnification of the fore- and hindlimb are from samples stored in methanol whilst wholemounts were stored in 70% ethanol. A diagrammatic representation of the fore and hindlimb at day 24 and day 25 is provided showing dorsal and ventral views. All limbs are viewed from the dorsal aspect unless indicated. NB: images not to scale.

The more developed forelimb can be observed in the tammar wallaby (Macropus eugenii) which clearly demonstrates that the hindlimb development clearly lags behind the forelimb development. Some other marsupials such as the South American didelphid Monodelphis domestica also known as grey short-tailed opossum, has significantly less difference between forelimb and hindlimb development at birth, however there is still more development in the forelimbs than the hindlimb. This may be because Mondelphis domestica does not have to climb such a distance to the pouch as the tammar wallaby. This recent study [51] has demonstrated that the key patterning T box genes TBX4, TBX5, PITX1, FGF8, and SHH are also involved in the developing limb buds in the tammar wallaby. The results show that all the T box genes examined were highly conserved in the tammar wallaby with orthologues from opossum and mouse. TBX4 expression appeared earlier in development in the tammar wallaby than in the mouse, but appeared later in the tammar wallaby than in the opossum. Other results demonstrate that SHH expression is restricted to the zone of polarising activity, while TBX5 (forelimb) and PITX1 (hindlimb) showed diffuse mRNA expression. FGF8 is specifically localised to the apical ectodermal ridge, which is more prominent than in the opossum. The conclusions of this study demonstrate that in kangaroos and wallabies there is a very marked difference in limb size when the forelimb is compared to the hindlimb. The faster development of the fore limb compared to that of the hind limb correlates with the early timing of the expression of the key patterning genes in these limbs.

Evolution of the T-box family: Insights from the living chordate: Amphioxus

The lancelets ( also known as amphioxus -singular or amphioxi- plural) consist of 32 species of fish-like marine chordates and all are placed in the order Amphioxiformes. They have a distribution in shallow temperate and tropical seas. The amphioxus is a bilaterian cephalochordate and is considered a close relative of vertebrates. They are important in the study of zoology as they provide indications of the evolutionary origin of vertebrates and how vertebrate organisms have evolved. Lancelets have split from vertebrate more than 520 million years ago, however their genomes give us insite on how vertebrates evolved and how vertebrates have employed old genes for new functions.They are regarded as similar to the archetypal vertebrate form.

Phylogenetic analyses indicate that two genome duplications have occurred in the vertebrate linage after cephalochordates diverged so that each amphioxus gene corresponds to two or three vertebrate genes [1] For example the amphioxus gene AmphiTbx1/10 corresponds to two vertebrate T box genes Tbx1 and Tbx10 so these have arose presumably during genome duplications. AmphiTbx1/10 is expressed in amphioxus during gastrulation in the ventral somites and branchial arches [52] and this corresponds to the mammalian mouse T box gene Tbx1 and the expression of this gene in the ventromedial somites and pharyngeal arches. While the mouse Tbx10 is only expressed in the developing hindbrain [53] Therefore the function of Tbx1/10 in chordates might originally have been involved in branchial arch patterning and ventral somite specification. These functions are retained by the Tbx1 gene while the Tbx10 has lost its role in pharyngeal arch patterning and instead have gained a novel new role in hindbrain development.

File:Branchiostoma lanceolatum copy.jpg

This photo above is a photo of a Lancelet (or Amphioxus) specimen —Subphylum: Cephalochordate. It was collected in coarse sand sediments (600 µm) on the Belgian continental shelf. Total Length: approximately 22 mm. It is a chordate and considered one of the closest living relatives to all vertebrates. Image from here]]


Homologous: existence of shared ancestry between a pair of structures, or genes, in different taxa.

Homologue: something homologous.

Congenital: a condition attributable to events prior to birth, could be related to a baby born with disease.

Glossary doesn't have to be referenced

Good places to look

PubMed Biomed Central OMIM PNAS PLOS

DO A QUIZ AT THE BOTTOM OF THE PAGE - maybe collapse it in a tab to make the page look cleaner etc.


Check this for doubling up! shows more teamwork and if we've worked together as a team

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