The evolution of left-right asymmetry in chordates

Streamlining asymmetry quantification in fetal mouse imaging: A semi-automated pipeline supported by expert guidance

BACKGROUND

Asymmetry is a key feature of numerous developmental disorders and in phenotypic screens is often used as a readout for environmental or genetic perturbations. A better understanding of the genetic basis of asymmetry and its relationship to disease susceptibility will help unravel the complex genetic and environmental factors and their interactions that increase risk in a range of developmental disorders. Large-scale imaging datasets offer opportunities to work with sample sizes necessary to detect and quantify differences in morphology beyond severe deformities but also pose challenges to manual phenotyping protocols.

RESULTS

We introduce a tool for quantifying asymmetry in 3D images and apply it to explore the role of genes contributing to abnormal asymmetry by deep phenotyping 3D fetal microCT images from knockout strains acquired as part of the Knockout Mouse Phenotyping Program. Four knockout strains: Ccdc186, Acvr2a, Nhlh1, and Fam20c were identified with highly significant asymmetry in craniofacial regions, making them good candidates for further analysis.

CONCLUSION

In this work, we demonstrate an open-source, semi-automated tool to quantify the asymmetry of craniofacial structures that integrates expert anatomical knowledge. This tool can detect abnormally asymmetric phenotypes in fetal mice to explore the relationship between facial asymmetry, perturbed development, and developmental instability.

Sidedness in Unilateral Orofacial Clefts: A Systematic Scoping Review

ObjectiveAn overview of the literature relating to the sidedness of unilateral cleft lip with or without cleft palate to map current knowledge on the cause and impact of directional asymmetry.DesignScoping review with a systematic search of Medline and Embase from inception to May 2023.Patients, ParticipantsHumans born with a left or right unilateral cleft lip with or without a cleft palate.Main Outcome MeasuresCleft sidedness as a co-occurrence, an outcome or an exposure.ResultsForty studies were eligible for inclusion and confirmed the predilection for the occurrence of left sided cleft lips; 12 studies reported cleft sidedness co-occurring with another phenotype, 11 studies report sidedness as an outcome and 17 studies as an exposure. Phenotypes which were reported to co-occur with either left or right sided clefts included congenital dental anomalies, handedness and additional congenital anomalies. Variables investigated as a potential cause of left or right sided clefts as an outcome included chromosomal anomalies, genetic variants and environmental factors. Outcomes investigated in relation to cleft sidedness as an exposure included facial anatomical features, facial growth, educational attainment, functional and psychological characteristics. More studies showed worse outcomes in right sided clefts versus left sided clefts than vice versa, although studies were inconsistent, and a quality assessment was not performed.ConclusionsThe field of cleft sidedness research is expanding and there are promising early findings to differentiate cause and outcome by sidedness of the cleft.

Streamlining Asymmetry Quantification in Fetal Mouse Imaging: A Semi-Automated Pipeline Supported by Expert Guidance

Asymmetry is a key feature of numerous developmental disorders and in phenotypic screens is often used as a readout for environmental or genetic perturbations to normal development. A better understanding of the genetic basis of asymmetry and its relationship to disease susceptibility will help unravel the complex genetic and environmental factors and their interactions that increase risk in a range of developmental disorders. Large-scale imaging datasets offer opportunities to work with sample sizes needed to detect and quantify differences in morphology beyond severe deformities while also posing challenges to manual phenotyping protocols. In this work, we introduce a semi-automated open-source workflow to quantify abnormal asymmetry of craniofacial structures that integrates expert anatomical knowledge. We apply this workflow to explore the role of genes contributing to abnormal asymmetry by deep phenotyping 3D fetal microCT images from knockout strains acquired as part of the Knockout Mouse Phenotyping Program (KOMP2). Four knockout strains: Ccdc186, Acvr2a, Nhlh1, and Fam20c were identified with highly significant asymmetry in craniofacial regions, making them good candidates for further analysis into their potential roles in asymmetry and developmental disorders.

Core genes of biomineralization and cis-regulatory long non-coding RNA regulate shell growth in bivalves

INTRODUCTION

Bivalve molluscs are abundant in marine and freshwater systems and contribute essential ecosystem services. They are characterized by an exuberant diversity of biomineralized shells and typically have two symmetric valves (a.k.a shells), but oysters (Ostreidae), some clams (Anomiidae and Chamidae) and scallops (Pectinida) have two asymmetrical valves. Predicting and modelling the likely consequences of ocean acidification on bivalve survival, biodiversity and aquaculture makes understanding shell biomineralization and its regulation a priority.

OBJECTIVES

This study aimed to a) exploit the atypical asymmetric shell growth of some bivalves and through comparative analysis of the genome and transcriptome pinpoint candidate biomineralization-related genes and regulatory long non-coding RNAs (LncRNAs) and b) demonstrate their roles in regulating shell biomineralization/growth.

METHODS

Meta-analysis of genomes, de novo generated mantle transcriptomes or transcriptomes and proteomes from public databases for six asymmetric to symmetric bivalve species was used to identify biomineralization-related genes. Bioinformatics filtering uncovered genes and regulatory modules characteristic of bivalves with asymmetric shells and identified candidate biomineralization-related genes and lncRNAs with a biased expression in asymmetric valves. A shell regrowth model in oyster and gene silencing experiments, were used to characterize candidate gene function.

RESULTS

Shell matrix genes with asymmetric expression in the mantle of the two valves were identified and unique cis-regulatory lncRNA modules characterized in Ostreidae. LncRNAs that regulate the expression of the tissue inhibitor of metalloproteinases gene family (TIMPDR) and of the shell matrix protein domain family (SMPDR) were identified. In vitro and in vivo silencing experiments revealed the candidate genes and lncRNA were associated with divergent shell growth rates and modified the microstructure of calcium carbonate (CaCO3) crystals.

CONCLUSION

LncRNAs are putative regulatory factors of the bivalve biomineralization toolbox. In the Ostreidae family of bivalves biomineralization-related genes are cis-regulated by lncRNA and modify the planar growth rate and spatial orientation of crystals in the shell.

The Implications of Laterality in Unilateral Cleft Lip Reconstruction: A Global Survey of Cleft Surgeons

OBJECTIVE

To explore the impact of directional laterality in complete Unilateral Cleft Lip (UCL) amongst the global cleft surgeon community.

DESIGN

Cross-sectional survey study.

SETTING

Global distribution of online survey distributed in English and Spanish.

PARTICIPANTS

Cleft surgeons from around the world.

MAIN OUTCOME MEASURES

Survey participant perception of the impact of laterality on: (1) cleft presentation (2) surgical challenge and (3) surgical outcomes.

RESULTS

Responses were received from 453 cleft surgeons located in 54 countries around the world. 221 (49%) had previously considered differences in patients presenting with a left- versus right-sided UCL. 95 (21%) considered right-sided clefts more difficult to reconstruct, 37 (8%) reported left-sided clefts to be more difficult and 321 (71%) reported no difference in difficulty between the cleft sides. Higher volume cleft surgeons, characterised by those reporting cleft as their principal area of practice and performing >20 cleft operations per year, were more likely to have both previously considered differences in laterality in cleft and to report right-sided unilateral cleft lip to be more difficult to primarily reconstruct. 395 (87%) did not consider surgical outcomes to be influenced by cleft laterality.

CONCLUSIONS

This survey reports perceptions on cleft laterality from a large body of global surgeons and suggests a trend for increased difficulty in right-sided compared to left-sided cleft lip reconstruction, where such laterality-associated difficulty is perceived.

Seasonal, Geographic, and Ethnic Influence on the Prevalence of Orofacial Clefts in Kuwait: A Nationwide Study

OBJECTIVE

Accurate nationwide epidemiological evidence is vital to study the seasonal, geographic, and ethnic influence on the trends of orofacial cleft prevalence in Kuwait.

DESIGN

Data obtained from the National Center for Health Information and Ministry of Health Hospital digital records were reviewed retrospectively to identify patients with orofacial clefts (OFC) using the ICD-10 diagnostic codes. The Jonckheere-Terpstra test was used to assess the trend of birth prevalence across the different years. The associations of types of OFC with ethnic and geographic influences were tested with chi-square or Fisher's exact tests, while the strength of that association was tested with multi-nominal logistic regression.

RESULTS

Birth prevalence in Kuwait ranged from 0.75-2.55 per 1000 live births (0.73- 2.73 among Kuwaitis, 0.60-3.27 among non-Kuwaitis), with no statistically significant change observed during the past 28 years. The risk of unilateral cleft lip and palate was eight times higher during summer compared to Autumn, while the risk of cleft lip and palate remained lower during winter. Jahra (OR-7.76, CI- 1.51-39.80), Farwaneya (OR-6.65, CI- 1.34-33.06), and Hawalli (OR-6.72, CI- 1.26-35.98) governorates had higher odds of bilateral cleft lip when compared to Mubarak Alkabeer.

CONCLUSIONS

The study outcome is an indicator to improve patient care and customize healthcare infrastructure in the Ministry of Health. It also provides insight to develop projections of future needs. Future studies should focus on understanding the factors that might be a potential contributor to the seasonal change observed in the prevalence of OFC.

Comparison of Presurgical Anthropometric Measures of Right and Left Complete Unilateral Cleft Lip and/or Palate

BACKGROUND

Left clefts occur twice as frequently as right ones. The sidedness has been suggested to influence certain outcomes. Some surgeons consider a right cleft more challenging to repair. This is often attributed to their reduced prevalence. The authors question whether this may be caused by morphologic differences. The authors' hypothesis is that there are anthropometric differences between left and right complete cleft lips.

METHODS

Patients with complete unilateral cleft lip, with or without cleft palate, operated on at the age of 3 to 6 months, between 2000 and 2018, by a single surgeon, were included. Eight standardized anthropometric measurements of the cleft lip, collected just before cleft lip repair, compare lip and vermillion dimensions and ratios between left and right clefts.

RESULTS

One hundred thirty-nine left and 80 right unilateral cleft lips were compared. A significant difference was found between left and right clefts for cleft-side to non-cleft-side ratios comparing the lateral lip element vertical heights and vermillion heights.

CONCLUSIONS

Patients with right cleft lips have a greater degree of lateral lip element hypoplasia, demonstrating greater deficiencies of lateral lip element vertical height and vermillion height when compared to patients with left clefts. This has clinical implications for preoperative assessment, choice of surgical technique, and postoperative and long-term outcomes.

Pitx controls amphioxus asymmetric morphogenesis by promoting left-side development and repressing right-side formation

Background

Left-right (LR) asymmetry is an essential feature of bilateral animals. Studies in vertebrates show that LR asymmetry formation comprises three major steps: symmetry breaking, asymmetric gene expression, and LR morphogenesis. Although much progress has been made in the first two events, mechanisms underlying asymmetric morphogenesis remain largely unknown due to the complex developmental processes deployed by vertebrate organs.

Results

We here addressed this question by studying Pitx gene function in the basal chordate amphioxus whose asymmetric organogenesis, unlike that in vertebrates, occurs essentially in situ and does not rely on cell migration. Pitx null mutation in amphioxus causes loss of all left-sided organs and incomplete ectopic formation of all right-sided organs on the left side, whereas Pitx partial loss-of-function leads to milder phenotypes with only some LR organs lost or ectopically formed. At the N1 to N3 stages, Pitx expression is gradually expanded from the dorsal anterior domain to surrounding regions. This leads to activation of genes like Lhx3 and/or Prop1 and Pit, which are essential for left-side organs, and downregulation of genes like Hex and/or Nkx2.1 and FoxE4, which are required for right-side organs to form ectopically on the left side. In Pitx mutants, the left-side expressed genes are not activated, while the right-side genes fail to decrease expression on the left side. In contrast, in embryos overexpressing Pitx genes, the left-side genes are induced ectopically on the right side, and the right-side genes are inhibited. Several Pitx binding sites are identified in the upstream sequences of the left-side and right-side genes which are essential for activation of the former and repression of the latter by Pitx.

Conclusions

Our results demonstrate that (1) Pitx is a major (although not the only) determinant of asymmetric morphogenesis in amphioxus, (2) the development of different LR organs have distinct requirements for Pitx activity, and (3) Pitx controls amphioxus LR morphogenesis probably through inducing left-side organs and inhibiting right-side organs directly. These findings show much more dependence of LR organogenesis on Pitx in amphioxus than in vertebrates. They also provide insight into the molecular developmental mechanism of some vertebrate LR organs like the lungs and atria, since they show a right-isomerism phenotype in Pitx2 knockout mice like right-sided organs in Pitx mutant amphioxus. Our results also explain why some organs like the adenohypophysis are asymmetrically located in amphioxus but symmetrically positioned in vertebrates.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01095-0.

Cardiac Development: A Glimpse on Its Translational Contributions

Cardiac development is a complex developmental process that is initiated soon after gastrulation, as two sets of precardiac mesodermal precursors are symmetrically located and subsequently fused at the embryonic midline forming the cardiac straight tube. Thereafter, the cardiac straight tube invariably bends to the right, configuring the first sign of morphological left–right asymmetry and soon thereafter the atrial and ventricular chambers are formed, expanded and progressively septated. As a consequence of all these morphogenetic processes, the fetal heart acquired a four-chambered structure having distinct inlet and outlet connections and a specialized conduction system capable of directing the electrical impulse within the fully formed heart. Over the last decades, our understanding of the morphogenetic, cellular, and molecular pathways involved in cardiac development has exponentially grown. Multiples aspects of the initial discoveries during heart formation has served as guiding tools to understand the etiology of cardiac congenital anomalies and adult cardiac pathology, as well as to enlighten novels approaches to heal the damaged heart. In this review we provide an overview of the complex cellular and molecular pathways driving heart morphogenesis and how those discoveries have provided new roads into the genetic, clinical and therapeutic management of the diseased hearts.

Filter feeding, deviations from bilateral symmetry, developmental noise, and heterochrony of hemichordate and cephalochordate gills

We measured gill slit fluctuating asymmetry (FA), a measure of developmental noise, in adults of three invertebrate deuterostomes with different feeding modes: the cephalochordate Branchiostoma floridae (an obligate filter feeder), the enteropneusts Protoglossus graveolens (a facultative filter feeder/deposit feeder) and Saccoglossus bromophenolosus (a deposit feeder). FA was substantially and significantly low in B. floridae and P. graveolens and high in S. bromophenolosus. Our results suggest that the gills of species that have experienced a relaxation of the filter feeding trait exhibit elevated FA. We found that the timing of development of the secondary collagenous gill bars, compared to the primary gill bars, was highly variable in P. graveolens but not the other two species, demonstrating an independence of gill FA from gill bar heterochrony. We also discovered the occasional ectopic expression of a second set of paired gills posterior to the first set of gills in the enteropneusts and that these were more common in S. bromophenolosus. Moreover, our finding that gill slits in enteropneusts exhibit bilateral symmetry suggests that the left-sidedness of larval cephalochordate gills, and the directional asymmetry of Cambrian stylophoran echinoderm fossil gills, evolved independently from a bilaterally symmetrical ancestor.

Transcription Factors of the bHLH Family Delineate Vertebrate Landmarks in the Nervous System of a Simple Chordate

Tunicates are marine invertebrates whose tadpole-like larvae feature a highly simplified version of the chordate body plan. Similar to their distant vertebrate relatives, tunicate larvae develop a regionalized central nervous system and form distinct neural structures, which include a rostral sensory vesicle, a motor ganglion, and a caudal nerve cord. The sensory vesicle contains a photoreceptive complex and a statocyst, and based on the comparable expression patterns of evolutionarily conserved marker genes, it is believed to include proto-hypothalamic and proto-retinal territories. The evolutionarily conserved molecular fingerprints of these landmarks of the vertebrate brain consist of genes encoding for different transcription factors, and of the gene batteries that they control, and include several members of the bHLH family. Here we review the complement of bHLH genes present in the streamlined genome of the tunicate Ciona robusta and their current classification, and summarize recent studies on proneural bHLH transcription factors and their expression territories. We discuss the possible roles of bHLH genes in establishing the molecular compartmentalization of the enticing nervous system of this unassuming chordate.

Analysis of permanent second molar development in children born with cleft lip and palate

OBJECTIVE

The study of dental development in individuals born with cleft lip and palate (CLP) serves to determine when orthodontic intervention should start. To evaluate the permanent second molar development in children born with cleft lip and palate according to Demirjian's and Nolla's methods.

METHODOLOGY

Out of a total of 513 digital panoramic radiographs, 113 pairs of children aged 3 to 16 years were selected. The exams were from children born with or without cleft lip and palate, of the same sex, with an age difference of up to 30 days. The images were analyzed by three examiners and reliability was checked through intra-examiner agreement by the Kappa test. The data were analyzed by Wilcoxon's and Mann-Whitney tests according to each dataset.

RESULTS

The findings indicated delayed development of the permanent second molars in children with CLP (P<0.001). The development of the right permanent second molar was delayed compared to the left molar in children with CLP. Moreover, mandibular teeth showed significantly earlier development than maxillary teeth in both the case and control groups. There was no significant difference in the development of permanent second molars between sexes.

CONCLUSION

Children with CLP presented delay in the development of permanent second molars.

Brain and Behavioral Asymmetry: A Lesson From Fish

It is widely acknowledged that the left and right hemispheres of human brains display both anatomical and functional asymmetries. For more than a century, brain and behavioral lateralization have been considered a uniquely human feature linked to language and handedness. However, over the past decades this idea has been challenged by an increasing number of studies describing structural asymmetries and lateralized behaviors in non-human species extending from primates to fish. Evidence suggesting that a similar pattern of brain lateralization occurs in all vertebrates, humans included, has allowed the emergence of different model systems to investigate the development of brain asymmetries and their impact on behavior. Among animal models, fish have contributed much to the research on lateralization as several fish species exhibit lateralized behaviors. For instance, behavioral studies have shown that the advantages of having an asymmetric brain, such as the ability of simultaneously processing different information and perform parallel tasks compensate the potential costs associated with poor integration of information between the two hemispheres thus helping to better understand the possible evolutionary significance of lateralization. However, these studies inferred how the two sides of the brains are differentially specialized by measuring the differences in the behavioral responses but did not allow to directly investigate the relation between anatomical and functional asymmetries. With respect to this issue, in recent years zebrafish has become a powerful model to address lateralization at different level of complexity, from genes to neural circuitry and behavior. The possibility of combining genetic manipulation of brain asymmetries with cutting-edge in vivo imaging technique and behavioral tests makes the zebrafish a valuable model to investigate the phylogeny and ontogeny of brain lateralization and its relevance for normal brain function and behavior.

Orthodontics and Genetics

Introduction:

Genetics has been suggested as an explanation for the etiology of malocclusions, although some questions, due to the perception that genetic inheritance is tied to a monogenic or Mendelian form of inheritance.

Objective:

This paper describes the inheritance of malocclusions, highlighting the areas of knowledge where research has explored mechanisms that explain deviations in patterns of craniofacial growth.

Conclusion:

Malocclusions have a complex or multifactorial pattern of inheritance, where more than one gene is involved in the development of the phenotype. There is also the possibility that the environment influences malocclusions.

A chordate species lacking Nodal utilizes calcium oscillation and Bmp for left-right patterning

Larvaceans are chordates with a tadpole-like morphology. In contrast to most chordates of which early embryonic morphology is bilaterally symmetric and the left-right (L-R) axis is specified by the Nodal pathway later on, invariant L-R asymmetry emerges in four-cell embryos of larvaceans. The asymmetric cell arrangements exist through development of the tailbud. The tail thus twists 90° in a counterclockwise direction relative to the trunk, and the tail nerve cord localizes on the left side. Here, we demonstrate that larvacean embryos have nonconventional L-R asymmetries: 1) L- and R-cells of the two-cell embryo had remarkably asymmetric cell fates; 2) Ca²⁺ oscillation occurred through embryogenesis; 3) Nodal, an evolutionarily conserved left-determining gene, was absent in the genome; and 4) bone morphogenetic protein gene (Bmp) homolog Bmp.a showed right-sided expression in the tailbud and larvae. We also showed that Ca²⁺ oscillation is required for Bmp.a expression, and that BMP signaling suppresses ectopic expression of neural genes. These results indicate that there is a chordate species lacking Nodal that utilizes Ca²⁺ oscillation and Bmp.a for embryonic L-R patterning. The right-side Bmp.a expression may have arisen via cooption of conventional BMP signaling in order to restrict neural gene expression on the left side.

On the Variable Clinical Presentation of Molar-Incisor Hypomineralization

Molar-incisor hypomineralization (MIH) is a condition that is defined based on its peculiar clinical presentation. Original reports on the etiology of the condition and possible risk factors were inconclusive, and we refuted the original suggestion that MIH is an idiopathic condition and suggested that MIH has complex inheritance and is due to the interaction of more than one gene and the environment. Our group was the first to suggest MIH has a genetic component that involves genetic variation in genes expressed during dental enamel formation. Later we expanded this work to include genes related to the immune response. In this report, we provide a rationale to explain the variation seen in the clinical presentation of MIH, which can affect just one molar out of the four or just a portion of a particular molar.

Automated behavioural analysis reveals the basic behavioural repertoire of the urochordate Ciona intestinalis

Quantitative analysis of animal behaviour in model organisms is becoming an increasingly essential approach for tackling the great challenge of understanding how activity in the brain gives rise to behaviour. Here we used automated image-based tracking to extract behavioural features from an organism of great importance in understanding the evolution of chordates, the free-swimming larval form of the tunicate Ciona intestinalis, which has a compact and fully mapped nervous system composed of only 231 neurons. We analysed hundreds of videos of larvae and we extracted basic geometric and physical descriptors of larval behaviour. Importantly, we used machine learning methods to create an objective ontology of behaviours for C. intestinalis larvae. We identified eleven behavioural modes using agglomerative clustering. Using our pipeline for quantitative behavioural analysis, we demonstrate that C. intestinalis larvae exhibit sensory arousal and thigmotaxis. Notably, the anxiotropic drug modafinil modulates thigmotactic behaviour. Furthermore, we tested the robustness of the larval behavioural repertoire by comparing different rearing conditions, ages and group sizes. This study shows that C. intestinalis larval behaviour can be broken down to a set of stereotyped behaviours that are used to different extents in a context-dependent manner.

Spatio-temporal regulation of Rx and mitotic patterns shape the eye-cup of the photoreceptor cells in Ciona

The ascidian larva has a pigmented ocellus comprised of a cup-shaped array of approximately 30 photoreceptor cells, a pigment cell, and three lens cells. Morphological, physiological and molecular evidence has suggested evolutionary kinship between the ascidian larval photoreceptors and vertebrate retinal and/or pineal photoreceptors. Rx, an essential factor for vertebrate photoreceptor development, has also been suggested to be involved in the development of the ascidian photoreceptor cells, but a recent revision of the photoreceptor cell lineage raised a crucial discrepancy between the reported expression patterns of Rx and the cell lineage. Here, we report spatio-temporal expression patterns of Rx at single-cell resolution along with mitotic patterns up to the final division of the photoreceptor-lineage cells in Ciona. The expression of Rx commences in non-photoreceptor a-lineage cells on the right side of the anterior sensory vesicle at the early tailbud stage. At the mid tailbud stage, Rx begins to be expressed in the A-lineage photoreceptor cell progenitors located on the right side of the posterior sensory vesicle. Thus, Rx is specifically but not exclusively expressed in the photoreceptor-lineage cells in the ascidian embryo. Two cis-regulatory modules are shown to be important for the photoreceptor-lineage expression of Rx. The cell division patterns of the photoreceptor-lineage cells rationally explain the generation of the cup-shaped structure of the pigmented ocellus. The present findings demonstrate the complete cell lineage of the ocellus photoreceptor cells and provide a framework elucidating the molecular and cellular mechanisms of photoreceptor development in Ciona.

Nodal and Hedgehog synergize in gill slit formation during development of the cephalochordate Branchiostoma floridae

The larval pharynx of the cephalochordate Branchiostoma (amphioxus) is asymmetrical. The mouth is on the left, and endostyle and gill slits are on the right. At the neurula, Nodal and Hedgehog (Hh) expression becomes restricted to the left. To dissect their respective roles in gill slit formation, we inhibited each pathway separately for 20 min at intervals during the neurula stage, before gill slits penetrate, and monitored the effects on morphology and expression of pharyngeal markers. The results pinpoint the short interval spanning the gastrula/neurula transition as the critical period for specification and positioning of future gill slits. Thus, reduced Nodal signaling shifts the gill slits ventrally, skews the pharyngeal domains of Hh, Pax1/9, Pax2/5/8, Six1/2 and IrxC towards the left, and reduces Hh and Tbx1/10 expression in endoderm and mesoderm, respectively. Nodal auto-regulates. Decreased Hh signaling does not affect gill slit positions or Hh or Nodal expression, but it does reduce the domain of Gli, the Hh target, in the pharyngeal endoderm. Thus, during the neurula stage, Nodal and Hh cooperate in gill slit development - Hh mediates gill slit formation and Nodal establishes their left-right position.

Pitx2c orchestrates embryonic axis extension via mesendodermal cell migration

Pitx2c, a homeodomain transcription factor, is classically known for its left-right patterning role. However, an early wave of pitx2 expression occurs at the onset of gastrulation in several species, indicating a possible earlier role that remains relatively unexplored. Here we show that in zebrafish, maternal-zygotic (MZ) pitx2c mutants exhibit a shortened body axis indicative of convergence and extension (CE) defects. Live imaging reveals that MZpitx2c mutants display less persistent mesendodermal migration during late stages of gastrulation. Transplant data indicate that Pitx2c functions cell non-autonomously to regulate this cell behavior by modulating cell shape and protrusive activity. Using transcriptomic analyses and candidate gene approaches, we identify transcriptional changes in components of the chemokine-ECM-integrin dependent mesendodermal migration network. Together, our results define pathways downstream of Pitx2c that are required during early embryogenesis and reveal novel functions for Pitx2c as a regulator of morphogenesis.

A Conserved Role of the Unconventional Myosin 1d in Laterality Determination

Anatomical and functional asymmetries are widespread in the animal kingdom [1, 2]. In vertebrates, many visceral organs are asymmetrically placed [3]. In snails, shells and inner organs coil asymmetrically, and in Drosophila, genitalia and hindgut undergo a chiral rotation during development. The evolutionary origin of these asymmetries remains an open question [1]. Nodal signaling is widely used [4], and many, but not all, vertebrates use cilia for symmetry breaking [5]. In Drosophila, which lacks both cilia and Nodal, the unconventional myosin ID (myo1d) gene controls dextral rotation of chiral organs [6, 7]. Here, we studied the role of myo1d in left-right (LR) axis formation in Xenopus. Morpholino oligomer-mediated myo1d downregulation affected organ placement in >50% of morphant tadpoles. Induction of the left-asymmetric Nodal cascade was aberrant in >70% of cases. Expression of the flow-target gene dand5 was compromised, as was flow itself, due to shorter, fewer, and non-polarized cilia at the LR organizer. Additional phenotypes pinpointed Wnt/planar cell polarity signaling and suggested that myo1d, like in Drosophila [8], acted in the context of the planar cell polarity pathway. Indeed, convergent extension of gastrula explant cultures was inhibited in myo1d morphants, and the ATF2 reporter gene for non-canonical Wnt signaling was downregulated. Finally, genetic interference experiments demonstrated a functional interaction between the core planar cell polarity signaling gene vangl2 and myo1d in LR axis formation. Thus, our data identified myo1d as a common denominator of arthropod and chordate asymmetry, in agreement with a monophyletic origin of animal asymmetry.

Associations between laterality of orofacial clefts and medical and academic outcomes

Patients with oral clefts have an increased risk of other malformations, syndromes, and lower academic performance in school. Few studies have investigated if laterality of clefts is associated with medical and academic outcomes. Oral clefts have nonrandom laterality, with left-sided clefts occurring approximately twice as often as right-sided clefts. Using a retrospective study design, we examined potential associations of cleft attributes and outcomes in patients with cleft lip with or without cleft palate (CL/P) born in 2003-2010 who were treated at the Seattle Children's Craniofacial Center. The following variables were extracted from medical records: cleft type, medical history, maternal hyperglycemia, other malformations, and the need for academic support at school. We used logistic regression to examine risk of associations with outcomes of interest. Relative to patients with left-sided clefts, patients with bilateral CL/P were more likely to have a syndrome. Patients with nonsyndromic right-sided CL/P had a higher risk (OR and 95%CI: 3.5, 1.3-9.5, and 5.5, 1.9-16.0, respectively) of having other malformations and requiring academic support at school, when compared to patients with left-sided CL/P. Understanding the etiology of oral clefts is complicated, in part because both genetic and environmental factors contribute to the risk of developing a cleft. However, the different outcomes associated with cleft laterality suggest that right-sided clefts may have a distinct etiology. Using laterality to study cleft subgroups may advance our understanding of the etiology of this common birth defect.

Multiple Roles of Pitx2 in Cardiac Development and Disease

Cardiac development is a complex morphogenetic process initiated as bilateral cardiogenic mesoderm is specified at both sides of the gastrulating embryo. Soon thereafter, these cardiogenic cells fuse at the embryonic midline configuring a symmetrical linear cardiac tube. Left/right bilateral asymmetry is first detected in the forming heart as the cardiac tube bends to the right, and subsequently, atrial and ventricular chambers develop. Molecular signals emanating from the node confer distinct left/right signalling pathways that ultimately lead to activation of the homeobox transcription factor Pitx2 in the left side of distinct embryonic organ anlagen, including the developing heart. Asymmetric expression of Pitx2 has therefore been reported during different cardiac developmental stages, and genetic deletion of Pitx2 provided evidence of key regulatory roles of this transcription factor during cardiogenesis and thus congenital heart diseases. More recently, impaired Pitx2 function has also been linked to arrhythmogenic processes, providing novel roles in the adult heart. In this manuscript, we provide a state-of-the-art review of the fundamental roles of Pitx2 during cardiogenesis, arrhythmogenesis and its contribution to congenital heart diseases.

Cerberus-Nodal-Lefty-Pitx signaling cascade controls left-right asymmetry in amphioxus

Many bilaterally symmetrical animals develop genetically programmed left-right asymmetries. In vertebrates, this process is under the control of Nodal signaling, which is restricted to the left side by Nodal antagonists Cerberus and Lefty. Amphioxus, the earliest diverging chordate lineage, has profound left-right asymmetry as a larva. We show that Cerberus, Nodal, Lefty, and their target transcription factor Pitx are sequentially activated in amphioxus embryos. We then address their function by transcription activator-like effector nucleases (TALEN)-based knockout and heat-shock promoter (HSP)-driven overexpression. Knockout of Cerberus leads to ectopic right-sided expression of Nodal, Lefty, and Pitx, whereas overexpression of Cerberus represses their left-sided expression. Overexpression of Nodal in turn represses Cerberus and activates Lefty and Pitx ectopically on the right side. We also show Lefty represses Nodal, whereas Pitx activates Nodal These data combine in a model in which Cerberus determines whether the left-sided gene expression cassette is activated or repressed. These regulatory steps are essential for normal left-right asymmetry to develop, as when they are disrupted embryos may instead form two phenotypic left sides or two phenotypic right sides. Our study shows the regulatory cassette controlling left-right asymmetry was in place in the ancestor of amphioxus and vertebrates. This includes the Nodal inhibitors Cerberus and Lefty, both of which operate in feedback loops with Nodal and combine to establish asymmetric Pitx expression. Cerberus and Lefty are missing from most invertebrate lineages, marking this mechanism as an innovation in the lineage leading to modern chordates.

Current Perspectives in Cardiac Laterality

The heart is the first organ to break symmetry in the developing embryo and onset of dextral looping is the first indication of this event. Looping is a complex process that progresses concomitantly to cardiac chamber differentiation and ultimately leads to the alignment of the cardiac regions in their final topology. Generation of cardiac asymmetry is crucial to ensuring proper form and consequent functionality of the heart, and therefore it is a highly regulated process. It has long been known that molecular left/right signals originate far before morphological asymmetry and therefore can direct it. The use of several animal models has led to the characterization of a complex regulatory network, which invariably converges on the Tgf-β signaling molecule Nodal and its downstream target, the homeobox transcription factor Pitx2. Here, we review current data on the cellular and molecular bases of cardiac looping and laterality, and discuss the contribution of Nodal and Pitx2 to these processes. A special emphasis will be given to the morphogenetic role of Pitx2 and to its modulation of transcriptional and functional properties, which have also linked laterality to atrial fibrillation.

The CNS connectome of a tadpole larva of Ciona intestinalis (L.) highlights sidedness in the brain of a chordate sibling

Left-right asymmetries in brains are usually minor or cryptic. We report brain asymmetries in the tiny, dorsal tubular nervous system of the ascidian tadpole larva, Ciona intestinalis. Chordate in body plan and development, the larva provides an outstanding example of brain asymmetry. Although early neural development is well studied, detailed cellular organization of the swimming larva's CNS remains unreported. Using serial-section EM we document the synaptic connectome of the larva's 177 CNS neurons. These formed 6618 synapses including 1772 neuromuscular junctions, augmented by 1206 gap junctions. Neurons are unipolar with at most a single dendrite, and few synapses. Some synapses are unpolarised, others form reciprocal or serial motifs; 922 were polyadic. Axo-axonal synapses predominate. Most neurons have ciliary organelles, and many features lack structural specialization. Despite equal cell numbers on both sides, neuron identities and pathways differ left/right. Brain vesicle asymmetries include a right ocellus and left coronet cells.

Left-right axis asymmetry determining human Cryptic gene is transcriptionally repressed by Snail

BACKGROUND

Establishment of the left-right axis is important for positioning organs asymmetrically in the developing vertebrate-embryo. A number of factors like maternally deposited molecules have emerged essential in initiating the specification of the axis; the downstream events, however, are regulated by signal-transduction and gene-expression changes identifying which remains a crucial challenge. The EGF-CFC family member Cryptic, that functions as a co-receptor for some TGF-beta ligands, is developmentally expressed in higher mammals and mutations in the gene cause loss or change in left-right axis asymmetry. Despite the strong phenotype, no transcriptional-regulator of this gene is known till date.

RESULTS

Using promoter-analyses tools, we found strong evidence that the developmentally essential transcription factor Snail binds to the human Cryptic-promoter. We cloned the promoter-region of human Cryptic in a reporter gene and observed decreased Cryptic-promoter activation upon increasing Snail expression. Further, the expression of Cryptic is down-regulated upon exogenous Snail expression, validating the reporter assays and the previously identified role of Snail as a transcriptional repressor. Finally, we demonstrate using gel-shift assay that Snail in nuclear extract of PANC1 cells interacts with the promoter-construct bearing putative Snail binding sites and confirm this finding using chromatin immunoprecipitation assay.

CONCLUSIONS

Snail represses the expression of human Cryptic and therefore, might affect the signaling via Nodal that has previously been demonstrated to specify the left-right axis using the EGF-CFC co-receptors.

Deployment of regulatory genes during gastrulation and germ layer specification in a model spiralian mollusc Crepidula

BACKGROUND

During gastrulation, endoderm and mesoderm are specified from a bipotential precursor (endomesoderm) that is argued to be homologous across bilaterians. Spiralians also generate mesoderm from ectodermal precursors (ectomesoderm), which arises near the blastopore. While a conserved gene regulatory network controls specification of endomesoderm in deuterostomes and ecdysozoans, little is known about genes controlling specification or behavior of either source of spiralian mesoderm or the digestive tract.

RESULTS

Using the mollusc Crepidula, we examined conserved regulatory factors and compared their expression to fate maps to score expression in the germ layers, blastopore lip, and digestive tract. Many genes were expressed in both ecto- and endomesoderm, but only five were expressed in ectomesoderm exclusively. The latter may contribute to epithelial-to-mesenchymal transition seen in ectomesoderm.

CONCLUSIONS

We present the first comparison of genes expressed during spiralian gastrulation in the context of high-resolution fate maps. We found variation of genes expressed in the blastopore lip, mouth, and cells that will form the anus. Shared expression of many genes in both mesodermal sources suggests that components of the conserved endomesoderm program were either co-opted for ectomesoderm formation or that ecto- and endomesoderm are derived from a common mesodermal precursor that became subdivided into distinct domains during evolution.

Anticlockwise or clockwise? A dynamic Perception-Action-Laterality model for directionality bias in visuospatial functioning

Orientation bias and directionality bias are two fundamental functional characteristics of the visual system. Reviewing the relevant literature in visual psychophysics and visual neuroscience we propose here a three-stage model of directionality bias in visuospatial functioning. We call this model the 'Perception-Action-Laterality' (PAL) hypothesis. We analyzed the research findings for a wide range of visuospatial tasks, showing that there are two major directionality trends in perceptual preference: clockwise versus anticlockwise. It appears these preferences are combinatorial, such that a majority of people fall in the first category demonstrating a preference for stimuli/objects arranged from left-to-right rather than from right-to-left, while people in the second category show an opposite trend. These perceptual biases can guide sensorimotor integration and action, creating two corresponding turner groups in the population. In support of PAL, we propose another model explaining the origins of the biases - how the neurogenetic factors and the cultural factors interact in a biased competition framework to determine the direction and extent of biases. This dynamic model can explain not only the two major categories of biases in terms of direction and strength, but also the unbiased, unreliably biased or mildly biased cases in visuosptial functioning.

Asymmetry of the Brain: Development and Implications

Although the left and right hemispheres of our brains develop with a high degree of symmetry at both the anatomical and functional levels, it has become clear that subtle structural differences exist between the two sides and that each is dominant in processing specific cognitive tasks. As the result of evolutionary conservation or convergence, lateralization of the brain is found in both vertebrates and invertebrates, suggesting that it provides significant fitness for animal life. This widespread feature of hemispheric specialization has allowed the emergence of model systems to study its development and, in some cases, to link anatomical asymmetries to brain function and behavior. Here, we present some of what is known about brain asymmetry in humans and model organisms as well as what is known about the impact of environmental and genetic factors on brain asymmetry development. We specifically highlight the progress made in understanding the development of epithalamic asymmetries in zebrafish and how this model provides an exciting opportunity to address brain asymmetry at different levels of complexity.

Are there conserved roles for the extracellular matrix, cilia, and junctional complexes in left-right patterning?

Many different types of molecules have essential roles in patterning the left-right axis and directing asymmetric morphogenesis. In particular, the relationship between signaling molecules and transcription factors has been explored extensively. Another group of proteins implicated in left-right patterning are components of the extracellular matrix, apical junctions, and cilia. These structural molecules have the potential to participate in the conversion of morphogenetic cues from the extracellular environment into morphogenetic patterning via their interactions with the actin cytoskeleton. Although it has been relatively easy to temporally position these proteins within the hierarchy of the left-right patterning pathway, it has been more difficult to define how they mechanistically fit into these pathways. Consequently, our understanding of how these factors impart patterning information to influence the establishment of the left-right axis remains limited. In this review, we will discuss those structural molecules that have been implicated in early phases of left-right axis development.

A potential molecular pathogenesis of cardiac/laterality defects in Oculo-Facio-Cardio-Dental syndrome

Pitx2 is the last effector of the left-right (LR) cascade known to date and plays a crucial role in the patterning of LR asymmetry. In Xenopus embryos, the expression of Pitx2 gene in the left lateral plate mesoderm (LPM) is directly regulated by Xnr1 signaling, which is mediated by Smads and FoxH1. Previous studies suggest that the suppression of Pitx2 gene in the left LPM is a potential cause of cardiac/laterality defects in Oculo-Facio-Cardio-Dental (OFCD) syndrome, which is known to be caused by mutations in BCL6 co-repressor (BCOR) gene. Recently, our work has revealed that the BCL6/BCOR complex blocks Notch-dependent transcriptional activity to protect the expression of Pitx2 in the left LPM from the inhibitory activity of Notch signaling. These studies indicated that uncontrolled Notch activity in the left LPM caused by dysfunction of BCOR may result in cardiac/laterality defects of OFCD syndrome. However, this Notch-dependent inhibitory mechanism of Pitx2 gene transcription still remains unknown. Here we report that transcriptional repressor ESR1, which acts downstream of Notch signaling, inhibits the expression of Pitx2 gene by binding to a left side-specific enhancer (ASE) region in Pitx2 gene and recruiting histone deacetylase 1 (HDAC1) to this region. Once HDAC1 is tethered, histone acetyltransferase p300 is no longer recruited to the Xnr1-dependent transcriptional complex on the ASE region, leading to the suppression of Pitx2 gene in the left LPM. The study presented here uncovers the regulatory mechanism of Pitx2 gene transcription which may contribute to an understanding of pathogenesis of OFCD syndrome.

Homeobox transcription factor Pitx2: The rise of an asymmetry gene in cardiogenesis and arrhythmogenesis

The homeobox transcription factor Pitx2 displays a highly specific expression pattern during embryogenesis. Gain and loss of function experiments have unraveled its pivotal role in left-right signaling. Conditional deletion in mice has demonstrated a complex and intricate role for Pitx2 in distinct aspects of cardiac development and more recently a link to atrial fibrillation has been proposed based on genome-wide association studies. In this review we will revise the role of Pitx2 in the developing heart, starting from the early events of left-right determination followed by its role in cardiac morphogenesis and ending with its role in cardiac arrhythmogenesis.

Brain and behavioral lateralization in invertebrates

Traditionally, only humans were thought to exhibit brain and behavioral asymmetries, but several studies have revealed that most vertebrates are also lateralized. Recently, evidence of left–right asymmetries in invertebrates has begun to emerge, suggesting that lateralization of the nervous system may be a feature of simpler brains as well as more complex ones. Here I present some examples in invertebrates of sensory and motor asymmetries, as well as asymmetries in the nervous system. I illustrate two cases where an asymmetric brain is crucial for the development of some cognitive abilities. The first case is the nematode Caenorhabditis elegans, which has asymmetric odor sensory neurons and taste perception neurons. In this worm left/right asymmetries are responsible for the sensing of a substantial number of salt ions, and lateralized responses to salt allow the worm to discriminate between distinct salt ions. The second case is the fruit fly Drosophila melanogaster, where the presence of asymmetry in a particular structure of the brain is important in the formation or retrieval of long-term memory. Moreover, I distinguish two distinct patterns of lateralization that occur in both vertebrates and invertebrates: individual-level and population-level lateralization. Theoretical models on the evolution of lateralization suggest that the alignment of lateralization at the population level may have evolved as an evolutionary stable strategy in which individually asymmetrical organisms must coordinate their behavior with that of other asymmetrical organisms. This implies that lateralization at the population-level is more likely to have evolved in social rather than in solitary species. I evaluate this new hypothesis with a specific focus on insects showing different level of sociality. In particular, I present a series of studies on antennal asymmetries in honeybees and other related species of bees, showing how insects may be extremely useful to test the evolutionary hypothesis.

A detailed description of the development of the hemichordate Saccoglossus kowalevskii using SEM, TEM, Histology and 3D-reconstructions

INTRODUCTION

Traditionally, the origin of the third germ layer and its special formation of coelomic cavities by enterocoely is regarded to be an informative character in phylogenetic analyses. In early deuterostomes such as sea urchins, the mesoderm forms through a single evagination pinching off from the apical end of the archenteron which then gives off mesocoela and metacoela on each side. This echinoid-type coelom formation has conventionally been assumed to be ancestral for Deuterostomia. However, recent phylogenetic analyses show that Echinodermata hold a more derived position within Deuterostomia. In this regard a subgroup of Hemichordata, namely enteropneusts, seem to host promising candidates, because they are supposed to have retained many ancestral deuterostome features on the one hand, and furthermore share some characteristics with chordates on the other hand. In enteropneusts a wide range of different modes of coelom formation has been reported and in many cases authors of the original observations carefully detailed the limitations of their descriptions, while these doubts disappeared in subsequent reviews. In the present study, we investigated the development of all tissues in an enteropneust, Saccoglossus kowalevskii by using modern morphological techniques such as complete serial sectioning for LM and TEM, and 3D-reconstructions, in order to contribute new data to the elucidation of deuterostome evolution.

RESULTS

Our data show that in the enteropneust S. kowalevskii all main coelomic cavities (single protocoel, paired mesocoela and metacoela) derive from the endoderm via enterocoely as separate evaginations, in contrast to the aforementioned echinoid-type. The anlagen of the first pair of gill slits emerge at the late kink stage (~96 h pf). From that time onwards, we documented a temporal left-first development of the gill slits and skeletal gill rods in S. kowalevskii until the 2 gill slit juvenile stage.

CONCLUSIONS

The condition of coelom formation from separate evaginations is recapitulated in the larva of amphioxus and can be observed in crinoid echinoderms in a similar way. Therefore, coelom formation from separated pouches, rather than from a single apical pouch with eventual subdivision is suggested as the ancestral type of coelom formation for Deuterostomia. Left-right asymmetries are also present in echinoderms (rudiment formation), cephalochordates (larval development), tunicates (gut coiling) and vertebrates (visceral organs), and it is known from other studies applying molecular genetic analyses that genes such as nodal, lefty and pitx are involved during development. We discuss our findings in S. kowalevskii in the light of morphological as well as molecular genetic data.

Reciprocal signaling between the ectoderm and a mesendodermal left-right organizer directs left-right determination in the sea urchin embryo

During echinoderm development, expression of nodal on the right side plays a crucial role in positioning of the rudiment on the left side, but the mechanisms that restrict nodal expression to the right side are not known. Here we show that establishment of left-right asymmetry in the sea urchin embryo relies on reciprocal signaling between the ectoderm and a left-right organizer located in the endomesoderm. FGF/ERK and BMP2/4 signaling are required to initiate nodal expression in this organizer, while Delta/Notch signaling is required to suppress formation of this organizer on the left side of the archenteron. Furthermore, we report that the H(+)/K(+)-ATPase is critically required in the Notch signaling pathway upstream of the S3 cleavage of Notch. Our results identify several novel players and key early steps responsible for initiation, restriction, and propagation of left-right asymmetry during embryogenesis of a non-chordate deuterostome and uncover a functional link between the H(+)/K(+)-ATPase and the Notch signaling pathway.

Opposing nodal and BMP signals regulate left-right asymmetry in the sea urchin larva

Nodal and BMP signals are important for establishing left-right (LR) asymmetry in vertebrates. In sea urchins, Nodal signaling prevents the formation of the rudiment on the right side. However, the opposing pathway to Nodal signaling during LR axis establishment is not clear. Here, we revealed that BMP signaling is activated in the left coelomic pouch, specifically in the veg2 lineage, but not in the small micromeres. By perturbing BMP activities, we demonstrated that BMP signaling is required for activating the expression of the left-sided genes and the formation of the left-sided structures. On the other hand, Nodal signals on the right side inhibit BMP signaling and control LR asymmetric separation and apoptosis of the small micromeres. Our findings show that BMP signaling is the positive signal for left-sided development in sea urchins, suggesting that the opposing roles of Nodal and BMP signals in establishing LR asymmetry are conserved in deuterostomes.

Neurula rotation determines left-right asymmetry in ascidian tadpole larvae

Tadpole larvae of the ascidian Halocynthia roretzi show morphological left-right asymmetry. The tail invariably bends towards the left side within the vitelline membrane. The structure of the larval brain is remarkably asymmetric. nodal, a conserved gene that shows left-sided expression, is also expressed on the left side in H. roretzi but in the epidermis unlike in vertebrates. We show that nodal signaling at the late neurula stage is required for stereotypic morphological left-right asymmetry at later stages. We uncover a novel mechanism to break embryonic symmetry, in which rotation of whole embryos provides the initial cue for left-sided expression of nodal. Two hours prior to the onset of nodal expression, the neurula embryo rotates along the anterior-posterior axis in a counterclockwise direction when seen in posterior view, and then this rotation stops when the left side of the embryo is oriented downwards. It is likely that epidermis monocilia, which appear at the neurula rotation stage, generate the driving force for the rotation. When the embryo lies on the left side, protrusion of the neural fold physically prevents it from rotating further. Experiments in which neurula rotation is perturbed by various means, including centrifugation and sandwiching between glass, indicate that contact of the left epidermis with the vitelline membrane as a consequence of neurula rotation promotes nodal expression in the left epidermis. We suggest that chemical, and not mechanical, signals from the vitelline membrane promote nodal expression. Neurula rotation is also conserved in other ascidian species.

Left-sided predominance of hypodontia irrespective of cleft sidedness in a French population

OBJECTIVE

Individuals with oral clefts exhibit considerably more dental anomalies than do individuals without clefts. Our aim was to evaluate the prevalence of tooth agenesis in a sample composed of 124 children (81 boys and 43 girls, mean age 12.5 years) with clefts registered with the Cleft Palate Center in Strasbourg (France).

DESIGN

Cleft types and dental agenesis were assessed by clinical and radiographic examination. Cleft types were divided into four groups according to the clinical extent of the cleft (cleft lip only [CL, 12.9%], cleft lip and alveolus [CLA, 4%], cleft lip and palate [CLP, 49.2%], and cleft palate only [CP, 33.9%]).

RESULTS

Of the subjects 63% had evidence of hypodontia: maxillary lateral incisors (54%) and upper and lower premolars (32%) were the most common missing teeth. The number of dental ageneses associated with CP (54%) and CLP (79%) was significantly higher than that with CL (33%). All these anomalies were found in proportionately higher frequencies as the severity of the cleft increased, and we found left side predominance for hypodontia (p < .01) irrespective of cleft sidedness.

CONCLUSIONS

Both right-sided and left-sided clefts were more frequently correlated with left-sided dental agenesis. This left-sided prevalence suggests that common signaling malfunctions might be involved, both in dental development anomalies and cleft genesis.

Asexual propagation and regeneration in colonial ascidians

Regeneration is widely distributed among the metazoans. However, clear differences exist as to the degree of regenerative capacity: some phyla can only replace missing body parts, whereas others can generate entirely new individuals. Ascidians are animals that possess a remarkable regenerative plasticity and exhibit a great diversity of mechanisms for asexual propagation and survival. They are marine invertebrate members of the subphylum Tunicata and represent modern-day descendants of the chordate ancestor; in their tadpole stage they exhibit a chordate body plan that is resorbed during metamorphosis. Solitary species grow into an adult that can reach several centimeters in length, whereas colonial species grow by asexual propagation, creating a colony of genetically identical individuals. In this review, we present an overview of the biology of colonial ascidians as a paradigm for study in stem cell and regenerative biology. Focusing on botryllid ascidians, we introduce the potential roles played by multipotent epithelia and multipotent/pluripotent stem cells as source of asexual propagation and regenerative plasticity in the different budding mechanisms, and consider the putative mechanism of body repatterning in a non-embryonic scenario. We also discuss the involvement of intra-colony homeostatic processes in regulating budding potential, and the functional link between allorecognition, chimerism, and regenerative potential.

Xenopus: An emerging model for studying congenital heart disease

Congenital heart defects affect nearly 1% of all newborns and are a significant cause of infant death. Clinical studies have identified a number of congenital heart syndromes associated with mutations in genes that are involved in the complex process of cardiogenesis. The African clawed frog, Xenopus, has been instrumental in studies of vertebrate heart development and provides a valuable tool to investigate the molecular mechanisms underlying human congenital heart diseases. In this review, we discuss the methodologies that make Xenopus an ideal model system to investigate heart development and disease. We also outline congenital heart conditions linked to cardiac genes that have been well studied in Xenopus and describe some emerging technologies that will further aid in the study of these complex syndromes.

Repression of Rx gene on the left side of the sensory vesicle by Nodal signaling is crucial for right-sided formation of the ocellus photoreceptor in the development of Ciona intestinalis

Nodal signaling plays an essential role in the establishment of left-right asymmetry in various animals. However, it is largely unknown how Nodal signaling is involved in the establishment of the left-right asymmetric morphology. In this study, the role of Nodal signaling in the left-right asymmetric ocellus formation in the ascidian, Ciona intestinalis was dealt with. During the development of C. intestinalis, the ocellus pigment cell forms on the midline and moves to the right side of the midline. Then, the photoreceptor cells form on the right side of the sensory vesicle (SV). Ci-Nodal is expressed on the left side of the SV in the developing tail bud embryo. When Nodal signaling is inhibited, the ocellus pigment cell form but remain on the midline, and expression of marker genes of the ocellus photoreceptor cells is ectopically detected on the left side as well as on the right side of the SV in the larva. Furthermore, Ci-Rx, which is essential for the ocellus differentiation, turns out to be negatively regulated by the Nodal signaling on the left side of the SV, even though it is required for the right-sided photoreceptor formation. These results indicate that Nodal signaling controls the left-right asymmetric ocellus formation in the development of C. intestinalis.

Directional asymmetry in the limbs, skull and pelvis of the silver fox (V. vulpes)

Directional asymmetry (DA) is a characteristic of most vertebrates, most strikingly exhibited by the placement of various organs (heart, lungs, liver, etc.) but also noted in small differences in the metrics of skeletal structures such as the pelvis of certain fish or sauropsids. We have analyzed DA in the skeleton of the fox (V. vulpes), using ∼1,000 radiographs of foxes from populations used in the genetic analysis of behavior and morphology. Careful measurements from this robust data base demonstrate that: 1) DA occurs in the limb bones, the ileum, and ischium and in the mandible; 2) regardless of the direction of the length asymmetry vector of a particular skeletal unit, the vectorial direction of length is always opposite to that of width; 3) with the exception of the humerus and radius, there is no correlation or inverse correlation between vectorial amplitudes or magnitudes of bone asymmetries. 4) Postnatal measurements on foxes demonstrate that the asymmetry increases after birth and continues to change (increasing or decreasing) during postnatal growth. 5) A behavior test for preferential use of a specific forelimb exhibited fluctuating asymmetry but not DA. None of the skeletal asymmetries were significantly correlated with a preferential use of a specific forelimb. We suggest that for the majority of fox skeletal parameters, growth on the right and left side of the fox are differentially biased resulting in fixed differences between the two sides in either the rate of growth or the length of the period during which growth occurs. Random effects around these fixed differences perturb the magnitude of the effects such that the magnitudes of length and width asymmetries are not inversely correlated at the level of individual animals.