Mouse and zebrafish Hoxa3 orthologues have nonequivalent in vivo protein function

High expression and regulatory mechanisms of ANGPT1 and HOXA3 in acute myeloid leukemia

OBJECTIVE

Acute Myeloid Leukemia (AML) is a type of leukemia characterized by the malignant clonal proliferation of hematopoietic stem cells in the bone marrow. This study aims to investigate the role of ANGPT1 and HOXA3 in the leukemia cell line KG-1a.

METHODS

The expression patterns of ANGPT1 and HOXA3 in AML patients were determined by analyzing the TCGA database and clinical samples. Experiments were conducted using the KG-1a cell line, including flow cytometry and SA-β-Gal staining, to knock down ANGPT1 and HOXA3 and evaluate their functions.

RESULTS

ANGPT1 and HOXA3 were found to be highly expressed in AML patients. Knocking down ANGPT1 and HOXA3 promoted apoptosis and senescence in KG-1a cells by inhibiting proliferation-related genes and upregulating apoptosis-related genes. There is a reciprocal regulatory relationship between ANGPT1 and HOXA3, forming a positive feedback loop. Treatment with ATRA downregulated the expression of HOXA3 and induced apoptosis in KG-1a cells, highlighting the importance of HOXA3 as a therapeutic target in AML.

CONCLUSION

ANGPT1 and HOXA3 are highly expressed in AML, and knocking them down can promote apoptosis and senescence in leukemia cells. They exhibit a mutual regulatory relationship, forming a positive feedback loop. These findings contribute to a better understanding of the functional roles and regulatory mechanisms of ANGPT1 and HOXA3, and provide new scientific evidence for the treatment and prognosis improvement of AML patients.

Thymus Ontogeny and Development

The thymus is a primary lymphoid organ composed of a three-dimensional (3D) epithelial network that provides a specialized microenvironment for the phenotypical and functional maturation of lymphoid progenitors. The specification of the pharyngeal endoderm to thymus fate occurs during the early stages of thymic organogenesis, independent of the expression of the transcription factor Foxn1. However, Foxn1 governs the later organogenesis of thymus together with the colonizing lymphoid cells. In the present chapter, we will review recent evidence on the topic covered in our original chapter (Muñoz and Zapata 2019). It described the early development of thymus and its resemblance to the development of endoderm-derived epithelial organs based on tubulogenesis and branching morphogenesis as well as the molecules known to be involved in these processes.

Craniofacial Phenotypes and Genetics of DiGeorge Syndrome

The 22q11.2 deletion is one of the most common genetic microdeletions, affecting approximately 1 in 4000 live births in humans. A 1.5 to 2.5 Mb hemizygous deletion of chromosome 22q11.2 causes DiGeorge syndrome (DGS) and velocardiofacial syndrome (VCFS). DGS/VCFS are associated with prevalent cardiac malformations, thymic and parathyroid hypoplasia, and craniofacial defects. Patients with DGS/VCFS manifest craniofacial anomalies involving the cranium, cranial base, jaws, pharyngeal muscles, ear-nose-throat, palate, teeth, and cervical spine. Most craniofacial phenotypes of DGS/VCFS are caused by proximal 1.5 Mb microdeletions, resulting in a hemizygosity of coding genes, microRNAs, and long noncoding RNAs. TBX1, located on chromosome 22q11.21, encodes a T-box transcription factor and is a candidate gene for DGS/VCFS. TBX1 regulates the fate of progenitor cells in the cranial and pharyngeal apparatus during embryogenesis. Tbx1-null mice exhibit the most clinical features of DGS/VCFS, including craniofacial phenotypes. Despite the frequency of DGS/VCFS, there has been a limited review of the craniofacial phenotypes of DGC/VCFS. This review focuses on these phenotypes and summarizes the current understanding of the genetic factors that impact DGS/VCFS-related phenotypes. We also review DGS/VCFS mouse models that have been designed to better understand the pathogenic processes of DGS/VCFS.

The alx3 gene shapes the zebrafish neurocranium by regulating frontonasal neural crest cell differentiation timing

During craniofacial development, different populations of cartilage- and bone-forming cells develop in precise locations in the head. Most of these cells are derived from pluripotent cranial neural crest cells and differentiate with distinct developmental timing and cellular morphologies. The mechanisms that divide neural crest cells into discrete populations are not fully understood. Here, we use single-cell RNA sequencing to transcriptomically define different populations of cranial neural crest cells. We discovered that the gene family encoding the Alx transcription factors is enriched in the frontonasal population of neural crest cells. Genetic mutant analyses indicate that alx3 functions to regulate the distinct differentiation timing and cellular morphologies among frontonasal neural crest cell subpopulations. This study furthers our understanding of how genes controlling developmental timing shape craniofacial skeletal elements.

T-Cell Immunodeficiencies With Congenital Alterations of Thymic Development: Genes Implicated and Differential Immunological and Clinical Features

Combined Immunodeficiencies (CID) are rare congenital disorders characterized by defective T-cell development that may be associated with B- and NK-cell deficiency. They are usually due to alterations in genes expressed in hematopoietic precursors but in few cases, they are caused by impaired thymic development. Athymia was classically associated with DiGeorge Syndrome due to TBX1 gene haploinsufficiency. Other genes, implicated in thymic organogenesis include FOXN1, associated with Nude SCID syndrome, PAX1, associated with Otofaciocervical Syndrome type 2, and CHD7, one of the genes implicated in CHARGE syndrome. More recently, chromosome 2p11.2 microdeletion, causing FOXI3 haploinsufficiency, has been identified in 5 families with impaired thymus development. In this review, we will summarize the main genetic, clinical, and immunological features related to the abovementioned gene mutations. We will also focus on different therapeutic approaches to treat SCID in these patients.

Thymus Inception: Molecular Network in the Early Stages of Thymus Organogenesis

The thymus generates central immune tolerance by producing self-restricted and self-tolerant T-cells as a result of interactions between the developing thymocytes and the stromal microenvironment, mainly formed by the thymic epithelial cells. The thymic epithelium derives from the endoderm of the pharyngeal pouches, embryonic structures that rely on environmental cues from the surrounding mesenchyme for its development. Here, we review the most recent advances in our understanding of the molecular mechanisms involved in early thymic organogenesis at stages preceding the expression of the transcription factor Foxn1, the early marker of thymic epithelial cells identity. Foxn1-independent developmental stages, such as the specification of the pharyngeal endoderm, patterning of the pouches, and thymus fate commitment are discussed, with a special focus on epithelial–mesenchymal interactions.

Molecular Insights Into the Causes of Human Thymic Hypoplasia With Animal Models

22q11.2 deletion syndrome (DiGeorge), CHARGE syndrome, Nude/SCID and otofaciocervical syndrome type 2 (OTFCS2) are distinct clinical conditions in humans that can result in hypoplasia and occasionally, aplasia of the thymus. Thymic hypoplasia/aplasia is first suggested by absence or significantly reduced numbers of recent thymic emigrants, revealed in standard-of-care newborn screens for T cell receptor excision circles (TRECs). Subsequent clinical assessments will often indicate whether genetic mutations are causal to the low T cell output from the thymus. However, the molecular mechanisms leading to the thymic hypoplasia/aplasia in diverse human syndromes are not fully understood, partly because the problems of the thymus originate during embryogenesis. Rodent and Zebrafish models of these clinical syndromes have been used to better define the underlying basis of the clinical presentations. Results from these animal models are uncovering contributions of different cell types in the specification, differentiation, and expansion of the thymus. Cell populations such as epithelial cells, mesenchymal cells, endothelial cells, and thymocytes are variably affected depending on the human syndrome responsible for the thymic hypoplasia. In the current review, findings from the diverse animal models will be described in relation to the clinical phenotypes. Importantly, these results are suggesting new strategies for regenerating thymic tissue in patients with distinct congenital disorders.

Embryology of the Parathyroid Glands

The parathyroid glands are essential for regulating calcium homeostasis in the body. The genetic programs that control parathyroid fate specification, morphogenesis, differentiation, and survival are only beginning to be delineated, but are all centered around a key transcription factor, GCM2. Mutations in the Gcm2 gene as well as in several other genes involved in parathyroid organogenesis have been found to cause parathyroid disorders in humans. Therefore, understanding the normal development of the parathyroid will provide insight into the origins of parathyroid disorders.

Chd7 Is Critical for Early T-Cell Development and Thymus Organogenesis in Zebrafish

Coloboma, heart defect, atresia choanae, retarded growth and development, genital hypoplasia, ear anomalies/deafness (CHARGE) syndrome is a congenital disorder affecting multiple organs and mainly caused by mutations in CHD7, a gene encoding a chromatin-remodeling protein. Immunodeficiency and reduced T cells have been noted in CHARGE syndrome. However, the mechanisms underlying T lymphopenia are largely unexplored. Herein, we observed dramatic decrease of T cells in both chd7knockdown and knockout zebrafish embryos. Unexpectedly, hematopoietic stem and progenitor cells and, particularly, lymphoid progenitor cells were increased peripherally in nonthymic areas in chd7-deficient embryos, unlikely to contribute to the T-cell decrease. Further analysis demonstrated that both the organogenesis and homing function of the thymus were seriously impaired. Chd7 might regulate thymus organogenesis through modulating the development of both neural crest cell-derived mesenchyme and pharyngeal endoderm-derived thymic epithelial cells. The expression of foxn1, a central regulator of thymic epithelium, was remarkably down-regulated in the pharyngeal region in chd7-deficient embryos. Moreover, the T-cell reduction in chd7-deficient embryos was partially rescued by overexpressing foxn1, suggesting that restoring thymic epithelium may be a potential therapeutic strategy for treating immunodeficiency in CHARGE syndrome. Collectively, the results indicated that chd7 was critical for thymic development and T-lymphopenia in CHARGE syndrome may be mainly attributed to the defects of thymic organogenesis. The current finding may benefit the diagnosis and therapy of T lymphopenia and immunodeficiency in CHARGE syndrome.

Morphological and molecular evolution of the ultimobranchial gland of nonmammalian vertebrates, with special reference to the chicken C cells

This review summarizes the current understanding of the nonmammalian ultimobranchial gland from morphological and molecular perspectives. Ultimobranchial anlage of all animal species develops from the last pharyngeal pouch. The genes involved in the development of pharyngeal pouches are well conserved across vertebrates. The ultimobranchial anlage of nonmammalian vertebrates and monotremes does not merge with the thyroid, remaining as an independent organ throughout adulthood. Although C cells of all animal species secrete calcitonin, the shape, cellular components and location of the ultimobranchial gland vary from species to species. Avian ultimobranchial gland is unique in several phylogenic aspects; the organ is located between the vagus and recurrent laryngeal nerves at the upper thorax and is densely innervated by branches emanating from them. In chick embryos, TuJ1-, HNK-1-, and PGP 9.5-immunoreactive cells that originate from the distal vagal (nodose) ganglion, colonize the ultimobranchial anlage and differentiate into C cells; neuronal cells give rise to C cells. Like C cells of mammals, the cells of fishes, amphibians, reptiles, and also a subset of C cells of birds, appear to be derived from the endodermal epithelium forming ultimobranchial anlage. Thus, the avian ultimobranchial C cells may have dual origins, neural progenitors and endodermal epithelium. Developmental Dynamics 246:719-739, 2017. © 2017 Wiley Periodicals, Inc.

Temporal and spatial requirements for Hoxa3 in mouse embryonic development

Hoxa3(null) mice have severe defects in the development of pharyngeal organs including athymia, aparathyroidism, thyroid hypoplasia, and ultimobranchial body persistence, in addition to defects of the throat cartilages and cranial nerves. Some of the structures altered in the Hoxa3(null) mutant embryos are anterior to the described Hoxa3 gene expression boundary: the thyroid, soft palate, and lesser hyoid horn. All of these structures develop over time and through the interactions of multiple cell types. To investigate the specific cellular targets for HOXA3 function in these structures across developmental time, we performed a comprehensive analysis of the temporal and tissue-specific requirements for Hoxa3, including a lineage analysis using Hoxa3(Cre). The combination of these approaches showed that HOXA3 functions in both a cell autonomous and non-cell autonomous manner during development of the 3rd and 4th arch derivatives, and functions in a neural crest cell (NCC)-specific, non-cell autonomous manner for structures that were Hoxa3-negative by lineage tracing. Our data indicate that HOXA3 is required for tissue organization and organ differentiation in endodermal cells (in the tracheal epithelium, thymus, and parathyroid), and contributes to organ migration and morphogenesis in NCCs. These data provide a detailed picture of where and when HOXA3 acts to promote the development of the diverse structures that are altered in the Hoxa3(null) mutant. Data presented here, combined with our previous studies, indicate that the regionally restricted defects in Hoxa3 mutants do not reflect a role in positional identity (establishment of cell or tissue fate), but instead indicate a wider variety of functions including controlling distinct genetic programs for differentiation and morphogenesis in different cell types during development.

Unraveling the Tangled Skein: The Evolution of Transcriptional Regulatory Networks in Development

The molecular and genetic basis for the evolution of anatomical diversity is a major question that has inspired evolutionary and developmental biologists for decades. Because morphology takes form during development, a true comprehension of how anatomical structures evolve requires an understanding of the evolutionary events that alter developmental genetic programs. Vast gene regulatory networks (GRNs) that connect transcription factors to their target regulatory sequences control gene expression in time and space and therefore determine the tissue-specific genetic programs that shape morphological structures. In recent years, many new examples have greatly advanced our understanding of the genetic alterations that modify GRNs to generate newly evolved morphologies. Here, we review several aspects of GRN evolution, including their deep preservation, their mechanisms of alteration, and how they originate to generate novel developmental programs.

Dynamic epithelia of the developing vertebrate face

A segmental series of endoderm-derived pouch and ectoderm-derived cleft epithelia act as signaling centers in the developing face. Their precise morphogenesis is therefore essential for proper patterning of the vertebrate head. Intercellular adhesion and polarity are highly dynamic within developing facial epithelial cells, with signaling from the adjacent mesenchyme controlling both epithelial character and directional migration. Endodermal and ectodermal epithelia fuse to form the primary mouth and gill slits, which involves basement membrane dissolution, cell intercalations, and apoptosis, as well as undergo further morphogenesis to generate the middle ear cavity and glands of the neck. Recent studies of facial epithelia are revealing both core programs of epithelial morphogenesis and insights into the coordinated assembly of the vertebrate head.

Modular evolution of DNA-binding preference of a Tbrain transcription factor provides a mechanism for modifying gene regulatory networks

Gene regulatory networks (GRNs) describe the progression of transcriptional states that take a single-celled zygote to a multicellular organism. It is well documented that GRNs can evolve extensively through mutations to cis-regulatory modules (CRMs). Transcription factor proteins that bind these CRMs may also evolve to produce novelty. Coding changes are considered to be rarer, however, because transcription factors are multifunctional and hence are more constrained to evolve in ways that will not produce widespread detrimental effects. Recent technological advances have unearthed a surprising variation in DNA-binding abilities, such that individual transcription factors may recognize both a preferred primary motif and an additional secondary motif. This provides a source of modularity in function. Here, we demonstrate that orthologous transcription factors can also evolve a changed preference for a secondary binding motif, thereby offering an unexplored mechanism for GRN evolution. Using protein-binding microarray, surface plasmon resonance, and in vivo reporter assays, we demonstrate an important difference in DNA-binding preference between Tbrain protein orthologs in two species of echinoderms, the sea star, Patiria miniata, and the sea urchin, Strongylocentrotus purpuratus. Although both orthologs recognize the same primary motif, only the sea star Tbr also has a secondary binding motif. Our in vivo assays demonstrate that this difference may allow for greater evolutionary change in timing of regulatory control. This uncovers a layer of transcription factor binding divergence that could exist for many pairs of orthologs. We hypothesize that this divergence provides modularity that allows orthologous transcription factors to evolve novel roles in GRNs through modification of binding to secondary sites.

Multiple roles for HOXA3 in regulating thymus and parathyroid differentiation and morphogenesis in mouse

Hoxa3 was the first Hox gene to be mutated by gene targeting in mice and is required for the development of multiple endoderm and neural crest cell (NCC)-derived structures in the pharyngeal region. Previous studies have shown that the Hoxa3 null mutant lacks third pharyngeal pouch derivatives, the thymus and parathyroids by E18.5, and organ-specific markers are absent or downregulated during initial organogenesis. Our current analysis of the Hoxa3 null mutant shows that organ-specific domains did undergo initial patterning, but the location and timing of key regional markers within the pouch, including Tbx1, Bmp4 and Fgf8, were altered. Expression of the parathyroid marker Gcm2 was initiated but was quickly downregulated and differentiation failed; by contrast, thymus markers were delayed but achieved normal levels, concurrent with complete loss through apoptosis. To determine the cell type-specific roles of Hoxa3 in third pharyngeal pouch development, we analyzed tissue-specific mutants using endoderm and/or NCC-specific Cre drivers. Simultaneous deletion with both drivers resulted in athymia at E18.5, similar to the null. By contrast, the individual tissue-specific Hoxa3 deletions resulted in small, ectopic thymi, although each had a unique phenotype. Hoxa3 was primarily required in NCCs for morphogenesis. In endoderm, Hoxa3 temporally regulated initiation of the thymus program and was required in a cell-autonomous manner for parathyroid differentiation. Furthermore, Hoxa3 was required for survival of third pharyngeal pouch-derived organs, but expression in either tissue was sufficient for this function. These data show that Hoxa3 has multiple complex and tissue-specific functions during patterning, differentiation and morphogenesis of the thymus and parathyroids.

Hox transcription factors: modulators of cell-cell and cell-extracellular matrix adhesion

Hox genes encode homeodomain-containing transcription factors that determine cell and tissue identities in the embryo during development. Hox genes are also expressed in various adult tissues and cancer cells. In Drosophila, expression of cell adhesion molecules, cadherins and integrins, is regulated by Hox proteins operating in hierarchical molecular pathways and plays a crucial role in segment-specific organogenesis. A number of studies using mammalian cultured cells have revealed that cell adhesion molecules responsible for cell-cell and cell-extracellular matrix interactions are downstream targets of Hox proteins. However, whether Hox transcription factors regulate expression of cell adhesion molecules during vertebrate development is still not fully understood. In this review, the potential roles Hox proteins play in cell adhesion and migration during vertebrate body patterning are discussed.

Combined function of HoxA and HoxB clusters in neural crest cells

The evolution of chordates was accompanied by critical anatomical innovations in craniofacial development, along with the emergence of neural crest cells. The potential of these cells to implement a craniofacial program in part depends upon the (non-)expression of Hox genes. For instance, the development of jaws requires the inhibition of Hox genes function in the first pharyngeal arch. In contrast, Hox gene products induce craniofacial structures in more caudal territories. To further investigate which Hox gene clusters are involved in this latter role, we generated HoxA;HoxB cluster double mutant animals in cranial neural crest cells. We observed the appearance of a supernumerary dentary-like bone with an endochondral ossification around a neo-Meckel's cartilage matrix and an attachment of neo-muscle demonstrating that HoxB genes enhance the phenotype induced by the deletion of the HoxA cluster alone. In addition, a cervical and hypertrophic thymus was associated with the supernumerary dentary-like bone, which may reflect its ancestral position near the filtrating system. Altogether these results show that the HoxA and HoxB clusters cooperated during evolution to lead to present craniofacial diversity.

Early development of the thymus in Xenopus laevis

BACKGROUND

Although Xenopus laevis has been a model of choice for comparative and developmental studies of the immune system, little is known about organogenesis of the thymus, a primary lymphoid organ in vertebrates. Here we examined the expression of three transcription factors that have been functionally associated with pharyngeal gland development, gcm2, hoxa3, and foxn1, and evaluated the neural crest contribution to thymus development.

RESULTS

In most species Hoxa3 is expressed in the third pharyngeal pouch endoderm where it directs thymus formation. In Xenopus, the thymus primordium is derived from the second pharyngeal pouch endoderm, which is hoxa3-negative, suggesting that a different mechanism regulates thymus formation in frogs. Unlike other species foxn1 is not detected in the epithelium of the pharyngeal pouch in Xenopus, rather, its expression is initiated as thymic epithelial cell starts to differentiate and express MHC class II molecules. Using transplantation experiments we show that while neural crest cells populate the thymus primordia, they are not required for the specification and initial development of this organ or for T-cell differentiation in frogs.

CONCLUSIONS

These studies provide novel information on early thymus development in Xenopus, and highlight a number of features that distinguish Xenopus from other organisms.

Evolution of thymus organogenesis

The thymus is the primary organ for functional T lymphocyte development in jawed vertebrates. A new study in the jawless fish, lampreys, indicates the existence of a primitive thymus in these surviving representatives of the most ancient vertebrates, providing strong evidence of co-evolution of T cells and thymus. This review summarizes the wealth of data that have been generated towards understanding the evolution of the thymus in the vertebrates. Progress in identifying genetic networks and cellular mechanisms that control thymus organogenesis in mammals and their evolution in lower species may inspire the development of new strategies for medical interventions targeting faulty thymus functions.

Regulatory mechanisms of thymus and T cell development

The thymus is a central hematopoietic organ which produces mature T lymphocytes with diverse antigen specificity. During development, the thymus primordium is derived from the third pharyngeal endodermal pouch, and then differentiates into cortical and medullary thymic epithelial cells (TECs). TECs represent the primary functional cell type that forms the unique thymic epithelial microenvironment which is essential for intrathymic T-cell development, including positive selection, negative selection and emigration out of the thymus. Our understanding of thymopoiesis has been greatly advanced by using several important animal models. This review will describe progress on the molecular mechanisms involved in thymus and T cell development with particular focus on the signaling and transcription factors involved in this process in mouse and zebrafish.

The TP53 fertility network

The TP53 gene, first described in 1979, was identified as a tumor suppressor gene in 1989, when it became clear that its product, the p53 nuclear phosphoprotein, was frequently inactivated in many different forms of cancers. Nicknamed "guardian of the genome", TP53 occupies a central node in stress response networks. The p53 protein has a key role as transcription factor in limiting oncogenesis through several growth suppressive functions, such as initiating apoptosis, senescence, or cell cycle arrest. The p53 protein is directly inactivated in about 50% of all tumors as a result of somatic gene mutations or deletions, and over 80% of tumors demonstrate dysfunctional p53 signaling. Beyond the undeniable importance of p53 as a tumor suppressor, an increasing number of new functions for p53 have been reported, including its ability to regulate energy metabolism, to control autophagy, and to participate in various aspects of differentiation and development. Recently, studies on genetic variations in TP53 among different populations have led to the notion that the p53 protein might play an important role in regulating fertility. This review summarizes current knowledge on the basic functions of different genes of the TP53 family and TP53 pathway with respect to fertility. We also provide original analyses based on genomic and genotype databases, providing further insights into the possible roles of the TP53 pathway in human reproduction.

Comparative embryology of the carotid body

Vertebrate carotid bodies and related structures (branchial arch oxygen chemoreceptors in fishes, carotid labyrinth in amphibians, chemoreceptors in the wall of the common carotid and its branches in birds) develop in embryos when neural crest cells, blood vessels, and nerve fibers from sympathetic and cranial nerve ganglia invade mesenchymal primordia in the wall of the 3rd branchial arch. This review focuses on literature published since the 1970s investigating similarities and differences in the embryological development of 3rd arch oxygen chemoreceptors, especially between mammals and birds, but also considering reptiles, amphibians and fishes.

Regulatory evolution through divergence of a phosphoswitch in the transcription factor CEBPB

There is an emerging consensus that gene regulation evolves through changes in cis-regulatory elements and transcription factors. Although it is clear how nucleotide substitutions in cis-regulatory elements affect gene expression, it is not clear how amino-acid substitutions in transcription factors influence gene regulation. Here we show that amino-acid changes in the transcription factor CCAAT/enhancer binding protein-β (CEBPB, also known as C/EBP-β) in the stem-lineage of placental mammals changed the way it responds to cyclic AMP/protein kinase A (cAMP/PKA) signalling. By functionally analysing resurrected ancestral proteins, we identify three amino-acid substitutions in an internal regulatory domain of CEBPB that are responsible for the novel function. These amino-acid substitutions reorganize the location of key phosphorylation sites, introducing a new site and removing two ancestral sites, reversing the response of CEBPB to GSK-3β-mediated phosphorylation from repression to activation. We conclude that changing the response of transcription factors to signalling pathways can be an important mechanism of gene regulatory evolution.

Mechanisms of thymus organogenesis and morphogenesis

The thymus is the primary organ responsible for generating functional T cells in vertebrates. Although T cell differentiation within the thymus has been an area of intense investigation, the study of thymus organogenesis has made slower progress. The past decade, however, has seen a renewed interest in thymus organogenesis, with the aim of understanding how the thymus develops to form a microenvironment that supports T cell maturation and regeneration. This has prompted modern revisits to classical experiments and has driven additional genetic approaches in mice. These studies are making significant progress in identifying the molecular and cellular mechanisms that control specification, early organogenesis and morphogenesis of the thymus.

Evo-devo and accounting for Darwin's endless forms

Evo-devo has led to dramatic advances in our understanding of how the processes of development can contribute to explaining patterns of evolutionary diversification that underlie the endless forms of animal life on the Earth. This is increasingly the case not only for the origins of evolutionary novelties that permit new functions and open up new adaptive zones, but also for the processes of evolutionary tinkering that occur within the subsequent radiations of related species. Evo-devo has time and again yielded spectacular examples of Darwin's notions of common ancestry and of descent with modification. It has also shown that the evolution of endless forms is more about the evolution of the regulatory machinery of ancient genes than the origin and elaboration of new genes. Evolvability, especially with respect to the capacity of a developmental system to evolve and to generate the variation in form for natural selection to screen, has become a pivotal focus of evo-devo. As a consequence, a balancing of the concept of endless forms in morphospace with a greater awareness of the potential for developmental constraints and bias is becoming more general. The prospect of parallel horizons opening up for the evolution of behaviour is exciting; in particular, does Sean Carroll's phrase referring to old genes learning new tricks in the evolution of endless forms apply equally as well to patterns of diversity and disparity in behavioural trait-space?

argC Orthologs from Rhizobiales show diverse profiles of transcriptional efficiency and functionality in Sinorhizobium meliloti

Several factors can influence ortholog replacement between closely related species. We evaluated the transcriptional expression and metabolic performance of ortholog substitution complementing a Sinorhizobium meliloti argC mutant with argC from Rhizobiales (Agrobacterium tumefaciens, Rhizobium etli, and Mesorhizobium loti). The argC gene is necessary for the synthesis of arginine, an amino acid that is central to protein and cellular metabolism. Strains were obtained carrying plasmids with argC orthologs expressed under the speB and argC (S. meliloti) and lac (Escherichia coli) promoters. Complementation analysis was assessed by growth, transcriptional activity, enzymatic activity, mRNA levels, specific detection of ArgC proteomic protein, and translational efficiency. The argC orthologs performed differently in each complementation, reflecting the diverse factors influencing gene expression and the ability of the ortholog product to function in a foreign metabolic background. Optimal complementation was directly related to sequence similarity with S. meliloti, and was inversely related to species signature, with M. loti argC showing the poorest performance, followed by R. etli and A. tumefaciens. Different copy numbers of genes and amounts of mRNA and protein were produced, even with genes transcribed from the same promoter, indicating that coding sequences play a role in the transcription and translation processes. These results provide relevant information for further genomic analyses and suggest that orthologous gene substitutions between closely related species are not completely functionally equivalent.