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The Drosophila visual system supports complex behaviors and shares many of its anatomical and molecular features with the vertebrate brain. Yet, it contains a much more manageable number of neurons and neuronal types. In addition to the extensive Drosophila genetic toolbox, this relative simplicity has allowed decades of work to yield a detailed account of its neuronal type diversity, morphology, connectivity and specification mechanisms. In the past three years, numerous studies have applied large scale single-cell transcriptomic approaches to the Drosophila visual system and have provided access to the complete gene expression profile of most neuronal types throughout development. This makes the fly visual system particularly well suited to perform detailed studies of the genetic mechanisms underlying the evolution and development of neuronal systems. Here, we highlight how these transcriptomic resources allow exploring long-standing biological questions under a new light. We first present the efforts made to characterize neuronal diversity in the Drosophila visual system and suggest ways to further improve this description. We then discuss current advances allowed by the single-cell datasets, and envisage how these datasets can be further leveraged to address fundamental questions regarding the regulation of neuronal identity, neuronal circuit development and the evolution of neuronal diversity.

Deciphering how neuronal diversity is established and maintained requires a detailed knowledge of neuronal gene expression throughout development. In contrast to mammalian brains1,2, the large neuronal diversity of the Drosophila optic lobe3 and its connectome4-6 are almost completely characterized. However, a molecular characterization of this neuronal diversity, particularly during development, has been lacking. Here we present insights into brain development through a nearly complete description of the transcriptomic diversity of the optic lobes of Drosophila. We acquired the transcriptome of 275,000 single cells at adult and at five pupal stages, and built a machine-learning framework to assign them to almost 200 cell types at all time points during development. We discovered two large neuronal populations that wrap neuropils during development but die just before adulthood, as well as neuronal subtypes that partition dorsal and ventral visual circuits by differential Wnt signalling throughout development. Moreover, we show that the transcriptomes of neurons that are of the same type but are produced days apart become synchronized shortly after their production. During synaptogenesis we also resolved neuronal subtypes that, although differing greatly in morphology and connectivity, converge to indistinguishable transcriptomic profiles in adults. Our datasets almost completely account for the known neuronal diversity of the Drosophila optic lobes, and serve as a paradigm to understand brain development across species.

International audience; Dans l’objectif de mettre à jour la Plateforme d’Aide à la Facture Instrumentale Pafi, nous proposons ici de recenser et comparer les outils existants permettant de calculer l’impédance d’entrée d’instruments à vent. Un groupe de travail a été constitué, réunissant une dizaine d’équipes de recherche françaises expertes dans la modélisation et les méthodes de résolution par matrice de transfert, éléments finis 1D, éléments finis 3D, méthodes multimodales, guides d’ondes, etc. Ce groupe a défini un ensemble de géométries étalons de résonateurs mettant en jeu différentes conditions de propagation (cylindre, cône, coude, trou latéral, etc.) ou de rayonnement (fermé, avec baffle, etc.). Les impédances d’entrée de ces tubes ont été calculées en utilisant l'expertise des membres du groupe de travail puis mises en commun sur une plateforme collaborative. La base de données ainsi constituée peut être utilisée pour mener des études comparatives, la vérification de modèles et la comparaison avec une base expérimentale. Son analyse permettra notamment de faire un choix éclairé des méthodes et modèles à implémenter dans un outil d’aide à la facture en considérant leur validité, leur versatilité, leur complexité d’implémentation et les ressources de calcul nécessaires. Après avoir présenté la mise en place de ce projet et l’accessibilité de la base de données, nous discuterons des résultats obtenus sur quelques cas particuliers.

The brain consists of thousands of neuronal types that are generated by stem cells producing different neuronal types as they age. In Drosophila, this temporal patterning is driven by the successive expression of temporal transcription factors (tTFs)(1-3) We used single-cell mRNA sequencing to identify the complete series of tTFs that specify most Drosophila optic lobe neurons. We verify that tTFs regulate the progression of the series by activating the next tTF(s) and repressing the previous one(s), and also identify more complex regulations. Moreover, we establish the temporal window of origin and birth order of each neuronal type in the medulla and provide evidence that these tTFs are sufficient to explain the generation of all the neuronal diversity in this brain region. Finally, we describe the first steps of neuronal differentiation. We find that terminal differentiation genes, such as neurotransmitter-related genes, are present as transcripts, but not as proteins, in immature larval neurons; we show that these steps are conserved in humans. This comprehensive analysis of a temporal series of tTFs in the optic lobe offers mechanistic insights into how tTF series are regulated, and how they can lead to the generation of a complete set of neurons.

Répondre aux problématiques d'optimisation de perce ou de synthèse sonore par modèle physique nécessite une grande précision des modèles d'instruments à vent. La prise en compte des particularités géométriques de ces instruments est donc nécessaire pour modéliser le plus fidèlement possible leurs résonances. Ainsi, ce travail s'intéresse à l'effet sur l'impédance d'entrée de la courbure des instruments de la famille des cuivres. Une méthode multimodale est alors utilisée pour sa capacité à modéliser sur une large gamme de fréquence la propagation et le rayonnement de guides d'ondes cylindriques courbes de section variable. Une interpolation par plusieurs splines est appliquée à la géométrie d'un cor naturel Lawson dont la section a été mesurée. Une comparaison entre des simulations et la mesure de l'impédance d'entrée du cor révèle que la courbure de l'instrument peut induire une altération des fréquences de résonance ainsi qu'une atténuation de l'amplitude des pics d'impédance à hautes fréquences.

Une caractéristique du régime localisé dans un milieu désordonné est l’insensibilité du speckle transmis à l’onde incidente. En optique, l’image sur un écran du champ transmis à travers un milieu désordonné opaque - le speckle - est la même quelles que soient les conditions d’éclairage. Ce phénomène remarquable peut être expliqué par l’analyse des modes propres de transmission du matériau étudié. Le régime localisé se caractérise par la prédominance d’un unique mode, la transmission de tous les autres étant très significativement plus faible. Le motif du champ transmis est alors déterminé par ce seul mode, indépendamment de la source. Un phénomène analogue est possible dans un milieu ordonné, périodique, lorsque l’onde propagée dans le milieu est principalement portée par un unique mode de Bloch. L’onde propagée dans le milieu périodique est alors progressivement « gelée », présentant un même motif, quelque soit la source qui l’a générée. Le travail présenté vise à caractériser ce phénomène dans le cas de la propagation dans un guide d’onde périodique.

The brain consists of thousands of different neuronal types that are generated through multiple divisions of neuronal stem cells. These stem cells have the capacity to generate different neuronal types at different stages of their development. In Drosophila, this temporal patterning is driven by the successive expression of temporal transcription factors (tTFs). While a number of tTFs are known in different animals and across various parts of the nervous system, these have been mostly identified by informed guesses and antibody availability. We used single-cell mRNA sequencing to identify the complete series of tTFs that specify most Drosophila medulla neurons in the optic lobe. We tested the genetic interactions among these tTFs. While we verify the general principle that tTFs regulate the progression of the series by activating the next tTFs in the series and repressing the previous ones, we also identify more complex regulations. Two of the tTFs, Eyeless and Dichaete, act as hubs integrating the input of several upstream tTFs before allowing the series to progress and in turn regulating the expression of several downstream tTFs. Moreover, we show that tTFs not only specify neuronal identity by controlling the expression of cell type-specific genes. Finally, we describe the very first steps of neuronal differentiation and find that terminal differentiation genes, such as neurotransmitter-related genes, are present as transcripts, but not as proteins, in immature larval neurons days before they are being used in functioning neurons; we show that these mechanisms are conserved in humans. Our results offer a comprehensive description of a temporal series of tTFs in a neuronal system, offering mechanistic insights into the regulation of the progression of the series and the regulation of neuronal diversity. This represents a proof-of-principle for the use of single-cell mRNA sequencing for the comparison of temporal patterning across phyla that can lead to an understanding of how the human brain develops and how it has evolved.

International audience; The standard multimodal formulation in waveguides consists in discretizing the transverse problem into transverse modes, so that the solving can be reduced to the one longitudinal dimension. In addition to provide an efficient numerical method, one of the main advantages of this method is to provide a direct access to the physical quantities that are relevant to the understanding of the problem, such as impedances and scattering coefficients. In 3D waveguides with circular cross-section, the Bessel's function satisfying Neumann boudary conditions are naturally chosen to be the transverse modes of the waveguides. They indeed have the useful property of being uncoupled in the straight parts of the guide. However, they do not have the ability to fulfill the exact boundary condition in the varying radius parts of the waveguide, as the radial vector is no longer the normal vector to the wall. This has a strong impact on the convergence, that is significantly lowered. To tackle this issue, we add an orthogonalized boundary mode to our transverse basis (hence the *improved* multimodal formulation), that makes the fulfilling of the boundary condition at the wall much easier. As we will show in our presentation, this both keeps the propagation equation unchanged and restores the fast convergence.

International audience; We started in September 2019 a new master curriculum at Le Mans University, as part of a plan of actions in the frame of the Institut d?Acoustique - Graduate School. This Institute is related to a project funded by the French state, launched in 2018 in the frame of the national call ?Ecole Universitaire de Recherche? and which intends to offer ?Universities the possibility to reinforce the international impact and attractiveness of their research and training activities in one or several scientific domains, by integrating master and doctoral trainings with high level laboratories? and to introduce in France the model of Graduate Schools. One proposed action to implement all this, was to launch a new master curriculum focused on training by and through research, rethinking usual ways of organizing the training at master level in France, and inspired from several US and European universities. In this presentation, we will describe this new master curriculum on Wave Physics & Acoustics and in particular how we implemented a plan to favor education by and through research. We will highlight the necessary adjustments we had to make and the expected outputs for the students and the laboratory.

International audience; In the beginning of the 20th century, Dayton C. Miller conducted an experiment that was aiming at showing the vibrations impact on the sound that is produced by organ pipes. The experiment consists in surrounding a pipe by a leaking container that is filled with water. With the water level going down, some accidents are heard, that have long been attributed to the coupling between the wall vibrations and the acoustic field. However, to ensure that these accidents are unrelated to the varying radiation condition induced by the changing water level, the acoustic field and the radiation impedance in the Miller's experiment have been numerically computed. The results shows that some accidents are actually caused by acoustic resonances in the container, which have been confirmed experimentally. The presentation will insist on the multimodal method that has been used to solve the acoustic problem. It indeed provides an efficient alternative to the classical numerical methods, and is especially suited for this kind of radiation modeling.

International audience; In this work, we study the propagation of sound waves in a honeycomb waveguide network loaded with Helmholtz resonators (HRs). By using a plane wave approximation in each waveguide we obtain a first-principle modeling of the network, which is an exact mapping to the graphene tight-binding Hamiltonian. We show that additional Dirac points appear in the band diagram when HRs are introduced at the network nodes. It allows to break the inversion (sub-lattice) symmetry by tuning the resonators, leading to the appearence of edge modes that reflect the configuration of the zigzag boundaries. Besides, the dimerization of the resonators also permits the formation of interface modes located in the band gap, and these modes are found to be robust against symmetry preserving defects. Our results and the proposed networks reveal the additional degree of freedom bestowed by the local resonance in tuning the properties of not only acoustical graphene-like structures but also of more complex systems.

International audience; In a recent paper we report on a significant and broadband enhancement of the transmission through an opaque barrier when placed between symmetric diffusive slabs. In order to show if this effect could be observed experimentally, we raise the questions of the robustness of the observed enhancement to absorption and symmetry defects. Introducing losses, we adapt a scaling model from the conservative case to the non-conservative one. We show that while weakened, the enhancement is still observed and is mainly driven by the absorption length of the media. The sensitivity of this phenomenon to symmetry defects is then investigated by associating the deterioration of the enhancement with the positions of the symmetry defects that could be particularly interesting for non-destructive testing applications. We also propose a probabilistic model to estimate the conductance of a medium with a number of randomly distributed defects.

International audience

Two studies are presented in this work, both concerned with modal propagation in waveguides : the acoustic propagation in curved waveguides and the problem of the field generated by a source in a parallel shear flow.A multimodal formulation of the sound propagation in 2 or 3-dimensionnal curved waveguides is proposed and validated. Two infinite first-order differential equations are constructed for the pressure and axial velocity in the bend, as well as a Riccati equation for the impedance matrix. When the section of the bend is constant and the wall admittance is uniform, an algebraic formulation of the reflection and transmission matrices can be obtained, which enables us to characterize the scattering properties of bends and every complex duct system composed with curved and straight parts. A comparison between this wave approach and a scattering matrix constructed with a ray method shows a great similarity between these two results at high frequencies. Sound attenuation in lined bends is also considered using the multimodal method, and the properties of such ducts treated with absorbing materials are studied.In a second part, the sound field generated by a source in duct carrying a shear flow is derived. We first derive the Green function of the Pridmore-Brown equation, governing modes in a rectilinear duct with shear flow. When applying the inverse Fourier transform, the poles of this Green function lead to the acoustic modes, which amplitude can then be calculated. The presence of a continuous spectra due to the singularity of thePridmore-Brown equation is also shown, and the contribution of corresponding continuum of hydrodynamic modes is evaluated., Les travaux de cette thèse ont pour cadre l'approche modale de la propagation d'ondes en conduit. Deux aspects sont abordés : la propagation acoustique dans les guides d'ondes courbes, sans écoulement, et le problème d'une source dans un écoulement cisaillé, dans un conduit rectiligne.Pour un guide courbe à deux ou trois dimensions, une formulation multimodale de la propagation acoustique est mise en place et validée, et des équations différentielles matricielles sont établies pour la pression et la vitesse longitudinale, ainsi qu'une équation de Riccati pour la matrice impédance. Si les caractéristiques du coude (section, admittance aux parois) sont constantes, un calcul algébrique de la matrice de diffusion est possible, qui permet l'étude des propriétés de diffusion de ce coude et de tout système complexe composé de conduits droits et courbes. Une comparaison de cette approche, ondulatoire, et d'une matrice de diffusion construite par la méthode des rayons montre une très grande similitude entre ces résultats à haute fréquence. L'atténuation dans un coude traité en parois par un matériau absorbant est enfin étudiée dans le cadre de l'approche multimodale et nous mettons en évidence plusieurs propriétés des conduits courbes traités.La seconde partie de ces travaux concerne le problème de la propagation acoustique dans un guide droit siège d'un écoulement parallèle cisaillé, en présence d'une source. La fonction de Green de l'équation de Pridmore-Brown est calculée dans un premier temps. Par transformée de Fourier spatiale inverse, les pôles de la fonction de Green font apparaître les modes acoustiques perturbés par l'écoulement, dont il est alors possible de calculer l'amplitude. La présence d'un spectre continu dû à la singularité de l'équation de Pridmore-Brown est également mise en évidence, et la contribution du continuum de modes hydrodynamiques correspondant est étudiée et décrite.