Climate and biodiversity : application of recent methodological developments in the study and the modelling of transepidermal evaporative water loss in an amphibian

Dans un contexte de changement climatique, il est essentiel de prévoir comment les organismes peuvent être affectés dans leurs relations physiques avec l'environnement. Les amphibiens représentent le groupe des vertébrés le plus sensible aux changements environnementaux, en raison d'une perte en eau par évaporation transépidermique (PEET) permanente. Ma thèse vise à mettre en place des approches pour mesurer et modéliser avec précision les surfaces de la peau contribuant à la PEET; établir les lois de transfert d'eau amphibien-environnement; et fournir une meilleure compréhension des réponses physiologiques face au pathogène Batrachochytrium dendrobatidis. Ces expériences ont été réalisées sur des tritons palmés (Lissotriton helveticus) mâles adultes en phase terrestre. Nos résultats suggèrent que L. helveticus ne présente aucune adaptation physiologique pour limiter la PEET, les moyens étant limités à l'expression d'une posture en forme de dollar. Une méthode de reconstruction 3D basée sur l'lmagerie par Résonance Magnétique a été utilisée pour générer des géométries 3D des tritons utilisables pour mesurer leur surface, et à des fins de simulation. Nous avons ainsi réalisé une analyse numérique en 3D des PEET, et avons proposé une relation pour estimer cette perte dans une large gamme de conditions climatiques. Enfin, nos résultats supportent l'hypothèse de dysfonctionnement épidermique, suggérant que B. dendrobatidis compromet la capacité des amphibiens à se réhydrater. Ma thèse devrait contribuer à développer de nouvelles approches en sciences des relations eau-amphibiens, et à améliorer nos connaissances sur les effets des changements environnementaux sur les organismesLn a context of climate change, it is critical to predict how organisms might be affected in their physical relations with environment. Amphibians are among the vertebrate groups the most affected by ecological changes, because of permanent transepidermal evaporative water loss (TEWL) through their skin. My thesis aims at setting up approaches to measure and model accurately the functional skin surface areas that contribute to TEWL, establishing laws of water transfer between an amphibian and its physical environment, and providing understanding on physiological responses to the skin pathogen Batrachochytrium dendrobatidis. We conducted experiments on adult males of the palmate newt (Lissotriton helveticus) during their terrestrial phase. Our results suggest that L. helveticus did not show any physiological adaptations to restrain TEWL. The ways to reduce TEWL result essentially in the expression of a stereotyped dollar-shaped water-conserving posture. We used a Magnetic Resonance lmaging-based 3D reconstruction method to generate 3D geometries of newt that is suitable for measuring skin surface areas, and for simulation purposes. We successfully performed 3D numerical analysis of TEWL, and proposed an original relationship to estimate TEWL rates in a large range of temperature and humidity. Finally, our results support the epidermal dysfunction hypothesis, which suggests that B. dendrobatidis compromises the ability of amphibians to rehydrate. My thesis would contribute to open new approaches to the science of amphibian water relations, and improve our knowledge of the effects of ecological changes on organism

Climat et biodiversité : application de récents développements méthodologiques à l'étude et la modélisation des pertes en eau par évaporation chez un amphibien

Ln a context of climate change, it is critical to predict how organisms might be affected in their physical relations with environment. Amphibians are among the vertebrate groups the most affected by ecological changes, because of permanent transepidermal evaporative water loss (TEWL) through their skin. My thesis aims at setting up approaches to measure and model accurately the functional skin surface areas that contribute to TEWL, establishing laws of water transfer between an amphibian and its physical environment, and providing understanding on physiological responses to the skin pathogen Batrachochytrium dendrobatidis. We conducted experiments on adult males of the palmate newt (Lissotriton helveticus) during their terrestrial phase. Our results suggest that L. helveticus did not show any physiological adaptations to restrain TEWL. The ways to reduce TEWL result essentially in the expression of a stereotyped dollar-shaped water-conserving posture. We used a Magnetic Resonance lmaging-based 3D reconstruction method to generate 3D geometries of newt that is suitable for measuring skin surface areas, and for simulation purposes. We successfully performed 3D numerical analysis of TEWL, and proposed an original relationship to estimate TEWL rates in a large range of temperature and humidity. Finally, our results support the epidermal dysfunction hypothesis, which suggests that B. dendrobatidis compromises the ability of amphibians to rehydrate. My thesis would contribute to open new approaches to the science of amphibian water relations, and improve our knowledge of the effects of ecological changes on organismsDans un contexte de changement climatique, il est essentiel de prévoir comment les organismes peuvent être affectés dans leurs relations physiques avec l'environnement. Les amphibiens représentent le groupe des vertébrés le plus sensible aux changements environnementaux, en raison d'une perte en eau par évaporation transépidermique (PEET) permanente. Ma thèse vise à mettre en place des approches pour mesurer et modéliser avec précision les surfaces de la peau contribuant à la PEET; établir les lois de transfert d'eau amphibien-environnement; et fournir une meilleure compréhension des réponses physiologiques face au pathogène Batrachochytrium dendrobatidis. Ces expériences ont été réalisées sur des tritons palmés (Lissotriton helveticus) mâles adultes en phase terrestre. Nos résultats suggèrent que L. helveticus ne présente aucune adaptation physiologique pour limiter la PEET, les moyens étant limités à l'expression d'une posture en forme de dollar. Une méthode de reconstruction 3D basée sur l'lmagerie par Résonance Magnétique a été utilisée pour générer des géométries 3D des tritons utilisables pour mesurer leur surface, et à des fins de simulation. Nous avons ainsi réalisé une analyse numérique en 3D des PEET, et avons proposé une relation pour estimer cette perte dans une large gamme de conditions climatiques. Enfin, nos résultats supportent l'hypothèse de dysfonctionnement épidermique, suggérant que B. dendrobatidis compromet la capacité des amphibiens à se réhydrater. Ma thèse devrait contribuer à développer de nouvelles approches en sciences des relations eau-amphibiens, et à améliorer nos connaissances sur les effets des changements environnementaux sur les organisme

Plasticity in metabolic rates and life history traits affects foraging behaviour in a parasitic wasp

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Modelling skin surface areas involved in water transfer in the Palmate Newt (Lissotriton helveticus)

International audienceMagnetic resonance imaging (MRI) based 3D reconstructions were used to derive accurate quantitative data on body volume and functional skin surface areas involved in water transfer in the Palmate Newt (Lissotriton helveticus (Razoumovsky, 1789)). Body surface area can be functionally divided into evaporative surface area that interacts with the atmosphere and controls the transepidermal evaporative water loss (TEWL); ventral surface area in contact with the substratum that controls transepidermal water absorption (TWA); and skin surface area in contact with other skin surfaces when amphibians adopt water-conserving postures. We generated 3D geometries of the newts via volume-rendering by a “segmentation” process carried out using a graph-cuts algorithm and a Web-based interface. The geometries reproduced the two postures adopted by the newts, i.e., an I-shaped posture characterized by a straight body without tail coiling and an S-shaped posture where the body is huddled up with the tail coiling along it. As a guide to the quality of the surface area estimations, we compared measurements of TEWL rates between living newts and their agar replicas (reproducing their two postures) at 20 °C and 60% relative humidity. Whereas the newts did not show any physiological adaptations to restrain evaporation, they expressed an efficient S-shaped posture with a resulting water economy of 22.9%, which is very close to the 23.6% reduction in evaporative surface area measured using 3D analysis

Plasticity in metabolic rates and life history traits affects foraging behaviour in a parasitic wasp

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La température affecte le comportement d'exploitation des ressources hôtes chez un parasitoïde

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Impact of both desiccation and exposure to an emergent skin pathogen on transepidermal water exchange in the palmate newt Lissotriton helveticus

International audienceAmphibians are the vertebrate group most affected by global change. Their highly permeable skin is involved in maintaining homeostasis (e.g. water and electrolyte equilibrium), which makes them particularly vulnerable to climate warming and skin pathogens. This study focused on the impacts of both desiccation (as a potential consequence of climate warming) and exposure to Batrachochytrium dendrobatidis (Bd), an emergent skin pathogen of amphibians. Bd causes chytridiomycosis, a lethal skin disease of amphibians, and is responsible for mass mortality events in several regions of the world. Because Bd colonizes the superficial layers of the epider- mis, it is assumed to affect water transfer across the skin. We investigated the behavioural pos- tures of the palmate newt Lissotriton helveticus expressed in response to desiccation and their influence on transepidermal water loss (TEWL) rate. We also investigated the effects of repeated 24 h exposure to Bd (i.e. every 4 d for 16 d) on the TEWL and ventral water absorption (VWA) rates of these newts. Our results suggest an efficient behavioural water-conserving mechanism, i.e. an 'S'-shaped posture associated with a restricted activity rate, not affected by repeated exposure to Bd. Similarly, TEWL was not significantly affected in exposed newts. VWA was significantly reduced after just 24 h exposure to Bd without modification until the end of the experiments. Our results suggest that Bd could rapidly inhibit rehydration of L. helveticus through fungal toxins and disrupt an essential function for survival