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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.

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; 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 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.

We discuss various situations where the formation of rocky coast morphology can be attributed to the retro-action of the coast morphology itself on the erosive power of the sea. Destroying the weaker elements of the coast, erosion can creates irregular seashores. In turn, the geometrical irregularity participates in the damping of sea-waves, decreasing their erosive power. There may then exist a mutual self-stabilization of the wave amplitude together with the irregular morphology of the coast. A simple model of this type of stabilization is discussed. The resulting coastline morphologies are diverse, depending mainly on the morphology/damping coupling. In the limit case of weak coupling, the process spontaneously builds fractal morphologies with a dimension close to 4/3. This provides a direct connection between the coastal erosion problem and the theory of percolation. For strong coupling, rugged but non-fractal coasts may emerge during the erosion process, and we investigate a geometrical characterization in these cases. The model is minimal, but can be extended to take into account heterogeneity in the rock lithology and various initial conditions. This allows to mimic coastline complexity, well beyond simple fractality. Our results suggest that the irregular morphology of coastlines as well as the stochastic nature of erosion are deeply connected with the critical aspects of percolation phenomena., Comment: 34 pages, 15 figures. arXiv admin note: substantial text overlap with arXiv:cond-mat/0311509