Life-history traits in fish populations are highly correlated. A subset of these correlations are called allometric scaling, they refer to biological processes which can be described using body size as independent variable. Particularly, allometric scaling related with food consumption (Q) and biomass production (P) has gained the attention of ecologists for several decades. This thesis proposes a quantitative framework for food consumption, which allows both the identification of the mechanisms underlying the allometric scaling for Q and the development of a predictive model for consumption to biomass ratio (Q/B) in fish populations. This thesis is based on the fact that food consumption can be inferred from first principles underlying the von Bertalanffy growth model. In addition, it has been noticed in the literature that biomass production and food consumption show similar allometric scaling dependence, therefore, both can be derived from these first principles. Thus, a similar quantitative framework was used to produce models for P/B in fish populations. Once functional forms for production and food consumption were identified, a third model was developed for the ratio between production and consumption (P/Q). This ratio is usually named ecological efficiency because it determines how efficiently a population can transform ingested food into biomass. Several authors have noticed that P/Q remains invariant (independent of body size) across species. From a theoretical point of view, the results presented here allow the first quantitative explanation for the existence of the allometric scaling for Q/B and the invariance of P/Q across fish species. These results, together with the explanation for allometry in P/B reported in the literature, suggest that the regular across-species pattern for the trio {P/B,Q/B,P/Q} can be explained by basic principles that underpin life-history in fish populations. This quantitative framework for the trio {P/B,Q/B,P/Q} is based on an explicit dependence with body size, which simplifies the estimation of these quantities. Model complexity depends, in part, on which data are available. Models were applied to real data from commercially important species fished in Chile. Statistical properties of the new models were evaluated by an intensive resampling approach. The simplest possible model for the trio {P/B,Q/B,P/Q} rests on the assumption of a stable age distribution. These quantities have a key importance in ecosystem modelling because they determine population energetics in terms of food intake by predation and the transformation of this energy into population biomass of predators. Application of the new models produces results which were comparable to those given by standard methods. This thesis is a result of multidisciplinary research which attempts to make a contribution to the understanding of the mechanisms underlying the allometric scaling of food consumption and production in fish populations. It proposes models for the trio {P/B,Q/B,P/Q} and thus, has the potential to be widely applicable in fisheries science.

The trade-off between gametes and soma is central to life-history evolution. Oosorption has been proposed as a mechanism that can mediate this trade-off. When conditions are not conducive to successful reproduction, females are expected to be able to recoup nutrients from unfertilized oocytes and reinvest them into the somatic processes that promote survival and hence future reproduction. Although positive correlations between oocyte degradation and lifespan have been documented in oviparous insects, the adaptive significance of this process in species with more complex reproductive biology has not been explored. Oocyte degradation via apoptosis (programmed cell death) occurs in response to enforced virginity in females of the ovoviviparous cockroach, Nauphoeta cinerea. Observed apoptosis may represent oosorption, however, an alternative but not mutually exclusive argument is that oocyte apoptosis may represent oocyte ageing and clearance in order to maintain reproductive synchrony. The aim of this thesis was to test the hypothesis that the function of oocyte apoptosis is oosorption in N. cinerea. I found that in addition to enforced virginity, starvation induces oocyte apoptosis. However, the life history outcome following one form of stress is the opposite of the other. Hence, the functional role of oocyte apoptosis appears to be different depending on whether apoptosis is induced through starvation or age. Following a period of starvation-induced apoptosis females exhibit the increase in survival and future reproduction predicted by oosorption. Whereas, following a period of age-induced apoptosis females suffer fecundity and longevity cuts. However, age-induced apoptosis does not appear to simply be cellular ageing and clearance. In conjugation with age-induced apoptosis, ovariole number declines whilst the size of surviving oocytes increases. Hence, it appears that resources from sacrificed oocytes are being recycled into the survivors, and that this reinvestment in current reproduction trade-offs with future reproductive capacity. My thesis shows the importance of studying proximal mechanisms alongside more traditional measures of life history, as the relationship between isolated biological levels is not always clear.

In seawater, drinking is a fundamental part of the osmoregulatory strategy for teleost fish, and presents a unique challenge. The intestine has an established role in osmoregulation, and its ability to effectively absorb fluid from imbibed seawater is crucial to compensating for water losses to the surrounding hyperosmotic environment. Alongside solute-linked water transport (driven by NaCl cotransport), intestinal bicarbonate (HCO3-) secretion also benefits fluid absorption directly (via apical Cl-/HCO3- exchange), and indirectly through the formation of calcium carbonate (CaCO3) thus removing the osmotic influence of Ca2+ within the gut fluid. For the European flounder (Platichthys flesus), elevated luminal Ca2+ has proven to be a specific, potent stimulator of HCO3- secretion both in vitro and in vivo where these actions are presumably modulated by an extracellular Ca2+-sensing receptor (CaR). The focus of this work was to learn more about how intestinal HCO3- secretion is regulated, the role of Ca2+, and more specifically the CaR. To achieve this, in vitro ‘gut sac’ experiments investigated how luminal Ca2+ influenced HCO3- secretion, and associated ion and fluid transport. Contrary to expectation, increasing Ca2+ from 5 to 20 mM did not stimulate HCO3- secretion. In an attempt to elucidate the role of CaCO3 precipitation in fluid absorption, and further explore the physiological implications of HCO3- secretion, the intestine was perfused in vivo with salines containing varying concentrations of Ca2+ (10, 40 and 90 mM). The production and secretion of HCO3-, in addition to CaCO3 formation increased accordingly with Ca2+, and was associated with a dramatic 25 % rise in the fraction of fluid absorbed by the gut. Additional in vitro experiments, utilising the Ussing chamber, helped establish some of the characteristics of intestinal HCO3- secretion by the euryhaline killifish (Fundulus heteroclitus), but was unresponsive to elevated mucosal Ca2+. Further attempts to potentiate the activity of the CaR, and application of the receptor agonists gadolinium (Gd3+) and neomycin, failed to produce responses consistent with the effect of Ca2+ observed previously, either in vitro or in vivo. With no evidence supporting a direct role for an extracellular, intestinal CaR in HCO3- secretion it was argued that secretion would be principally regulated by two factors, the ability of the epithelia to generate high levels of intracellular HCO3- and the rate of CaCO3 formation.

Le danio zébré est un modèle bien établi pour étudier la biologie du développement des vertébrés. Ses larves transparentes sont favorables à des approches de microscopie non invasive, qui permettent de réaliser des observations à l’échelle d’un individu entier à des niveaux de résolution cellulaire et subcellulaire. Ces atouts font du danio zébré un excellent modèle pour étudier les infections virales in vivo. Au cours de mon projet, j’ai etudié l’entrée et la colonisation du système nerveux central (SNC) par le virus Sindbis (SINV) dans le modèle danio zébré. Mon projet présentait plusieurs axes: 1) développer un modèle d’infection du virus Sindbis chez le danio zébré, 2) caractériser l’invasion du SNC par le virus par des techniques d’imagerie à haute résolution, 3) définir la voie d’entrée du virus dans le SNC, 4) évaluer la dynamique de la réponse immunitaire innée par l’étude de la réponse interféron. Le suivi de la propagation du virus a été rendu possible par l’utilisation d’un ARN viral recombinant exprimant la protéine fluorescente verte ‘GFP’. L’utilisation de cette construction m’a permis de caractériser la progression de SINV chez le danio zébré et d’identifier les organes/tissus cibles que sont le vitellus, le foie, le cœur et enfin, le cerveau. Les données rassemblées jusqu'à présent m’ont aussi permis d’identifier le mécanisme par lequel SINV se propage vers le cerveau: le virus se propage par un transport axonal du system nerveux périphérique vers le SNC. Dans le cadre de la réponse immunitaire au niveau cellulaire, j’ai pu observer le rôle joué par les leucocytes, en particulier les neutrophiles, comme cellules productrices d'interféron. / The zebrafish (Danio rerio) is an important model organism, particularly for studies of development and more recently host pathogen interactions. As opposed to other vertebrate model organisms, its optical clarity and ease of genetic manipulations allow to visualize highly dynamic cellular processes in vivo at the whole organism scale. These assets make the zebrafish a perfect model for the study of viral infections in vivo, such as those caused by neurotropic viruses. The aim of this project has been to gain insights in some of the interactions that determine encephalitis, by characterizing the neurotropic Sindbis virus (SINV). This Thesis project has consisted therefore in: 1) the development of a SINV infection model in zebrafish larvae, 2) the characterization of SINV neuroinvasion upon its inoculation in the bloodstream, thanks to the use of high resolution microscopy, 3) the study of SINV mechanism of entry in the CNS, 4) the characterization of the innate immune response, both at the whole organism and organ specific level. Thanks to the use of a SINV recombinant strain, engineered to express the green fluorescent protein “GFP” in infected cells upon viral replication, we have been able to follow the onset and the progression of the infection. We have suggested infection of peripheral neurons and subsequent axonal transport to the CNS as SINV entry mechanism. At the cellular level, we have identified neutrophils as the main IFN producing cells.

Le changement climatique devrait induire une augmentation des précipitations pendant l'hiver et le printemps dans les régions tempérées et la côte nord de l'Europe. Dans les écosystèmes fluviaux, les précipitations affectent directement le débit des eaux courantes et les rivières subiront des crues plus sévères. En outre, la température de l'air et de l'eau augmenteront à travers le monde. Ces nouvelles conditions environnementales vont avoir des conséquences sur la phénologie des espèces et les interactions prédateurs/proies. Les jeunes truites fario (Salmo trutta L.) commencent leur alimentation exogène en mars/avril. Cette étape critique de leur cycle de vie induit d’importants changements aussi bien physiologiques que comportementaux. Pour permettre un bon développement des individus et un taux de survie élevé, les proies doivent être disponibles et abondantes, en particulier à ce moment de l’ontogénèse où les juvéniles sont vulnérables au manque de nourriture et à la prédation. Des expériences en milieux contrôlés ont été menées pour quantifier la sensibilité à différentes modalités de vitesses de courant de trois espèces d’invertébrés couramment consommées par les salmonidés et pour évaluer l’effet de la température sur le métabolisme d’alevins en situation de jeûne. Des expériences en milieu semi-naturel ont été mises en place pour mieux comprendre les effets d’une crue sur la communauté d’invertébrés et sur la survie, le comportement et la croissance des alevins en première alimentation. Il apparaît que la crue impacte différemment les truites en fonction du moment de la saison auquel elles commencent à s’alimenter (au début ou à la fin du printemps) et de la productivité du système. / Global Climate Change will increase precipitations in the temperate and Northern coast of Europe during winter and spring. In riverine ecosystems, precipitation affects directly the discharge of running waters and, thus, it is predicted that streams will face more severe floods. Additionally, air and water temperature will increase all over the world. These new environmental conditions can alter the phenology of species and predator/prey interactions. Newborns of brown trout (Salmo trutta L.) start their exogenous feeding in March/April. This stage is a critical step as individuals undergo huge physiological and behavioral changes. To allow a good development and a high survival rate, prey has to be abundant, particularly during early ontogenesis when they are most vulnerable to food scarcity and predation. In this thesis, experiments in controlled-environment were conducted to estimate the effect of water velocity on the preferred prey taxa for salmonids and to understand the effect of temperature on the metabolism of alevins facing starvation. Experiments in semi-natural conditions were set up to better understand the effects of floods on invertebrate communities and on survival, behavior and growth of first-feeding alevins. Our data support that floods affect trout differently depending on when they start feeding (early or late spring) and the availability of prey in their environment.

In the present study, the ease of rearing, short generation time and molecular research tools available for the zebrafish model (Danio rerio, Hamilton) were exploited to investigate transcriptional regulation in relation to feeding, photoperiod and experimental selection. Chapter 2 describes transcriptional regulation in fast skeletal muscle following fasting and a single satiating meal of bloodworms. Changes in transcript abundance were investigated in relation to the food content in the gut. Using qPCR, the transcription patterns of 16 genes comprising the insulin-like growth factor (IGF) system were characterized, and differential regulation between some of the paralogues was recorded. For example, feeding was associated with upregulation of igf1a and igf2b at 3 and 6h after the single-meal was offered, respectively, whereas igf1b was not detected in skeletal muscle. On the other hand, fasting triggered the upregulation of the igf1 receptors and igfbp1a/b, the only binding proteins whose transcription was responsive to a single-satiating meal. In addition to the investigation of the IGF-axis, an agnostic approach was used to discover other genes involved in transcriptional response to nutritional status, by employing a whole-genome microarray containing 44K probes. This resulted in the discovery of 147 genes in skeletal muscle that were differentially expressed between fasting and satiation. Ubiquitin-ligases involved in proteasome-mediated protein degradation, and antiproliferative and pro-apoptotic genes were among the genes upregulated during fasting, whereas satiation resulted in an upregulation of genes involved in protein synthesis and folding, and a gene highly correlated with growth in mice and fish, the enzyme ornithine decarboxylase 1. Zebrafish exhibit circadian rhythms of breeding, locomotor activity and feeding that are controlled by molecular clock mechanisms in central and peripheral organs. In chapter 3 the transcription of 17 known clock genes was investigated in skeletal muscle in relation to the photoperiod and food content in the gut. The hypothesis that myogenic regulatory factors and components of the IGF-pathway were clock-controlled was also tested. Positive (clock1 and bmal1 paralogues) and negative oscillators (cry1a and per genes) showed a strong circadian pattern in skeletal muscle in anti-phase with each other. MyoD was not clock-controlled in zebrafish in contrast to findings in mice, whereas myf6 showed a circadian pattern of expression in phase with clock and bmal. Similarly, the expression of two IGF binding proteins (igfbp3 and 5b) was circadian and in phase with the positive oscillators clock and bmal. It was also found that some paralogues responded differently to photoperiod. For example, clock1a was 3-fold more responsive than clock1b. Cry1b did not show a circadian pattern of expression. These patterns of expression provide evidence that the molecular clock mechanisms in skeletal muscle are synchronized with the molecular clock in central pacemaker organs such as eyes and the pineal gland. Using the short generation time of zebrafish the effects of selective breeding for body size at age were investigated and are described in chapter 4. Three rounds of artificial selection for small (S-lineage) and large body size (L-lineage) resulted in zebrafish populations whose average standard length were, respectively, 2% lower and 10% higher than an unselected control lineage (U-lineage). Fish from the L-lineage showed an increased egg production and bigger egg size with more yolk, possibly contributing to the larger body size observed in the early larval stage (6dpf) of fish from this lineage. Fish from S- and L-lineage exposed to fasting and refeeding showed very similar feed intake, providing evidence that experimental selection did not cause significant changes in appetite control. Investigation of the expression of the IGF-axis and nutritionally-response in skeletal muscle after fasting and refeeding revealed that the pattern of expression was not different between the selected lineages, but that a differential responsiveness was observed in a limited number of genes, providing evidence that experimental selection might have changed the way fish allocate the energy acquired through feeding. For example, a constitutive higher expression of igf1a was recorded in skeletal muscle of fish from the L-lineage whereas igfbp1a/b transcripts were higher in muscle of fish from the S-lineage. These findings demonstrate the rapid changes in growth and transcriptional response in skeletal muscle of zebrafish after only three rounds of selection. Furthermore, it provides evidences that differences in growth during embryonic and larval stages might be related to higher levels of energy deposited during oogenesis, whereas differences in adult fish were better explained by changes in energy allocation instead of energy acquisition. In chapter 5 the main findings made during this study and their impact on the literature are discussed.

King penguins have successfully colonized cold ecosystems of the southern hemisphere by developing physiological mechanisms that are not well understood. The aim of this study was to investigate, at different integrative levels from the gene to the whole animal, the functional responses developed by penguins to overcome polar constrains. We focused on acclimatization mechanisms enabling the first departure to sea of king penguin immatures and the rapid growth of Adélie penguin chicks.To explore differentially expressed genes in pectoralis muscle during penguin’s first sea acclimatization, we used Affymetrix microarrays design for chicken. We first set up and validated a new method to analyze heterologous hybridization transcriptomic profiles. We highlighted a selective shift in metabolic pathways favoring the use of lipids as fuel to sustain highly energetic needs imposed by marine life-style. Our results revealed a development of a global antioxidant response, potential consequences of penguin marine life-style that imposes repeated dives under apnea.Secondly, our integrative study on Adélie penguin’s chick revealed the development of molecular and cellular mechanisms which sustain an original strategy by first allocating most of the energy to growth and then promoting thermogenic processes.Our results showed that both king and Adélie penguins develop complex and coordinated physiological responses to energetic constraints highlighting their high phenotypic plasticity. / King penguins have successfully colonized cold ecosystems of the southern hemisphere by developing physiological mechanisms that are not well understood. The aim of this study was to investigate, at different integrative levels from the gene to the whole animal, the functional responses developed by penguins to overcome polar constrains. We focused on acclimatization mechanisms enabling the first departure to sea of king penguin immatures and the rapid growth of Adélie penguin chicks.To explore differentially expressed genes in pectoralis muscle during penguin’s first sea acclimatization, we used Affymetrix microarrays design for chicken. We first set up and validated a new method to analyze heterologous hybridization transcriptomic profiles. We highlighted a selective shift in metabolic pathways favoring the use of lipids as fuel to sustain highly energetic needs imposed by marine life-style. Our results revealed a development of a global antioxidant response, potential consequences of penguin marine life-style that imposes repeated dives under apnea.Secondly, our integrative study on Adélie penguin’s chick revealed the development of molecular and cellular mechanisms which sustain an original strategy by first allocating most of the energy to growth and then promoting thermogenic processes.Our results showed that both king and Adélie penguins develop complex and coordinated physiological responses to energetic constraints highlighting their high phenotypic plasticity.

La fragmentation des habitats ainsi que la réduction de leur disponibilité sont des menaces importantes pour les écosystèmes, notamment aquatiques. La disponibilité et la qualité des sites de fraie du saumon Atlantique peut donc être limitante dans certaines rivières. Après la migration en rivière, les femelles saumon vont chercher à s'établir dans un des sites de fraie disponibles. La distribution spatiale des femelles va alors influer sur la distribution spatiale des mâles reproducteurs qui vont chercher soit à monopoliser plusieurs femelles (pour les mâles dominants), soit à les approcher en adoptant un comportement "furtif" (pour les mâles satellites). L'accès des mâles aux femelles va donc dépendre de la distribution spatiale des femelles et des déplacements des mâles dans la rivière. La distribution des femelles va également jouer sur la distribution spatiale des juvéniles, les concentrant à certains endroits. Cette concentration peut augmenter la mortalité densité-dépendante qui modifie la dynamique de la population. Cette thèse avait pour but d'évaluer comment l'utilisation de l'espace des potentiels reproducteurs, c'est à dire les individus matures, influence la dynamique de la population et la sélection sexuelle.Des séries temporelles de stock (oeufs déposés) et de recrutement (juvéniles) pour la population de saumon de la Nivelle ont été associées à des mesures de l'agrégation spatiale des nids creusés par les femelles. L'effet des déplacements et de la distribution des reproducteurs sur leur participation à la reproduction a notamment été testé via la reconstruction du réseau d'interactions sexuelles dans la Nivelle.J'ai ainsi démontré que l'agrégation spatiale des nids influa sur la variabilité du recrutement mais pas sur le recrutement moyen. Cette agrégation qui résulte de l'agrégation des femelles a également été liée au nombre de reproducteurs ayant engendrés des juvéniles. Le réseau d'interactions sexuelles a permis de mettre en évidence une structure sociale locale au sein de la population. Cette structure et les compétiteurs présents pouvaient par exemple diminuer le succès reproducteur des mâles. Notamment, les tacons matures qui participent à la reproduction avant de migrer en mer renforçaient la compétition sexuelle dont les mâles dominants faisaient l'objet. Enfin, ces mêmes tacons matures adoptèrent des comportements spatiaux du fait de leur statut de maturité avec des individus bougeant peu et des individus se déplaçant vers l'amont. Ces comportements spatiaux influaient leur participation à la reproduction, l'étendue des domaines de vie et les déplacements vers l'amont augmentaient la probabilité de rencontrer une femelle.Mes résultats m'ont permis de mettre en évidence l'effet de l'utilisation de l'espace des potentiels reproducteurs sur la dynamique de la population et sur la sélection sexuelle. Ainsi, en combinant différentes échelles spatiales et temporelles il a été montré que l'agrégation des femelles pouvait être bénéfique pour la dynamique de la population. Il semble que les femelles tendent à s'agréger d'abord sur les meilleurs sites de fraie avec une plus forte stabilité environnementale. Cette distribution des femelles affecta la capacité des mâles à monopoliser plusieurs femelles, une agrégation modérée permettant une monopolisation. La participation des mâles furtifs anadromes ou tacons matures était également dépendante de l'agrégation des femelles. Il apparait que la disponibilité d'habitats adéquats pour la reproduction semble limitante dans la Nivelle et notamment du fait de la présence de barrages. Ce manque semble affecter le système d'appariement de la population et la sélection sexuelle y opérant. Des mesures visant à améliorer la qualité des sites de fraie déjà disponibles ainsi que leur accessibilité pourraient être envisagées. / Habitat fragmentation and reduction in the availability of suitable habitats are significant threats to ecosystems in particularly for freshwater ones. For instance, accessibility, availability, and quality of breeding sites of Atlantic salmon, \textit{Salmo salar}, can be restricting in some rivers due to human activities and dams. Such threats may affect distribution of potential breeders because after their up-river migration, salmon females distribute within available breeding sites. Spatial distribution of females determines the spatial distribution of breeder males. Dominant males try to monopolise several females, whereas subordinate males adopt a sneaky behaviour. Access to females by males depends on the spatial distribution of females and on males' displacements within a river. The spatial distribution of females generates the spatial distribution of juveniles, aggregating them at some breeding sites. The latter aggregation of juveniles may raise the density-dependent mortality with potential consequences on population dynamics. The thesis aims to assess how space use of potential breeders, namely mature individuals able to spawn or fertilise eggs, influence population dynamics and sexual selection.I combined different approaches and different temporal and spatial scales to potential effects of space use of potential breeders. Time series of stock (deposited eggs) and of recruitment (juveniles) for the salmon population of the Nivelle River were combined with measures of spatial aggregation of nests. The effects of displacements and spatial distribution of potential breeders on their participation in reproduction were tested through inferring the sexual network of the population.I found that spatial aggregation of nests improved the recruitment variability but did not affect the average recruitment. The spatial aggregation of nests resulting from female aggregation was also linked to the number of effective breeders through a U-shaped curve. Such relationships suggested mate monopolization dependent on the spatial aggregation of females. The negative effect of the spatial isolation of females on their number of mates also suggest such mate monopolization. Then, the inferred sexual network enabled me to highlight a local social structure within the population. The latter structure and social competitors impacted the reproductive success of anadromous males. The participation of mature parr was one of the factors diminishing the sexual competition faced by anadromous males. Finally, mature parr were more mobile and moved more upstream than immature parr. Spatial behaviours of mature parr were linked to their participation in reproduction, individuals exhibiting wider home ranges had a higher probability to encounter females.Altogether my results show that space use of potential breeders affect both population dynamics and sexual selection. Then, spatial aggregation of females and of their nests can be beneficial for population dynamics as shown by combining different temporal and spatial scales. Females seemed to aggregate within the best breeding sites with a better environmental stability. The spatial distribution of females affected the ability of males to monopolise several females and participation of sneaky males. The availability of suitable good breeding habitats seemed restricting in the Nivelle, potentially due to the presence of impassable upstream dams. The lack of suitable breeding sites seemed impacting the mating system of the population and the sexual selection operating in. Some management actions to improve the quality of breeding sites and their availability could be planned.

Les jeunes oiseaux exposés au froid assurent leur homéothermie en stimulant les oxydations mitochondriales dans les muscles squelettiques. L’exposition prolongée au froid accroit les capacités de thermogenèse musculaire grâce à une plasticité bioénergétique mitochondriale dont le contrôle reste hypothétique. Chez les mammifères, des protéines de fusion (les mitofusines (Mfns) et OPA1(OPtic Atrophy 1)) participent au remaniement des réseaux dynamiques mitochondriaux dans de multiples types cellulaires. Le but de ce travail de thèse était de caractériser l’expression d’homologues aviaires des protéines de fusion mammaliennes et d’étudier leurs variations d’expression lors de la mise en place des processus bioénergétiques chez l’oiseau en croissance, lors d’une exposition aiguë ou prolongée au froid ou lors de challenges nutritionnels ou endocrines.Sur le plan méthodologique, une approche intégrative a été utilisée de l’animal entier (calorimétrie indirecte) à l’expression protéique (western blot) ou transcriptionnelle (RT-PCR) en passant par des mesures de la fonctionnalité bioénergétique sur des fibres musculaires perméabilisées et mitochondries isolées. Deux modèles animaux ont été utilisés, une espèce naturellement adaptée aux conditions extrêmes de l’Antarctique, le manchot Adélie (Pygoscelisadeliae), et un modèle de laboratoire, le canard de Barbarie (Cairina moschata). Nos résultats ont permis de caractériser chez l’oiseau l’expression de protéines de fusion (Mfn2, OPA1) immunoréactives homologues à celles des mammifères. Le séquençage d’une partie de la séquence codante des gènes codant les Mfns a montré une bonne similitude entre les espècesd’oiseaux et les mammifères. Chez le manchot, l’abondance relative de ces protéines dans lesmitochondries musculaires variait avec la croissance et l’exposition thermique en corrélation positiveavec les capacités bioénergétiques musculaires. Chez le canard, l’activité respiratoire et l’abondance relative de ces protéines étaient également corrélées suite à un jeûne de 60h ou, bien que dans une moindre mesure, après altération pharmacologique du statut thyroïdien.Ces résultats montrent pour la première fois chez l’oiseau l’expression de protéines homologues aux protéines de fusion des mammifères. L’association entre les variations d’expression de ces protéines et les modifications bioénergétiques du muscle squelettique indiquent qu’elles pourraient contribuer à la plasticité bioénergétique observée chez l’oiseau en croissance. Ces résultats suggèrent que des modifications potentielles de l’organisation des réseaux mitochondriaux musculaires pourraient contribuer aux réponses adaptatives des organismes face aux contraintes environnementales. / Cold-exposed young birds maintain their homeothermy by stimulating mitochondrial oxidations in skeletal muscle. Prolonged cold exposure enhances muscle thermogenic capacities through mitochondrial bioenergetics plasticity which control still remains hypothetical. In mammals, fusion proteins (mitofusins (Mfns) and OPA1 (Optic Atrophy 1)) contribute to the permanent and dynamic changes in mitochondrial networks in multiple cell types. The aim of our work was to characterize the expression of avian homologues of mammalian fusion proteins and to study the variations of their expression during the establishment of bioenergetics processes in growing birds, during an acute or a prolonged cold exposure and finally during nutritional or endocrine challenges. Methodologically, an integrative approach has been used from whole animal (indirect calorimetry) to protein (western-blot) or gene (RT-PCR) expression through measurements of the bioenergetics functionality of permeabilized muscle fibers and isolated mitochondria. Two animal models were used, a species naturally adapted to Antarctica harsh conditions, the Adélie penguin (Pygoscelis adeliae), and a laboratory model, the Muscovy duck (Cairina moschata).Our results allowed us to characterize, in birds, the expression of immunoreactive fusion proteins (Mfn2, OPA1) which were homologous to those of mammals. The sequencing of a part of the coding sequence of Mfns genes showed a great similitude between avian and mammalian species. In penguins, the relative abundance of these proteins in muscle mitochondria was modified by growth in the cold and was positively correlated with muscle bioenergetics capacities. In ducks, the respiratory activity and the relative abundance of these proteins were also correlated after a 60h fasting period or,though a lesser extent, after a pharmacological alteration of thyroid status. Our results show, for the first time in birds, the expression of proteins homologous to mammalian fusion proteins. The association between the changes in expression of these proteins and the bioenergetics modifications in skeletal muscle indicates that these proteins could contribute to thebioenergetics plasticity observed in growing chicks. These results suggest that potential modifications of the muscle mitochondrial network organization could play a role in the adaptive responses of organisms to the environmental constraints.