Local adaptation and matching habitat choice in female barn owls with respect to melanic coloration.

Local adaptation is a major mechanism underlying the maintenance of phenotypic variation in spatially heterogeneous environments. In the barn owl (Tyto alba), dark and pale reddish-pheomelanic individuals are adapted to conditions prevailing in northern and southern Europe, respectively. Using a long-term dataset from Central Europe, we report results consistent with the hypothesis that the different pheomelanic phenotypes are adapted to specific local conditions in females, but not in males. Compared to whitish females, reddish females bred in sites surrounded by more arable fields and less forests. Colour-dependent habitat choice was apparently beneficial. First, whitish females produced more fledglings when breeding in wooded areas, whereas reddish females when breeding in sites with more arable fields. Second, cross-fostering experiments showed that female nestlings grew wings more rapidly when both their foster and biological mothers were of similar colour. The latter result suggests that mothers should particularly produce daughters in environments that best match their own coloration. Accordingly, whiter females produced fewer daughters in territories with more arable fields. In conclusion, females displaying alternative melanic phenotypes bred in habitats providing them with the highest fitness benefits. Although small in magnitude, matching habitat selection and local adaptation may help maintain variation in pheomelanin coloration in the barn owl.

Indirect costs of parasitism are shaped by variation in the type of immune challenge and food availability

Parasites can inflict indirect fitness costs to their hosts by eliciting costly immune responses. These costs depend on the type and amount of immunostimulants presented to the host immune system but also on the amount of resources available to fuel host immune responses. Here, we investigated how the relative costs of two different types of immune challenge are modulated by variation in food availability. We injected nestling tawny owls (Strix aluco) with either 10 mu g of phytohaemagglutinin (PHA) or 20 mu g of lipopolysaccharide (LPS), and subsequently raised them under two different food regimes (food-restricted vs. ad libitum). After controlling for food consumption, we found that LPS-injected nestlings lost more body mass than PHA-injected ones only when food-restricted. We also found that body mass gain of owlets fed ad libitum decreased with the intensity of the skin swelling response against LPS, but not PHA. These experimental and correlative results suggest that nestling tawny owls suffered greater immune costs when treated with LPS than PHA, and that variation in the costs of two different types of immune challenge can be exacerbated under conditions of low food availability. Our study highlights the importance of taking into consideration the interplay between host immunity and nutrition in the study of indirect costs of parasitism.

Coloration signals the ability to cope with elevated stress hormones: effects of corticosterone on growth of barn owls are associated with melanism.

Stressful situations during development can shape the phenotype for life by provoking a trade-off between development and survival. Stress hormones, mainly glucocorticoids, play an important orchestrating role in this trade-off. Hence, how stress sensitive an animal is critically determines the phenotype and ultimately fitness. In several species, darker eumelanic individuals are less sensitive to stressful conditions than less eumelanic conspecifics, which may be due to the pleiotropic effects of genes affecting both coloration and physiological traits. We experimentally tested whether the degree of melanin-based coloration is associated with the sensitivity to an endocrine response to stressful situations in the barn owl. We artificially administered the mediator of a hormonal stress response, corticosterone, to nestlings to examine the prediction that corticosterone-induced reduction in growth rate is more pronounced in light eumelanic nestlings than in darker nest mates. To examine whether such an effect may be genetically determined, we swapped hatchlings between randomly chosen pairs of nests. We first showed that corticosterone affects growth and, thus, shapes the phenotype. Second, we found that under corticosterone administration, nestlings with large black spots grew better than nestlings with small black spots. As in the barn owl the expression of eumelanin-based coloration is heritable and not sensitive to environmental conditions, it is therefore a reliable, genetically based sign of the ability to cope with an increase in blood corticosterone level.

The costs of parasitism and anti-parasitic defense in the common vole

Résumé : Les relations entre un parasite et son hôte sont avant tout marquées par le coût pour l'hôte que représente la ponction de ressources au profit du parasite et ses conséquences sur les traits d'histoires de vie de l'hôte. Pour contenir la réduction de leur valeur reproductive, les hôtes ont acquis au cours de l'évolution des mécanismes soit de lutte contre les parasites, soit de réallocations des ressources. Curieusement les effets des ectoparasites sur la biologie de mammifères ont été peu étudiés. Dans une première expérience à long terme, nous avons examiné sous un angle intégratif si les puces Nosopsyllus fasciatus affectent certains paramètres physiologiques des campagnols des champs Microtus arvalis. Nous avons également testé si les puces peuvent réduire la longévité et si oui, si ce pourrait être dû à une accélération de la sénescence. Ensuite nous avons testé si la simple activation répétée du système immunitaire comme lors d'une infestation chronique pouvait aussi réduire la longévité. Dans une dernière expérience, nous avons d'abord testé si l'infestation par des puces de jeunes campagnols au stade néonatal (21 jours) pouvait modifier leur développement et leur phénotype adulte. Puis nous avons testé si la modification du phénotype adulte est une réponse prédite et potentiellement adaptative pour minimiser les effets des puces à l'âge adulte. Nos résultats montrent que l'infestation par des puces réduit la croissance subadulte, induit une forte anémie et une immunodépression, et augmente le métabolisme de repos. De plus les puces réduisent la longévité et la taille des testicules, réduisant fortement le succès reproducteur potentiel des individus parasités. La taille finale, c'est-à-dire le développement pré-adulte, détermine en grande part la longévité. La réduction de longévité ne devrait pas être due à l'investissement au profit du système immunitaire car l'activation chronique seule du système immunitaire ne réduit pas la longévité. L'infestation néonatale retarde légèrement le développement mais surtout modifie l'hématocrite et réduit les performances locomotrices des campagnols plus de 3 mois après l'infestation. Les effets immédiats du parasitisme sur la physiologie semblent bien supérieurs comparés aux effets à long terme. Nous n'avons pas d'éléments permettant d'affirmer que le parasitisme néonatal prépare les campagnols à faire face aux puces à l'âge adulte. Au contraire, le parasitisme néonatal interagit sur le parasitisme adulte pour augmenter le métabolisme de repos. Cette thèse offre une vision intégrative des mécanismes par lesquels les puces peuvent affecter la valeur reproductive de leurs hôtes. De façon générale, ces résultats 35 montrent l'importance des puces comme force de sélection chez les campagnols. Il est indispensable de prendre en compte les ectoparasites dans l'étude de l'écologie et des dynamiques de populations chez les mammifères. Summary : The relationship between a parasite and its host is fundamentally marked by the costs for host of the withdrawals of resources by parasite and the subsequent reduction in host life-history traits. Hosts have evolved a number of strategies to reduce these costs, either by fighting against the parasite directly or by reallocating resources to reduce costs on lifetime reproductive value. The effects of ectoparasites on burrowing mammals have been scarcely studied. In a first long-term experiment, we examined how fleas Nosopsyllus fasciatus affect physiological levels of the common vole, Microtus arvalis. We also examined whether fleas reduce longevity and if so, if it is due to an early senescence pattern. Then we tested if experimental activation of the immune system by repeated injections of an antigen could result in a shorter longevity. In the last experiment, we tested if short-lasting neonatal parasitism can have long-term effects on phenotype, and if these effects could induce a predictive response to reduce damages when parasitized at the adult stage. We found that parasitism by flea reduced subadult growth, induced anaemia and immunodepression, and increased energy consumption even when resting. Moreover fleas reduce longevity and testes size associated to splenomegaly, suggesting an overall reduction in fitness but we did not find any pattern of accelerated senescence explaining the early death of parasitized voles compared to non-parasitzed. The cost of mounting an immune response throughout life does not impair longevity, suggesting that it is the cost of parasitism that limits the longevity and not the immune investment. Neonatal infestation by fleas has long-term effects on physiology and reduces motor activity more than 3 months after infestation. The modification of physiology due to long-term effects seems weak compared to the immediate effects of adult infestation. We found no evidence that neonatal parasitism prepares voles to mount a predictive adaptive response in order to reduce effects of fleas on fitness components. On the contrary, neonatal parasitism seems to worsen the effect of adult parasitism. This thesis offers an integrative view of mechanisms by which fleas affect their host at the individual level. Overall, our results demonstrate the importance of fleas as a selective force in voles. These results highlight the importance of ectoparasitism in ecology of micromarnrnals and suggest a role in the dynamic of host populations.

Barn owls do not interrupt their siblings

Animals communicate with conspecifics to resolve conflicts over how resources are shared. Since signals reflect individuals' resource-holding potential and motivation to compete, it is crucial that opponents efficiently transmit and receive information to adjust investment optimally in competitive interactions. Acoustic communication is particularly flexible as it can be quickly modulated according to background noise and audience. Diverse mechanisms have evolved to minimize acoustic signal interference, one being the avoidance of signal overlap by adjusting the timing of call production to alternate calls with those of competitors. However, the occurrence and function of overlap avoidance in the resolution of competition among relatives have barely been studied. Using young barn owl siblings, Tyto alba, which vocally negotiate over who will have priority access to food provided by parents, we investigated the extent to which nestlings avoid calling simultaneously and the function of this behaviour. We found that nestlings overlapped both their live siblings' calls and experimentally broadcast calls at least five times less often than expected at random. Furthermore, a focal nestling engaged more intensely in vocal negotiation when competing with nestmates that called simultaneously compared to those that did not overlap their respective calls. This suggests that barn owl nestlings avoid calling simultaneously, as overlapped calls are less efficient at deterring siblings from competing. Overlap avoidance reduces signal interference and, as a consequence, would improve the efficiency of communication among kin.

Melanin pigmentation negatively correlates with plumage preening effort in barn owls

1. According to the handicap principle of sexual selection, colourful ornaments honestly signal absolute quality only if they entail fitness costs. The degree of melanism often covaries positively with aspects of individual quality, and hence melanin-based coloration should be costly to produce or to maintain in a good shape. This is, however, unlikely because melanin-based coloration is often strongly heritable and in birds the rate of feather wear decreases with the amount of melanin pigments packed in feathers. 2. The hypothesis that melanin pigments reduce the cost of maintaining colourful ornaments in a good shape predicts a negative correlation between the degree of melanism and both the size of the uropygial gland that produces preening secretions and the intensity of preening behaviour. 3. Using a correlative approach, I evaluated these two predictions in the barn owl Tyto alba in which the body underside varies from immaculate to heavily marked with black spots, a eumelanin-based trait, and from white to reddish-brown, a phaeomelanin-based trait. I correlated plumage traits with preening behaviour in nestlings and with the size and mass of the uropygial gland in dead adults. I also weighed nonornamental wing and tail feathers to assess whether the quality of nonornamental feathers is positively correlated with the degree of melanism of an ornamental plumage trait. 4. The degree of phaeomelanism was neither associated with preening behaviour nor with the size and mass of the uropygial gland. In line with the two predictions, individuals with more and larger black spots had a lighter uropygial gland and preened less frequently. Because nonornamental wing and tail feathers of spottier individuals were heavier per unit of surface area, the entire plumage of eumelanic individuals may be more robust and in turn require less care than the plumage of nonmelanic conspecifics. 5. In conclusion, the degree of eumelanism can be associated with aspects of individual quality even if eumelanic ornaments are neither costly to produce nor to maintain in a good shape. Document Type: Article

Effects of corticosterone pellets on baseline and stress-induced corticosterone and corticosteroid-binding-globulin.

Exogenous administration of glucocorticoids is a widely used and efficient tool to investigate the effects of elevated concentrations of these hormones in field studies. Because the effects of corticosterone are dose and duration-dependent, the exact course of plasma corticosterone levels after exogenous administration needs to be known. We tested the performance of self-degradable corticosterone pellets (implanted under the skin) in elevating plasma corticosterone levels. We monitored baseline (sampled within 3min after capture) total corticosterone levels and investigated potential interactions with corticosteroid-binding-globulin (CBG) capacity and the endogenous corticosterone response to handling in Eurasian kestrel Falco tinnunculus and barn owl Tyto alba nestlings. Corticosterone pellets designed for a 7-day-release in rodents elevated circulating baseline total corticosterone during only 2-3 days compared to placebo-nestlings. Highest levels occurred 1-2days after implantation and levels decreased strongly thereafter. CBG capacity was also increased, resulting in a smaller, but still significant, increase in baseline free corticosterone levels. The release of endogenous corticosterone as a response to handling was strong in placebo-nestlings, but absent 2 and 8 days after corticosterone pellet implantation. This indicates a potential shut-down of the hypothalamo-pituitary-adrenal axis after the 2-3 days of elevated baseline corticosterone levels. 20 days after pellet implantation, the endogenous corticosterone response to handling of nestlings implanted with corticosterone pellets attained similar levels as in placebo-nestlings. Self-degradable pellets proved to be an efficient tool to artificially elevate circulating baseline corticosterone especially in field studies, requiring only one intervention. The resulting peak-like elevation of circulating corticosterone, the concomitant elevation of CBG capacity, and the absence of an endogenous corticosterone response to an acute stressor have to be taken into account.

Habitat, breeding performance, diet and individual age in Swiss barn owls (Tyto alba)

Intensification of farming over the past 50 years has homogenised the landscape structure and contributed to the decline of bird populations in Europe. To better target the conservation of the Barn Owl Tyto alba, we assessed the influence of the landscape structure on breeding performance in western Switzerland. The analyses considered a 23-year data set of breeding parameters collected in an area dominated by intensive agriculture. Using a Geographic Information System approach, landscape characteristics were described around 194 nest sites. Our analyses showed that nest-box occupancy, laying date, clutch and brood size, egg volume and probability of producing a second annual clutch were not significantly associated with any of the eight principal landscape variables (agricultural land, woodland, urban area, hedgerows, cereals, sugar beet, maize and meadow). Nevertheless, the probability that a breeding pair occupied a nest-box decreased the more roads there were surrounding the nest-box. The absence of strong associations between habitat features and breeding parameters suggests that prey availability may be relatively similar between the different breeding sites. In our study area Barn Owls can always find suitable foraging habitats around most nest-boxes.

Ultraviolet reflectance in a melanin-based plumage trait is heritable

Question: Is ultraviolet (UV) reflectance of melanin-based plumage ornaments heritable? Data studied: We considered the barn owl (Two alba), a species that varies continuously from white to reddish-brown, a pheomelanin-based trait. Methods: To perform a partial cross-fostering experiment. we exchanged one to three hatchlings between 16 pairs of nests with a similar hatching date. This experiment allocated hatchlings randomly among rearing environments. Forty-nine days later, we collected three feathers per individual to measure UV reflectance. Conclusions: The cross-fostering experiment showed that. independently of human-visible coloration, variation in UV reflectance is significantly sensitive to origin-related factors.

THE USE OF SIMULATIONS IN EVOLUTIONARY POPULATION GENETICS: APPLICATIONS ON HUMANS, OWLS AND VIRTUAL ORGANISMS

Summary (in English) Computer simulations provide a practical way to address scientific questions that would be otherwise intractable. In evolutionary biology, and in population genetics in particular, the investigation of evolutionary processes frequently involves the implementation of complex models, making simulations a particularly valuable tool in the area. In this thesis work, I explored three questions involving the geographical range expansion of populations, taking advantage of spatially explicit simulations coupled with approximate Bayesian computation. First, the neutral evolutionary history of the human spread around the world was investigated, leading to a surprisingly simple model: A straightforward diffusion process of migrations from east Africa throughout a world map with homogeneous landmasses replicated to very large extent the complex patterns observed in real human populations, suggesting a more continuous (as opposed to structured) view of the distribution of modern human genetic diversity, which may play a better role as a base model for further studies. Second, the postglacial evolution of the European barn owl, with the formation of a remarkable coat-color cline, was inspected with two rounds of simulations: (i) determine the demographic background history and (ii) test the probability of a phenotypic cline, like the one observed in the natural populations, to appear without natural selection. We verified that the modern barn owl population originated from a single Iberian refugium and that they formed their color cline, not due to neutral evolution, but with the necessary participation of selection. The third and last part of this thesis refers to a simulation-only study inspired by the barn owl case above. In this chapter, we showed that selection is, indeed, effective during range expansions and that it leaves a distinguished signature, which can then be used to detect and measure natural selection in range-expanding populations. Résumé (en français) Les simulations fournissent un moyen pratique pour répondre à des questions scientifiques qui seraient inabordable autrement. En génétique des populations, l'étude des processus évolutifs implique souvent la mise en oeuvre de modèles complexes, et les simulations sont un outil particulièrement précieux dans ce domaine. Dans cette thèse, j'ai exploré trois questions en utilisant des simulations spatialement explicites dans un cadre de calculs Bayésiens approximés (approximate Bayesian computation : ABC). Tout d'abord, l'histoire de la colonisation humaine mondiale et de l'évolution de parties neutres du génome a été étudiée grâce à un modèle étonnement simple. Un processus de diffusion des migrants de l'Afrique orientale à travers un monde avec des masses terrestres homogènes a reproduit, dans une très large mesure, les signatures génétiques complexes observées dans les populations humaines réelles. Un tel modèle continu (opposé à un modèle structuré en populations) pourrait être très utile comme modèle de base dans l'étude de génétique humaine à l'avenir. Deuxièmement, l'évolution postglaciaire d'un gradient de couleur chez l'Effraie des clocher (Tyto alba) Européenne, a été examiné avec deux séries de simulations pour : (i) déterminer l'histoire démographique de base et (ii) tester la probabilité qu'un gradient phénotypique, tel qu'observé dans les populations naturelles puisse apparaître sans sélection naturelle. Nous avons montré que la population actuelle des chouettes est sortie d'un unique refuge ibérique et que le gradient de couleur ne peux pas s'être formé de manière neutre (sans l'action de la sélection naturelle). La troisième partie de cette thèse se réfère à une étude par simulations inspirée par l'étude de l'Effraie. Dans ce dernier chapitre, nous avons montré que la sélection est, en effet, aussi efficace dans les cas d'expansion d'aire de distribution et qu'elle laisse une signature unique, qui peut être utilisée pour la détecter et estimer sa force.

Oxygen consumption in offspring tawny owls Strix aluco is associated with colour morph of foster mother

In several colour polymorphic species, morphs differ in thermoregulation either because dark and pale surfaces absorb solar radiation to a different extent and/or because morphs differ in key metabolic processes. Morph-specific thermoregulation may potentially account for the observation that differently coloured individuals are frequently not randomly distributed among habitats, and differ in many respects, including behaviour, morphology, survival and reproductive success. In a wild population of the colour polymorphic tawny owl Strix aluco, a recent cross-fostering experiment showed that offspring raised and born from red mothers were heavier than those from grey mothers. In the present study, we tested in the same individuals whether these morph-specific offspring growth patterns were associated with a difference in metabolic rate between offspring of red and grey mothers. For this purpose, we measured nestling oxygen consumption under two different temperatures (laboratory measurements: 4 and 20 degrees C), and examined the relationships between these data sets and the colour morph of foster and biological mothers. After controlling for nestling body mass, oxygen consumption at 20 degrees C was greater in foster offspring raised by grey foster mothers. No relationship was found between nestling oxygen consumption and coloration of their biological mother. Therefore, our study indicates that in our experiment offspring raised by grey foster mothers showed not only a lower body mass than offspring raised by red foster mothers, but also consumed more oxygen under warm temperature. This further indicates that rearing conditions in nests of grey mothers were more stressful than in nests of red mothers.

Nest-box design for the study of diurnal raptors and owls is still an overlooked point in ecological, evolutionary and conservation studies: a review

The use of artificial nest-boxes has led to significant progress in bird conservation and in our understanding of the functional and evolutionary ecology of free-ranging birds that exploit cavities for roosting and reproduction. Nest-boxes and their improved accessibility have made it easier to perform comparative and experimental field investigations. However, concerns about the generality and applicability of scientific studies involving birds breeding in nest-boxes have been raised because the occupants of boxes may differ from conspecifics occupying other nest sites. Here we review the existing evidence demonstrating the importance of nest-box design to individual life-history traits in three falcon (Falconiformes) and seven owl (Strigiformes) species, as well as the extent to which publications on these birds describe the characteristics of exploited artificial nest-boxes in their 'methods' sections. More than 60% of recent publications did not provide any details on nest-box design (e.g. size, shape, material), despite several calls >15 years ago to increase the reporting of such information. We exemplify and discuss how variation in nest-box characteristics can affect or confound conclusions from nest-box studies and conclude that it is of overall importance to present details of nest-box characteristics in scientific publications.

The vigilance components of begging and sibling competition

The conflict between siblings over how parental resources are divided has promoted the evolution of specific behaviour to outcompete each other. Young animals look out for parents' arrival in order to start begging as quickly as possible, since a rapid begging reaction increases the likelihood of being fed before nestmates. If the young can physically intercept the parents, selection might be operating on the offspring ability to monitor parent arrival (vigilance towards parents) and any sudden modifications in siblings' behaviour (vigilance towards siblings). To investigate the adaptive value of nestling vigilance in the context of family interactions, we recorded which direction barn owl Tyto alba siblings were facing in 89 two-chick broods before the first parental feeding visit of the night. Nestlings were more vigilant towards nest entrance than expected by chance suggesting that vigilance towards parents is an important component of sibling competition. When positioned near the nest-box entrance where parents predictably deliver food, the younger individual (i.e. junior) looked more towards the entrance than its older sibling. Thus, when the likelihood of obtaining a food item is relatively high, juniors are more vigilant than seniors to detect the incoming parent. When positioned at the back of the nest, the senior looked relatively more frequently towards its sibling than the junior did in the same situation. This suggests that when the likelihood of obtaining a food item is relatively low, seniors are more vigilant than juniors to observe their sibling. Because vigilance was not related to hunger level and prey obtaining, we propose the hypothesis that vigilance towards parents and siblings only indirectly influences the outcome of sibling competition.

Evaluation of two minimally invasive techniques for electroencephalogram recording in wild or freely behaving animals.

Insight into the function of sleep may be gained by studying animals in the ecological context in which sleep evolved. Until recently, technological constraints prevented electroencephalogram (EEG) studies of animals sleeping in the wild. However, the recent development of a small recorder (Neurologger 2) that animals can carry on their head permitted the first recordings of sleep in nature. To facilitate sleep studies in the field and to improve the welfare of experimental animals, herein, we test the feasibility of using minimally invasive surface and subcutaneous electrodes to record the EEG in barn owls. The EEG and behaviour of four adult owls in captivity and of four chicks in a nest box in the field were recorded. We scored a 24-h period for each adult bird for wakefulness, slow-wave sleep (SWS), and rapid-eye movement (REM) sleep using 4 s epochs. Although the quality and stability of the EEG signals recorded via subcutaneous electrodes were higher when compared to surface electrodes, the owls' state was readily identifiable using either electrode type. On average, the four adult owls spent 13.28 h awake, 9.64 h in SWS, and 1.05 h in REM sleep. We demonstrate that minimally invasive methods can be used to measure EEG-defined wakefulness, SWS, and REM sleep in owls and probably other animals.

New insights into the long-term evolutionary history of the avian MHC

SummaryGene duplication and neofunctidnalization are important processes in the evolution of phenotypic complexity. They account for important evolutionary novelties that confer ecological adaptation, such as the major histocompatibility complex (MHC), a multigene family with a central role in vertebrates' adaptive immune system. Multigene families, which evolved in large part through duplication, represent promising systems to study the still strongly depbated relative roles of neutral and adaptive processes in the evolution of phenotypic complexity. Detailed knowledge on ecological function and a well-characterized evolutionary history place the mammals' MHC amongst ideal study systems. However mammalian MHCs usually encompass several million base pairs and hold a large number of functional and non-functional duplicate genes, which makes their study complex. Avian MHCs on the other hand are usually way more compact, but the reconstruction of. their evolutionary history has proven notoriously difficult. However, no focused attempt has been undertaken so far to study the avian MHC evolutionary history in a broad phylogenetic context and using adequate gene regions.In the present PhD, we were able to make important contributions to the understanding of the long-term evolution of the avian MHC class II Β (MHCI1B). First, we isolated and characterized MHCIIB genes in barn owl (Tyto alba?, Strigiformes, Tytonidae), a species from an avian lineage in which MHC has not been studied so far. Our results revealed that with only two functional MHCIIB genes the MHC organization of barn owl may be similar to the 'minimal essential' MHC of chicken (Gallus gallus), indicating that simple MHC organization may be ancestral to birds. Taking advantage of the sequence information from barn owl, we studied the evolution of MHCIIB genes in 13 additional species of 'typical' owls (Strigiformes, Strigidae). Phylogenetic analyses revealed that according to their function, in owls the peptide-binding region (PBR) encoding exon 2 and the non-PBR encoding exon 3 evolve by different patterns. Exon 2 exhibited an evolutionary history of positive selection and recombination, while exon 3 traced duplication history and revealed two paralogs evolving divergently from each other in owls, and in a shorebird, the great snipe {Gallinago media). The results from exon 3 were the first ever from birds to demonstrate gene orthology in species that diverged tens of millions of years ago, and strongly questioned whether the taxa studied before provided an adequate picture of avian MHC evolution. In a follow-up study, we aimed at explaining a striking pattern revealed by phylogenetic trees analyzing the owl sequences along with MHCIIB sequences from other birds: One owl paralog (termed DAB1) grouped with sequences of passerines and falcons, while the other (DAB2) grouped with wildfowl, penguins and birds of prey. This could be explained by either a duplication event preceding the evolution of these bird orders, or by convergent evolution of similar sequences in a number of orders. With extensive phylogenetic analyses we were able to show, that indeed a duplication event preceeded the major avian radiation -100 my ago, and that following this duplication, the paralogs evolved under positive selection. Furthermore, we showed that the divergently evolving amino acid residues in the MHCIIB-encoded β-chain potentially interact with the MHCI I α-chain, and that molecular coevolution of the interacting residues may have been involved in the divergent evolution of the MHCIIB paralogs.The findings of this PhD are of particular interest to the understanding of the evolutionary history of the avian MHC and, by providing essential information on long-term gene history in the avian MHC, open promising perspectives for advances in the understanding of the evolution of multigene families in general, and for avian MHC organization in particular. Amongst others I discuss the importance of including protein structure in the phylogenetic study of multigene families, and the roles of ecological versus molecular selection pressures. I conclude by providing a population genomic perspective on avian MHC, which may serve as a basis for future research to investigate the relative roles of neutral processes involving effective population size effects and of adaptation in the evolution of avian MHC diversity and organization.RésuméLa duplication de gènes et leur néo-fonctionnalisation sont des processus importants dans l'évolution de la complexité phénotypique. Ils sont impliqués dans l'apparition d'importantes nouveautés évolutives favorisant l'adaptation écologique, comme c'est le cas pour le complexe majeur d'histocompatibilité