Cavitation-induced force transition in confined viscous liquids under traction

We perform traction experiments on simple liquids highly confined between parallel plates. At small separation rates, we observe a simple response corresponding to a convergent Poiseuille flow. Dramatic changes in the force response occur at high separation rates, with the appearance of a force plateau followed by an abrupt drop. By direct observation in the course of the experiment, we show that cavitation accounts for these features which are reminiscent of the utmost complex behavior of adhesive films under traction. Surprisingly enough, this is observed here in purely viscous fluids.Comment: Submitted to Physical Review Letters on May 31, 2002. Related informations on http://www.crpp.u-bordeaux.fr/tack.htm

Functional odor classification through a medicinal chemistry approach

Crucial for any hypothesis about odor coding is the classification and prediction of sensory qualities in chemical compounds. The relationship between perceptual quality and molecular structure has occupied olfactory scientists throughout the 20th century, but details of the mechanism remain elusive. Odor molecules are typically organic compounds of low molecular weight that may be aliphatic or aromatic, may be saturated or unsaturated, and may have diverse functional polar groups. However, many molecules conforming to these characteristics are odorless. One approach recently used to solve this problem was to apply machine learning strategies to a large set of odors and human classifiers in an attempt to find common and unique chemical features that would predict a chemical’s odor. We use an alternative method that relies more on the biological responses of olfactory sensory neurons and then applies the principles of medicinal chemistry, a technique widely used in drug discovery. We demonstrate the effectiveness of this strategy through a classification for esters, an important odorant for the creation of flavor in wine. Our findings indicate that computational approaches that do not account for biological responses will be plagued by both false positives and false negatives and fail to provide meaningful mechanistic data. However, the two approaches used in tandem could resolve many of the paradoxes in odor perception

Applying medicinal chemistry strategies to understand odorant discrimination

Associating an odorant’s chemical structure with its percept is a long-standing challenge. One hindrance may come from the adoption of the organic chemistry scheme of molecular description and classification. Chemists classify molecules according to characteristics that are useful in synthesis or isolation, but which may be of little importance to a biological sensory system. Accordingly, we look to medicinal chemistry, which emphasizes biological function over chemical form, in an attempt to discern which among the many molecular features are most important for odour discrimination. Here we use medicinal chemistry concepts to assemble a panel of molecules to test how heteroaromatic ring substitution of the benzene ring will change the odour percept of acetophenone. This work allows us to describe an extensive rule in odorant detection by mammalian olfactory receptors. Whereas organic chemistry would have predicted the ring size and composition to be key features, our work reveals that the topological polar surface area is the key feature for the discrimination of these odorants

Spreading of Non-Newtonian and Newtonian Fluids on a Solid Substrate under Pressure

Strongly non-Newtonian fluids namely, aqueous gels of starch, are shown to exhibit visco-elastic behavior, when subjected to a load. We study arrowroot and potato starch gels. When a droplet of the fluid is sandwiched between two glass plates and compressed, the area of contact between the fluid and plates increases in an oscillatory manner. This is unlike Newtonian fluids, where the area increases monotonically in a similar situation. The periphery moreover, develops an instability, which looks similar to Saffman Taylor fingers. This is not normally seen under compression. The loading history is also found to affect the manner of spreading. We attempt to describe the non-Newtonian nature of the fluid through a visco-elastic model incorporating generalized calculus. This is shown to reproduce qualitatively the oscillatory variation in the surface strain.Comment: 11 page

A comparison of the olfactory gene repertoires of adults and larvae in the noctuid moth Spodoptera littoralis

International audienceTo better understand the olfactory mechanisms in a lepidopteran pest model species, the cotton leafworm Spodoptera littoralis, we have recently established a partial transcriptome from adult antennae. Here, we completed this transcriptome using next generation sequencing technologies, namely 454 and Illumina, on both adult antennae and larval tissues, including caterpillar antennae and maxillary palps. All sequences were assembled in 77,643 contigs. Their analysis greatly enriched the repertoire of chemosensory genes in this species, with a total of 57 candidate odorant-binding and chemosensory proteins, 47 olfactory receptors, 6 gustatory receptors and 17 ionotropic receptors. Using RT-PCR, we conducted the first exhaustive comparison of olfactory gene expression between larvae and adults in a lepidopteran species. All the 127 candidate olfactory genes were profiled for expression in male and female adult antennae and in caterpillar antennae and maxillary palps. We found that caterpillars expressed a smaller set of olfactory genes than adults, with a large overlap between these two developmental stages. Two binding proteins appeared to be larvae-specific and two others were adult-specific. Interestingly, comparison between caterpillar antennae and maxillary palps revealed numerous organ-specific transcripts, suggesting the complementary involvement of these two organs in larval chemosensory detection. Adult males and females shared the same set of olfactory transcripts, except two male-specific candidate pheromone receptors, two male-specific and two female-specific odorant-binding proteins. This study identified transcripts that may be important for sex-specific or developmental stage-specific chemosensory behaviors

Adhesion between elastic solids with randomly rough surfaces: comparison of analytical theory with molecular dynamics simulations

The adhesive contact between elastic solids with randomly rough, self affine fractal surfaces is studied by molecular dynamics (MD) simulations. The interfacial binding energy obtained from the simulations of nominally flat and curved surfaces is compared with the predictions of the contact mechanics theory by Persson. Theoretical and simulation results agree rather well, and most of the differences observed can be attributed to finite size effects and to the long-range nature of the interaction between the atoms in the block and the substrate in the MD model, as compared to the analytical theory which is for an infinite system with interfacial contact interaction. For curved surfaces (JKR-type of problem) the effective interfacial energy exhibit a weak hysteresis which may be due to the influence of local irreversible detachment processes in the vicinity of the opening crack tip during pull-off.Comment: 6 pages, 6 figure

A novel lineage of candidate pheromone receptors for sex communication in moths

Sex pheromone receptors (PRs) are key players in chemical communication between mating partners in insects. In the highly diversified insect order Lepidoptera, male PRs tuned to female-emitted type I pheromones (which make up the vast majority of pheromones identified) form a dedicated subfamily of odorant receptors (ORs). Here, using a combination of heterologous expression and in vivo genome editing methods, we bring functional evidence that at least one moth PR does not belong to this subfamily but to a distantly related OR lineage. This PR, identified in the cotton leafworm Spodoptera littoralis, is highly expressed in male antennae and is specifically tuned to the major sex pheromone component emitted by females. Together with a comprehensive phylogenetic analysis of moth ORs, our functional data suggest two independent apparitions of PRs tuned to type I pheromones in Lepidoptera, opening up a new path for studying the evolution of moth pheromone communication

Capacités olfactives des chenilles de la noctuelle Spodoptera littoralis et modulation par la prise alimentaire

Lepidoptera split their life cycle in two main phases: the adults assume the reproductive responsibilities and species dispersal whereas the larval stage consists of caterpillars that eat and grow in order to accumulate energy for metamorphosis. Within Lepidoptera, Noctuid caterpillars are responsible for 1/6 of agricultural losses around the world. Although caterpillars have been for long considered to have low mobility and to eat the plant they have been deposited on as eggs. Yet, different studies cumulated evidences that caterpillars have the olfactory ability to orient to and choose their hosts. However, the studies investigating the mechanisms of odorant detection are mainly focused on the adult stage. In this context, my work investigated the olfactory abilities of the caterpillars of the cotton leafworm Spodoptera littoralis, and their modulation by food intake. Indeed, several studies have described that starvation induces an increase in insect olfactory abilities, but the underlying mechanism are poorly understood. First, we searched for caterpillar relevant odorant signals using a behavioral approach. Through this work, we evidenced that caterpillars were attracted toward a sex pheromone source. This result was unexpected since the sex pheromone is described as an adult-specific signal, that they use for reproduction. What would be the biological relevance of such a signal for caterpillars? A possible hypothesis developed in this thesis would be that caterpillars use this singular signal to enhance food detection. Females may leave some pheromone traces on adequate host plants while calling or ovipositing, a trace the caterpillars may follow to find food.In parallel, we developed a transcriptomic approach based on next generation sequencing to: 1) complete, by sequencing caterpillar chemosensory organs, the olfactory gene repertoire previously described in S. littoralis adults, 2) establish a reference transcriptome to be used for RNAseq analyses of starvation in caterpillars.The analyses of the obtained transcriptome allowed us to identify a large repertoire of olfactory genes in this species, including odorant-binding proteins and chemosensory receptors. The description of a large array of olfactory genes in S. littoralis made it possible to investigate, for the first time in a crop pest species, the distribution of chemosensory genes between the developmental stages, revealing genes overlapping between caterpillar and larvae but also some stage-specific genes.The reference transcriptome was also used to evidence up-regulation of chemosensory proteins and antennal enzymes upon 24h of starvation. Thus, starvation may induce increased olfactory sensitivity in insects by increasing the odorant access to the receptor and the odorant clearance close to it. Through this work, we contributed to demonstrate that the peripheral chemosensory system of insect participates in olfactory plasticity.The established reference transcriptome could be further used for other profiling studies in this insect model, but also paves the way to future reverse-genetics experiments to identify regulators of feeding behavior or sex/stage specific olfactory behaviors, that mat represent new targets to fight against herbivorous pest insects.Le cycle de vie des lépidoptères se divise en deux phases principales : les adultes assurent la reproduction et la dispersion de l’espèce, tandis que les larves passent la majorité de leur temps à se nourrir afin d’accumuler assez d’énergie pour réussir leur métamorphose. Ainsi, les chenilles de noctuelles sont responsables de près d’1/6 des pertes des productions agricoles et sont le véritable stade ravageur. Longtemps considérées comme peu mobiles et ne se nourrissant que de la plante sur laquelle elles sont nées, des résultats récents montrent qu’au contraire les chenilles auraient la faculté de « choisir » leur hôte par perception olfactive. Cependant, l’étude des capacités olfactives des lépidoptères s’est essentiellement concentrée à ce jour sur le stade adulte. Ce travail s’est donc intéressé à caractériser les capacités olfactives des chenilles de la noctuelle du coton, Spodoptera littoralis, et à étudier l’impact de la prise alimentaire sur son système olfactif périphérique. En effet, de nombreux travaux ont décrit une augmentation des capacités olfactives des insectes en période de jeûne, mais les bases moléculaires sont encore assez méconnues. Dans un premier temps, nous avons cherché à identifier des signaux odorants pertinents pour les chenilles à travers une approche comportementale. Ces travaux ont permis de mettre en évidence un phénomène inattendu : les larves sont attirées par la phéromone sexuelle émise par les femelles. La question du rôle biologique de ce phénomène se pose désormais puisque la phéromone sexuelle était jusqu’alors décrite comme un signal olfactif spécifique du stade adulte en vue d’assurer la reproduction. Une hypothèse développée dans cette thèse serait une utilisation de ce signal par les chenilles pour localiser leurs plantes hôtes, marquées par les femelles lorsqu’elles sont en appel ou présélectionnées lors de l’oviposition. En parallèle, nous avons développé une approche transcriptomique par séquençage à haut débit pour : 1) compléter le répertoire des gènes chimiosensoriels de cette espèce préalablement obtenu au laboratoire sur les adultes, en y incluant les organes chimiosensoriels de chenilles, 2) établir un transcriptome de référence le plus complet possible pour étudier, par approches RNAseq, l’effet du jeûne sur l’expression du transcriptome sensoriel périphérique. Ainsi, l’analyse de ce transcriptome nous a permis d’identifier un large répertoire de gènes chimiosensoriels de S. littoralis, dont des protéines de liaison aux odorants et des récepteurs chimiosensoriels. Pour la première fois chez une espèce ravageuse, le profil d’expression de ces gènes a été comparé entre les stades développementaux, révélant de nombreux gènes partagés mais également des spécificités pour chaque stade. Ce transcriptome de référence nous a également permis de mettre en évidence qu’un jeûne de 24h entraine une surexpression de gènes codants pour des protéines de liaison des odorants et pour des protéines de dégradation des odorants. Cette surexpression entrainerait une meilleure dynamique de réception des odorants par les neurones olfactifs, ce qui pourrait expliquer en partie une augmentation des capacités olfactives des insectes. Ainsi, cette étude contribue à montrer que le système périphérique, bien que longtemps négligé au profit du système nerveux central, participerait à la plasticité olfactive. Grâce au transcriptome de référence établi, d’autres phénomènes de plasticité pourront à l’avenir être étudiés. Ces études ont permis de mettre en évidence certains gènes olfactifs régulés lors du développement ou selon la prise alimentaire, ouvrant la voie à leur caractérisation fonctionnelle. Impliqués dans les comportements olfactifs, ils apparaissent comme de nouvelles cibles pour perturber les comportements et lutter contre ces ravageurs