Formation of the Ascidian Epidermal Sensory Neurons: Insights into the Origin of the Chordate Peripheral Nervous System

The vertebrate peripheral nervous system (PNS) originates from neural crest and placodes. While its developmental origin is the object of intense studies, little is known concerning its evolutionary history. To address this question, we analyzed the formation of the larval tail PNS in the ascidian Ciona intestinalis. The tail PNS of Ciona is made of sensory neurons located within the epidermis midlines and extending processes in the overlying tunic median fin. We show that each midline corresponds to a single longitudinal row of epidermal cells and neurons sharing common progenitors. This simple organization is observed throughout the tail epidermis, which is made of only eight single-cell rows, each expressing a specific genetic program. We next demonstrate that the epidermal neurons are specified in two consecutive steps. During cleavage and gastrula stages, the dorsal and ventral midlines are independently induced by FGF9/16/20 and the BMP ligand ADMP, respectively. Subsequently, Delta/Notch–mediated lateral inhibition controls the number of neurons formed within these neurogenic regions. These results provide a comprehensive overview of PNS formation in ascidian and uncover surprising similarities between the fate maps and embryological mechanisms underlying formation of ascidian neurogenic epidermis midlines and the vertebrate median fin

Évaluation multicritère de systèmes de culture zéro-pesticides en grande culture et polyculture-élevage (Réseau Rés0Pest)

Dans le cadre du plan Ecophyto, un réseau d’expérimentations système a été mis en place en Francemétropolitaine, avec pour objectif de concevoir et d’évaluer des systèmes de culture sans pesticides.Les systèmes ont été mis en place dans des situations variées (différents climats, types de sol, filières)et concernent les grandes cultures et la polyculture élevage. Une évaluation multicritère (Criter 5.4,MASC 2.0) a été conduite sur la période 2013-2017. On n’observe pas de dégradation de la capacitéproductive à long-terme, ni de la qualité sanitaire des récoltes et la contribution au développementdurable des huit systèmes testés est considérée «faible à moyenne» pour la polyculture élevage et«moyenne à élevée» à «élevée» pour la grande culture. La durabilité environnementale est « élevée » à«très élevée» pour tous les systèmes et c’est la durabilité sociale qui est la moins favorable en raisond’une «très faible» contribution à l’emploi. La rentabilité montre de fortes différences entre systèmes,allant de « très faible » à « très élevée » mais aucune valorisation économique particulière des récoltesproduites sans pesticides n’a été appliquée. Les résultats présentés débouchent sur de nouvellesperspectives pour une réduction significative de l’utilisation des pesticides en France

Anteroposterior axis patterning by early canonical Wnt signaling during hemichordate development

The Wnt family of secreted proteins has been proposed to play a conserved role in early specification of the bilaterian anteroposterior (A/P) axis. This hypothesis is based predominantly on data from vertebrate embryogenesis as well as planarian regeneration and homeostasis, indicating that canonical Wnt (cWnt) signaling endows cells with positional information along the A/P axis. Outside of these phyla, there is strong support for a conserved role of cWnt signaling in the repression of anterior fates, but little comparative support for a conserved role in promotion of posterior fates. We further test the hypothesis by investigating the role of cWnt signaling during early patterning along the A/P axis of the hemichordate Saccoglossus kowalevskii. We have cloned and investigated the expression of the complete Wnt ligand and Frizzled receptor complement of S. kowalevskii during early development along with many secreted Wnt modifiers. Eleven of the 13 Wnt ligands are ectodermally expressed in overlapping domains, predominantly in the posterior, and Wnt antagonists are localized predominantly to the anterior ectoderm in a pattern reminiscent of their distribution in vertebrate embryos. Overexpression and knockdown experiments, in combination with embryological manipulations, establish the importance of cWnt signaling for repression of anterior fates and activation of mid-axial ectodermal fates during the early development of S. kowalevskii. However, surprisingly, terminal posterior fates, defined by posterior Hox genes, are unresponsive to manipulation of cWnt levels during the early establishment of the A/P axis at late blastula and early gastrula. We establish experimental support for a conserved role of Wnt signaling in the early specification of the A/P axis during deuterostome body plan diversification, and further build support for an ancestral role of this pathway in early evolution of the bilaterian A/P axis. We find strong support for a role of cWnt in suppression of anterior fates and promotion of mid-axial fates, but we find no evidence that cWnt signaling plays a role in the early specification of the most posterior axial fates in S. kowalevskii. This posterior autonomy may be a conserved feature of early deuterostome axis specification

Cryptococcal Neuroradiological Lesions Correlate with Severity during Cryptococcal Meningoencephalitis in HIV-Positive Patients in the HAART Era

Cryptococcal meningoencephalitis has an overall global mortality rate of 20% in AIDS patients despite antifungals. There is a need for additional means of precise assessment of disease severity. We thus studied the radiological brain images available from 62 HIV-positive patients with cryptococcocal meningoencephalitis to analyse the brain lesions associated with cryptococcosis in relationship with disease severity, and the respective diagnostic contribution of magnetic resonance (MR) versus computed tomography (CT)

Msxb is a core component of the genetic circuitry specifying the dorsal and ventral neurogenic midlines in the ascidian embryo

AbstractThe tail ascidian larval peripheral nervous system is made up of epidermal sensory neurons distributed more or less regularly in ventral and dorsal midlines. Their formation occurs in two-steps: the ventral and dorsal midlines are induced as neurogenic territories by Fgf9/16/20 and Admp respectively. The Delta2/Notch interaction then controls the number of neurons that form. The genetic machinery acting between the inductive processes taking place before gastrulation and neuron specification at tailbud stages are largely unknown. The analysis of seven transcription factors expressed in the forming midlines revealed an unexpected complexity and dynamic of gene expression. Their systematic overexpression confirmed that these genes do not interact following a linear cascade of activation. However, the integration of our data revealed the distinct key roles of the two upstream factors Msxb and Nkx-C that are the earliest expressed genes and the only ones able to induce neurogenic midline and ESN formation. Our data suggest that Msxb would be the primary midline gene integrating inputs from the ventral and dorsal inducers and launching a pan-midline transcriptional program. Nkx-C would be involved in tail tip specification, in maintenance of the pan-midline network and in a posterior to anterior wave controlling differentiation

Formation des structures axiales chez les chordés

AIX-MARSEILLE2-BU Sci.Luminy (130552106) / SudocSudocFranceF

An Otx/Nodal Regulatory Signature for Posterior Neural Development in Ascidians

International audienceIn chordates, neural induction is the first step of a complex developmental process through which ectodermal cells acquire a neural identity. In ascidians, FGF-mediated neural induction occurs at the 32-cell stage in two blastomere pairs, precursors respectively of anterior and posterior neural tissue. We combined molecular embryology and cis-regulatory analysis to unveil in the ascidian Ciona intestinalis the remarkably simple proximal genetic network that controls posterior neural fate acquisition downstream of FGF. We report that the combined action of two direct FGF targets, the TGFb factor Nodal, acting via Smad-and Fox-binding sites, and the transcription factor Otx suffices to trigger ascidian posterior neural tissue formation. Moreover, we found that this strategy is conserved in the distantly related ascidian Phallusia mammillata, in spite of extreme sequence divergence in the cis-regulatory sequences involved. Our results thus highlight that the modes of gene regulatory network evolution differ with the evolutionary scale considered. Within ascidians, developmental regulatory networks are remarkably robust to genome sequence divergence. Between ascidians and vertebrates, major fate determinants, such as Otx and Nodal, can be co-opted into different networks. Comparative developmental studies in ascidians with divergent genomes will thus uncover shared ascidian strategies, and contribute to a better understanding of the diversity of developmental strategies within chordates

Coupling molecular data and experimental crosses sheds light about species delineation: a case study with the genus Ciona

Abstract Molecular studies sometimes reveal evolutionary divergence within accepted species. Such findings can initiate taxonomic revision, as exemplified in the formerly recognized species Ciona intestinalis. While an increasing number of studies have examined the ecology, reproductive barriers and genetics of C. intestinalis and C. robusta, there are still much uncertainties regarding other species of this genus. Using experimental crosses and mitochondrial data, we investigated the evolutionary relationships among four native and introduced Ciona spp., found in sympatry in the Mediterranean Sea or English Channel. Outcome of 62 bi-parental reciprocal crosses between C. intestinalis, C. robusta, C. roulei and C. edwardsi showed that C. edwardsi is reproductively isolated from the other taxa, which is in agreement with its distinct location in the phylogenetic tree. Conversely, hybrids are easily obtained in both direction when crossing C. intestinalis and C. roulei, reinforcing the hypothesis of two genetically differentiated lineages but likely being from a same species. Altogether, this study sheds light on the evolutionary relationship in this complex genus. It also calls for further investigation notably based on genome-wide investigation to better describe the evolutionary history within the genus Ciona, a challenging task in a changing world where biological introductions are shuffling species distribution

Antero-posterior ectoderm patterning by canonical Wnt signaling during ascidian development

International audienceWnt/β-catenin signaling is an ancient pathway in metazoans and controls various developmental processes, in particular the establishment and patterning of the embryonic primary axis. In vertebrates, a graded Wnt activity from posterior to anterior endows cells with positional information in the central nervous system. Recent studies in hemichordates support a conserved role for Wnt/β-catenin in ectoderm antero-posterior patterning at the base of the deuterostomes. Ascidians are marine invertebrates and the closest relatives of vertebrates. By combining gain- and loss-of-function approaches, we have determined the role of Wnt/β-catenin in patterning the three ectoderm derivatives of the ascidian Ciona intestinalis, central nervous system, peripheral nervous system and epidermis. Activating Wnt/β-catenin signaling from gastrulation led to a dramatic transformation of the ectoderm with a loss of anterior identities and a reciprocal anterior extension of posterior identities, consistent with studies in other metazoans. Surprisingly, inhibiting Wnt signaling did not produce a reciprocal anteriorization of the embryo with a loss of more posterior identities like in vertebrates and hemichordate. Epidermis patterning was overall unchanged. Only the identity of two discrete regions of the central nervous system, the anteriormost and the posteriormost regions, were under the control of Wnt. Finally, the caudal peripheral nervous system, while being initially Wnt dependent, formed normally. Our results show that the Ciona embryonic ectoderm responds to Wnt activation in a manner that is compatible with the proposed function for this pathway at the base of the deuterostomes. However, possibly because of its fast and divergent mode of development that includes extensive use of maternal determinants, the overall antero-posterior patterning of the Ciona ectoderm is Wnt independent, and Wnt/β-catenin signaling controls the formation of some sub-domains. Our results thus indicate that there has likely been a drift in the developmental systems controlling ectoderm patterning in the lineage leading to ascidians

<i>Otx</i> is required for posterior neural tissue formation.

<p>Overexpressing Otx in the ectoderm using the pFOG driver is sufficient to activate <i>Msxb</i> (B) and <i>Delta2</i> (E) compared to control embryos (A, D) at stage 10. Upon injection of the <i>Otx</i> MO, <i>Msxb</i> (C) and <i>Delta2</i> (F) expression is abolished at stage 10. The dorsal expression of the tail midline marker <i>Klf1/2/4</i> is lost except in the posterior-most and ventral regions (I). The dorsal nerve cord marker <i>KH.C7.391</i> expression is also suppressed (J). Control MO-injected embryos at stage 10 (A, D) and stage 19 (G, H). Animal view with anterior to the top (B, D-F). Vegetal view with anterior to the top (A and C). Lateral view with dorsal to the top, anterior to the left (G-J). White arrows and arrowheads indicate sites with a loss of expression. (K) Summary of gene interactions reported in this study and from previous studies <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1004548#pgen.1004548-Bertrand2" target="_blank">[11]</a>, <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1004548#pgen.1004548-Hudson3" target="_blank">[29]</a>, <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1004548#pgen.1004548-Khoueiry1" target="_blank">[30]</a>, <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1004548#pgen.1004548-Imai2" target="_blank">[35]</a>.</p