Within an ecological community, interactions between species in a community occur directly, through physical contact, and indirectly, via other species or through abiotic environmental modification. Genetic variation within a species has the ability to alter the outcome of interactions between species. In other words, the specific genotypes of the interacting individuals are important for the outcome of the interaction. In this thesis, I begin by showing that indirect interactions between a soil rhizobacteria (Pseudomonas aeruginosa) and a parasitoid wasp (Aphidius rhopalosiphum) are mediated by genotypic interactions between the two linking species, aphids (Sitobion avenae) and barley (Hordeum vulgare). This means that the magnitude and direction of the indirect effect of rhizobacteria (presence/absence) on the wing size of the parasitoid wasp was different, depending on the combination of interacting aphid and barley genotypes. If such interactions were to have an evolutionary effect, there would need to be non-random association between the aphid and barley genotypes. In the next chapter, I demonstrated that different aphid genotypes actively choose (no effect on reproductive rate) to colonize particular barley genotypes. I then showed that host preference of an aphid genotype could be altered by the presence, and sometimes genotypic identity, of another competing aphid genotype. I confirmed that these interactions were indirect, via the plant, by showing that aphid growth rate can be reduced when a plant is pre-conditioned with a different aphid genotype. Further investigation, using microarrays, showed that the different aphid genotypes induced differential gene expression in a single barley genotype. Many of these sequences belonged to known plant defense pathways and suggest a possible mechanism for the observed genotypic interactions between aphid and barley. In order to further understand the influence of within-species genetic variation on species interactions there is a need to consider these interactions in a natural system. I therefore investigated the influence of genetic variation within a single tree species on the associated plant and invertebrate communities in a complex, natural tropical ecosystem. I found that more closely related trees were host to more similar communities of epiphytic plants, leaf litter invertebrates around the base of the tree and trunk-dwelling invertebrates. This shows that even in a highly diverse, naturally occurring ecosystem the effect of genetic variation within a species can be an important factor for the structure of associated communities of both plants and animals.Research on the influence of within-species genetic variation on species interactions at a community level has wide applications for understanding how communities and ecosystems function, which can benefit agriculture, disease management and conservation practices.

Introdução: Com o melhor controle das causas ambientais, as doenças genéticas e as malformações congênitas cada vez mais ganham destaque como fatores morbidade e mortalidade, assim como cada vez mais se reconhece a importância da base genética para as doenças comuns. Desta forma, é necessário desenvolver estratégias de prevenção e controle destas condições, assim como acesso aos cuidados de saúde para pessoas com ou em risco de desenvolver uma doença genética. A integração da genética na Atenção Primária à Saúde (APS) parece ser uma alternativa para se desenvolver ações de prevenção e controle, assim como facilitar o acesso da comunidade aos cuidados de saúde com base no conhecimento sobre a genética. Para tanto, é necessário que os profissionais da APS tenham um conhecimento básico sobre genética, as principais condições, as formas de manejo e prevenção. Objetivo: Avaliar se a aplicação de um programa educativo de genética médica direcionado às equipes de APS pode contribuir para integrar conceitos e atitudes relacionados à identificação, ao manejo e à prevenção de malformações congênitas e doenças genéticas. Métodos: O programa foi planejado por profissionais do Serviço de Genética Médica do Hospital de Clínicas de Porto Alegre e aplicado em três Unidades Básicas de Saúde (UBS) do Serviço de Saúde Comunitária do Grupo Hospitalar Conceição. O programa foi constituído por encontros realizados nas próprias UBS com ênfase nos seguintes temas: conceitos básicos em genética, avaliação genética de famílias, teratógenos e prevenção de defeitos congênitos, genética da deficiência intelectual, erros inatos do metabolismo e triagem neonatal, oncogenética, genética das doenças crônicas comuns e aconselhamento genético e préconcepcional. Foi desenvolvido material de apoio para os participantes e material educativo para a comunidade. O programa foi avaliado através de pré e pós-teste, bem como um questionário de auto-avaliação aplicado aos profissionais da APS sobre o conhecimento e confiança em prover cuidados com bases genéticas aos pacientes e famílias. Além disso, foi aplicado um questionário sobre a relevância do programa para a prática clínica e a utilização dos recursos apresentados ou disponibilizados durante o programa. Resultados: Quarenta e três profissionais participaram do programa. Em duas das três UBS encontramos um aumento estatisticamente significativo no número de acertos do pós-teste quando comparado ao préteste. Durante o programa foram desenvolvidos materiais educativos para a comunidade (folders) e para os profissionais (Manual de Genética Médica para a Atenção Primária à Saúde). A maioria dos profissionais relatou um maior conhecimento e confiança após o programa, referiu que o programa influenciou a sua prática clínica e informou utilizar os recursos de informação apresentados e o material educativo elaborado durante o programa. Conclusão: O programa proporcionou uma aproximação da genética médica com a APS, fazendo com que os profissionais atentem para a genética como determinante de doenças na comunidade. Entretanto percebeu-se a necessidade de estratégias de educação continuada e suporte contínuo à APS, com o intuito de fortalecer a integração das duas especialidades, colocando-se em prática a genética comunitária. / Introduction: Genetic disease and birth defects are increasingly gaining prominence as factors morbidity and mortality, as well as it is increasingly recognized the importance of the genetic basis for common diseases. So, it is necessary to develop strategies for the prevention and control of these conditions, as well for the access to health care for people with or at risk of developing a genetic disease. On this way, it is necessary to develop strategies for prevention and control of these conditions, as well as to provide access to health care for people with or at risk of developing a genetic disease. The integration of Medical Genetics and Primary Health Care (PHC) seems to be an alternative to develop actions of prevention and control, and to facilitate community access to health care based on knowledge about genetics. Therefore, it is necessary that primary care professionals have a basic knowledge about genetics, the main conditions, management measures and prevention of genetics conditions. Aim: To evaluate if the application of an educational program in Medical Genetics directed to PHC teams can help to integrate concepts and attitudes related to the identification, management and prevention of congenital malformations and genetic diseases. Method: The program was designed by health professional of the Medical Genetics Service/Hospital de Clínicas de Porto Alegre and applied to three Basic Health Unitis (BHU) of Community Health Service/Grupo Hospitalar Conceição. The program consisted of meetings that took place at the BHU, which emphasis on the following topics: basic concepts in genetics; genetic evaluation of families; teratogenic agents and prevention of birth defects; genetics of mental retardation; inborn errors of metabolism and neonatal screening; cancer genetics; genetics of common chronic diseases; and genetic and preconception counseling. It was developed support materials for participants and educational material for the community. The program was evaluated through pre and post-test, as well through self-assessment questionnaire applied to the PHC professionals based on the knowledge and confidence in providing care with genetic basis to patients and families. In addition, a questionnaire was applied about the relevance of the program for clinical practice and use of the resources presented or made available during the program. Results: Forty three professionals participated in the program. In two of the three BHU it was found a statistically significant increase in the number of hits from the post-test compared to pre-test. Most professionals reported increased knowledge and confidence after the program, said that the program influenced their clinical practice and informed that the information resources and educational material presented during the program were useful. Conclusions: The program provided an approximation between Genetic Medicine and PHC, so that professionals pay attention to the genetic determinants of disease in the community. However it was realized the need for strategies for continuing education and ongoing support to the APS, with the aim of strengthening the integration of the two specialties, bringing to practice the community genetics.

Introdução: Com o melhor controle das causas ambientais, as doenças genéticas e as malformações congênitas cada vez mais ganham destaque como fatores morbidade e mortalidade, assim como cada vez mais se reconhece a importância da base genética para as doenças comuns. Desta forma, é necessário desenvolver estratégias de prevenção e controle destas condições, assim como acesso aos cuidados de saúde para pessoas com ou em risco de desenvolver uma doença genética. A integração da genética na Atenção Primária à Saúde (APS) parece ser uma alternativa para se desenvolver ações de prevenção e controle, assim como facilitar o acesso da comunidade aos cuidados de saúde com base no conhecimento sobre a genética. Para tanto, é necessário que os profissionais da APS tenham um conhecimento básico sobre genética, as principais condições, as formas de manejo e prevenção. Objetivo: Avaliar se a aplicação de um programa educativo de genética médica direcionado às equipes de APS pode contribuir para integrar conceitos e atitudes relacionados à identificação, ao manejo e à prevenção de malformações congênitas e doenças genéticas. Métodos: O programa foi planejado por profissionais do Serviço de Genética Médica do Hospital de Clínicas de Porto Alegre e aplicado em três Unidades Básicas de Saúde (UBS) do Serviço de Saúde Comunitária do Grupo Hospitalar Conceição. O programa foi constituído por encontros realizados nas próprias UBS com ênfase nos seguintes temas: conceitos básicos em genética, avaliação genética de famílias, teratógenos e prevenção de defeitos congênitos, genética da deficiência intelectual, erros inatos do metabolismo e triagem neonatal, oncogenética, genética das doenças crônicas comuns e aconselhamento genético e préconcepcional. Foi desenvolvido material de apoio para os participantes e material educativo para a comunidade. O programa foi avaliado através de pré e pós-teste, bem como um questionário de auto-avaliação aplicado aos profissionais da APS sobre o conhecimento e confiança em prover cuidados com bases genéticas aos pacientes e famílias. Além disso, foi aplicado um questionário sobre a relevância do programa para a prática clínica e a utilização dos recursos apresentados ou disponibilizados durante o programa. Resultados: Quarenta e três profissionais participaram do programa. Em duas das três UBS encontramos um aumento estatisticamente significativo no número de acertos do pós-teste quando comparado ao préteste. Durante o programa foram desenvolvidos materiais educativos para a comunidade (folders) e para os profissionais (Manual de Genética Médica para a Atenção Primária à Saúde). A maioria dos profissionais relatou um maior conhecimento e confiança após o programa, referiu que o programa influenciou a sua prática clínica e informou utilizar os recursos de informação apresentados e o material educativo elaborado durante o programa. Conclusão: O programa proporcionou uma aproximação da genética médica com a APS, fazendo com que os profissionais atentem para a genética como determinante de doenças na comunidade. Entretanto percebeu-se a necessidade de estratégias de educação continuada e suporte contínuo à APS, com o intuito de fortalecer a integração das duas especialidades, colocando-se em prática a genética comunitária. / Introduction: Genetic disease and birth defects are increasingly gaining prominence as factors morbidity and mortality, as well as it is increasingly recognized the importance of the genetic basis for common diseases. So, it is necessary to develop strategies for the prevention and control of these conditions, as well for the access to health care for people with or at risk of developing a genetic disease. On this way, it is necessary to develop strategies for prevention and control of these conditions, as well as to provide access to health care for people with or at risk of developing a genetic disease. The integration of Medical Genetics and Primary Health Care (PHC) seems to be an alternative to develop actions of prevention and control, and to facilitate community access to health care based on knowledge about genetics. Therefore, it is necessary that primary care professionals have a basic knowledge about genetics, the main conditions, management measures and prevention of genetics conditions. Aim: To evaluate if the application of an educational program in Medical Genetics directed to PHC teams can help to integrate concepts and attitudes related to the identification, management and prevention of congenital malformations and genetic diseases. Method: The program was designed by health professional of the Medical Genetics Service/Hospital de Clínicas de Porto Alegre and applied to three Basic Health Unitis (BHU) of Community Health Service/Grupo Hospitalar Conceição. The program consisted of meetings that took place at the BHU, which emphasis on the following topics: basic concepts in genetics; genetic evaluation of families; teratogenic agents and prevention of birth defects; genetics of mental retardation; inborn errors of metabolism and neonatal screening; cancer genetics; genetics of common chronic diseases; and genetic and preconception counseling. It was developed support materials for participants and educational material for the community. The program was evaluated through pre and post-test, as well through self-assessment questionnaire applied to the PHC professionals based on the knowledge and confidence in providing care with genetic basis to patients and families. In addition, a questionnaire was applied about the relevance of the program for clinical practice and use of the resources presented or made available during the program. Results: Forty three professionals participated in the program. In two of the three BHU it was found a statistically significant increase in the number of hits from the post-test compared to pre-test. Most professionals reported increased knowledge and confidence after the program, said that the program influenced their clinical practice and informed that the information resources and educational material presented during the program were useful. Conclusions: The program provided an approximation between Genetic Medicine and PHC, so that professionals pay attention to the genetic determinants of disease in the community. However it was realized the need for strategies for continuing education and ongoing support to the APS, with the aim of strengthening the integration of the two specialties, bringing to practice the community genetics.

Introdução: Com o melhor controle das causas ambientais, as doenças genéticas e as malformações congênitas cada vez mais ganham destaque como fatores morbidade e mortalidade, assim como cada vez mais se reconhece a importância da base genética para as doenças comuns. Desta forma, é necessário desenvolver estratégias de prevenção e controle destas condições, assim como acesso aos cuidados de saúde para pessoas com ou em risco de desenvolver uma doença genética. A integração da genética na Atenção Primária à Saúde (APS) parece ser uma alternativa para se desenvolver ações de prevenção e controle, assim como facilitar o acesso da comunidade aos cuidados de saúde com base no conhecimento sobre a genética. Para tanto, é necessário que os profissionais da APS tenham um conhecimento básico sobre genética, as principais condições, as formas de manejo e prevenção. Objetivo: Avaliar se a aplicação de um programa educativo de genética médica direcionado às equipes de APS pode contribuir para integrar conceitos e atitudes relacionados à identificação, ao manejo e à prevenção de malformações congênitas e doenças genéticas. Métodos: O programa foi planejado por profissionais do Serviço de Genética Médica do Hospital de Clínicas de Porto Alegre e aplicado em três Unidades Básicas de Saúde (UBS) do Serviço de Saúde Comunitária do Grupo Hospitalar Conceição. O programa foi constituído por encontros realizados nas próprias UBS com ênfase nos seguintes temas: conceitos básicos em genética, avaliação genética de famílias, teratógenos e prevenção de defeitos congênitos, genética da deficiência intelectual, erros inatos do metabolismo e triagem neonatal, oncogenética, genética das doenças crônicas comuns e aconselhamento genético e préconcepcional. Foi desenvolvido material de apoio para os participantes e material educativo para a comunidade. O programa foi avaliado através de pré e pós-teste, bem como um questionário de auto-avaliação aplicado aos profissionais da APS sobre o conhecimento e confiança em prover cuidados com bases genéticas aos pacientes e famílias. Além disso, foi aplicado um questionário sobre a relevância do programa para a prática clínica e a utilização dos recursos apresentados ou disponibilizados durante o programa. Resultados: Quarenta e três profissionais participaram do programa. Em duas das três UBS encontramos um aumento estatisticamente significativo no número de acertos do pós-teste quando comparado ao préteste. Durante o programa foram desenvolvidos materiais educativos para a comunidade (folders) e para os profissionais (Manual de Genética Médica para a Atenção Primária à Saúde). A maioria dos profissionais relatou um maior conhecimento e confiança após o programa, referiu que o programa influenciou a sua prática clínica e informou utilizar os recursos de informação apresentados e o material educativo elaborado durante o programa. Conclusão: O programa proporcionou uma aproximação da genética médica com a APS, fazendo com que os profissionais atentem para a genética como determinante de doenças na comunidade. Entretanto percebeu-se a necessidade de estratégias de educação continuada e suporte contínuo à APS, com o intuito de fortalecer a integração das duas especialidades, colocando-se em prática a genética comunitária. / Introduction: Genetic disease and birth defects are increasingly gaining prominence as factors morbidity and mortality, as well as it is increasingly recognized the importance of the genetic basis for common diseases. So, it is necessary to develop strategies for the prevention and control of these conditions, as well for the access to health care for people with or at risk of developing a genetic disease. On this way, it is necessary to develop strategies for prevention and control of these conditions, as well as to provide access to health care for people with or at risk of developing a genetic disease. The integration of Medical Genetics and Primary Health Care (PHC) seems to be an alternative to develop actions of prevention and control, and to facilitate community access to health care based on knowledge about genetics. Therefore, it is necessary that primary care professionals have a basic knowledge about genetics, the main conditions, management measures and prevention of genetics conditions. Aim: To evaluate if the application of an educational program in Medical Genetics directed to PHC teams can help to integrate concepts and attitudes related to the identification, management and prevention of congenital malformations and genetic diseases. Method: The program was designed by health professional of the Medical Genetics Service/Hospital de Clínicas de Porto Alegre and applied to three Basic Health Unitis (BHU) of Community Health Service/Grupo Hospitalar Conceição. The program consisted of meetings that took place at the BHU, which emphasis on the following topics: basic concepts in genetics; genetic evaluation of families; teratogenic agents and prevention of birth defects; genetics of mental retardation; inborn errors of metabolism and neonatal screening; cancer genetics; genetics of common chronic diseases; and genetic and preconception counseling. It was developed support materials for participants and educational material for the community. The program was evaluated through pre and post-test, as well through self-assessment questionnaire applied to the PHC professionals based on the knowledge and confidence in providing care with genetic basis to patients and families. In addition, a questionnaire was applied about the relevance of the program for clinical practice and use of the resources presented or made available during the program. Results: Forty three professionals participated in the program. In two of the three BHU it was found a statistically significant increase in the number of hits from the post-test compared to pre-test. Most professionals reported increased knowledge and confidence after the program, said that the program influenced their clinical practice and informed that the information resources and educational material presented during the program were useful. Conclusions: The program provided an approximation between Genetic Medicine and PHC, so that professionals pay attention to the genetic determinants of disease in the community. However it was realized the need for strategies for continuing education and ongoing support to the APS, with the aim of strengthening the integration of the two specialties, bringing to practice the community genetics.

Community genetics research investigates the influence of intra-specific genetic variation on species interactions. This rapidly growing research field consists of more than one approach to explore how a significant portion of the environment of a focal species is differentially defined by the expressed genomes of other interacting species. While the basic concept of community genetics is well supported empirically, there is still a set of pertinent issues in need of further investigation. The initial research addressed herein focused on the extent to which the magnitude of a community genetic effect can be moderated when acting in concert with other forces in nature, i.e. the interaction between community genetic effects and the effects of other eco-evolutionary processes such as competition and parasitism. Subsequent research investigated the impact of genetic variation of host plants in agro-ecosystems on the performance (reproductive success) and behaviour (distribution and feeding-site choice) of plant-associated pests such as aphids, especially when pests and their hosts were subject to plant-mediated interactions. In addition, the differential effects of Indirect Ecological Effects (IEEs) and Indirect Genetic effects (IGEs) on the emergence of shared (extended) phenotypes between natural enemies (i.e. biological control agents and phytophagous insects) were examined. I provide clear evidence for significant effects of the genetic variation of host plant on aphid performance, behaviour and intra- and inter-specific competition. My findings also give credence to the concept of reciprocal moderation between plant genotype and aphid competition. I also provide observations on competition that segues into less antagonistic and possibly into a more cooperative form of interaction. In addition, I establish novel systems of economically important crop genotypes, noxious sap-feeding aphid species and root-galling nematodes. I also devise an amalgamated approach to interpret the interwoven set of mechanisms that underpin the observations presented and conclusions drawn. I also provide further investigation on the role of Indirect Ecological Effects (IEEs) between root-knot nematodes and sap-feeding aphids, and demonstrate the influence of in-plant variation on the interaction between the spatially separated plant consumers. Furthermore, I use a quantitative genetic experimental design in order to demonstrate a differential impact of parasitoid genotype on the behaviour of its aphid host. As such, I provide some of the clearest evidence to date that the phenotype of an organism can be the product of the genes expressed in another organism via Inter-specific Indirect Genetic Effects (IIGEs). Finally, I conducted research on epiphytic bromeliads and their associated faunal communities in the tropics. Here I demonstrate that the influence of intra-specific genetic variation of the host plant on the associated ecological communities may be more universal than previously conceived, with a plausible role for such variation in the maintenance of biological diversity. My research provides evidence for the genetic basis of species interactions and, interestingly, a genetic basis for the evolutionary arms-race between foragers and their hosts. My doctoral work adds new evidence to the increasing literature on the evolutionary importance of (Genotype x Genotype) interactions and (Genotype x Genotype x Environment) interactions in shaping the dynamics of pest communities, which in turn can affect plant phenotype and can influence the properties and services of the focal ecosystem in which the inter-players live and interact.

De nombreux et divers micro-organismes vivent dans les tissus interne et externe desfeuilles des plantes, la phyllosphère. Ils influencent de nombreux traits, les interactions biotiques,le flux d’énergie, la tolérance au stress de leur hôte et en fin de compte la valeur sélectivede leurs hôtes. Il a été montré que plusieurs traits quantitatifs de plantes structurentla communauté microbienne de la phyllosphère. Ainsi des Loci de ces traits quantitatifs(Quantitative Trait Loci QTL) liés à la structure de cette communauté étaient attendus.L’objectif principal de ce travail était de rechercher des régions génomiques chez le chêne(Quercus robur L.), dont l’effet se prolonge jusqu’au niveau de la communauté, influençantainsi le microbiote de la phyllosphère. Tout d’abord, nous avons étudié la composition etle réseau d’interactions du microbiote de la phyllosphère partant un intérêt particulier àErysiphe alphitoides, un agent pathogène majeur pour les chênes. Nous avons montré quel’infection par E. alphitoides est accompagnée par des changements dans la composition dela communauté fongique foliaire, mais pas dans le composition de la communauté bactérienne.Nous avons souligné certains partenaires d’interaction d’E. alphitoides et nous avonsmontré que le réseau d’interactions microbiennes, contrairement aux résultats précédents,été dominé par des interactions positives. Ensuite nous avons effectué une analyse QTLde descripteurs de la communauté microbienne dans une population de pleins frères. Nousavons trouvé 8 QTL correspondant à des traits de la communauté microbienne: compositionfongique et bactérienne, diversité fongique, pourcentage de Erysiphe alphitoides. Troisd’entre eux sont colocalisés avec un QTL de sensibilité à E. alphitoides, suggèrent un fortdéterminisme génétique de la résistance à l’oïdium chez Q. robur. Enfin, nous présentonsles résultats préliminaires d’une étude d’association génétique et discutons nos résultatsavec une perspective évolutive. / Numerous and various microorganisms inhabit inner and outer tissues of plant leaves, thephyllosphere. They influence many plant traits, biotic interactions, energy flux, host stresstolerance and ultimately the fitness of their hosts. Many plant quantitative traits wereshown to structure the phyllosphere microbial community. Hence quantitative trait loci(QTLs) linked to the structure of this community were expected. The main objective ofthis work was to search for genomic regions in oak (Quercus robur L.), whose effect extendsto the community level, influencing the phyllosphere microbiota. First, we studied thecomposition and the interaction network of oak phyllosphere microbiota with specific focuson Erysiphe alphitoides, a major biotrophic pathogen of oak. We showed that infection byE. alphitoides is accompanied by changes in the foliar fungal community composition butnot in the bacterial community composition. We highlighted likely interaction partners ofE. alphitoides and we showed that the complex microbial interaction network, in contrastto previous findings, was dominated by positive interactions. Next we performed QTLanalysis of microbial community descriptors in a full-sib mapping population of oak. Wefound 8 QTLs for microbial community traits: fungal and bacterial composition, fungaldiversity, and percentage of Erysiphe alphitoides reads. Three of these QTLs colocalizedwith a QTL for powdery mildew sensibility, suggesting for strong genetic determinism ofpowdery mildew resistance in Q. robur. Finally we present preliminary results of a geneticassociation study and discuss our findings within an evolutionary perspective.

Les relations entre plantes et herbivores occupent une part importante des travaux concernant le rôle de la biodiversité dans le fonctionnement des écosystèmes. Les effets de la diversité des plantes sur les herbivores et la phytophagie se répartissent le long d’un continuum entre résistance et susceptibilité par association, ces deux concepts décrivant respectivement la réduction ou au contraire l’augmentation des dégâts causés par les herbivores avec la diversité des plantes dans les communautés végétales. Si de nombreux travaux menés dans différents écosystèmes suggèrent que la résistance par association est plus fréquente que la susceptibilité par association, les mécanismes sous jacents restent discutés et semblent dépendre de la plus ou moins grande spécialisation des herbivores et de l’identité des espèces végétales associées. En particulier deux questions se posent : (1) quel est l’effet de la diversité des plantes sur la diversité des insectes, et principalement sur les insectes herbivores ? (2) Quelle composante de la diversité des plantes explique le mieux la résistance et la susceptibilité par association ? La méta-analyse présentée dans cette thèse montre que la diversité des animaux augmente avec la diversité des plantes, quels que soient les écosystèmes et les taxons considérés. Dans le cas des Arthropodes, la richesse spécifique des herbivores est mieux corrélée à la richesse spécifique des plantes que celle des prédateurs. De plus, la diversité des herbivores augmente plus rapidement que la diversité des prédateurs avec la diversité des plantes. Ces observations soulèvent un paradoxe : si la diversité des herbivores augmente avec celle des plantes, comment expliquer les nombreux cas où la phytophagie diminue (résistance par association) ? Cette question a fait l’objet d’une approche expérimentale centrée sur le chêne pédonculé et fondée sur la manipulation de la diversité spécifique et génétique des essences forestières. Il en ressort que ces deux niveaux de diversité ont des effets contrastés sur la phytophagie. Les dégâts causés par les insectes herbivores généralistes tendent à augmenter avec la diversité génétique chez leur hôte (susceptibilité par association), alors que les spécialistes ne répondent pas à la diversité génétique chez le chêne. Au contraire, la diversité spécifique est à l’origine d’une résistance par association pour les herbivores spécialistes mais elle est sans effet sur les généralistes. Un résultat original émerge des travaux présentés dans cette thèse : les effets de la diversité des plantes sur les insectes herbivores et leurs dégâts s’exprimeraient principalement à l’échelle locale – celle d’un arbre et de ses voisins immédiats – au travers de « l’apparence » des arbres, une variable conditionnée par leur taille et celle de leurs voisins. La prise en compte de la spécialisation des insectes herbivores et des différentes composantes de la diversité des arbres contribue donc à une meilleure compréhension des mécanismes impliqués dans la résistance et la susceptibilité par association des forêts mélangées. / Functional and structural consequences of plant diversity on insect communities and herbivoryThe relationship between biodiversity and ecosystem functioning remains a key issue in ecology. In particular it has been suggested that plant species diversity can trigger plant-insect interactions, ranging from associational resistance (higher herbivory in monocultures than in more diverse plant communities) to associational susceptibility (higher herbivory in plant mixtures). Although it is now widely acknowledged that associational resistance is more frequent than associational susceptibility, the underlying mechanisms are still debated and seem to depend on herbivore specialization and plant species identity. To contribute to this debate we addressed two questions: (1) What are the consequences of plant diversity for insect herbivore diversity? (2) Which component of plant diversity better accounts for herbivory? On the basis of a meta-analysis we show in this report that animal diversity significantly increase with increasing plant diversity, regardless of habitats or taxa. In arthropods, herbivores species richness increases more steeply with plant richness than predator richness. This result raises an apparent paradox: if herbivore diversity is positively correlated with plant diversity, why herbivory decreases with increasing plant diversity (associational resistance)? To solve this paradox we developed an experimental approach focused on pedunculate oak and based on the manipulation of tree genetic and species diversity. We showed that generalist and specialist herbivores respond differently to the two levels of tree diversity. Herbivory by generalist herbivores significantly increased with oak genetic diversity (i.e. associational susceptibility) while specialist herbivores did not respond to the genetic diversity of their host tree. Conversely, tree species diversity triggered associational resistance to specialist insect herbivores but had no effect on generalists.  The novelty of this works also lies in the identification of a new mechanism that may account for associational resistance in tree species mixtures: host apparency. More apparent oak trees – those that are taller than their neighbours – are more prone to insect damage. Increasing tree species diversity comes together with higher probability of associating faster growing species (such as pine or birch) than can hide neighbouring oaks, reducing their apparency and then their susceptibility. Our findings suggest that accounting for the diet breadth of insect herbivores and the different components of tree diversity can contribute to a better understanding of the mechanisms leading to associational resistance or susceptibility in mixed forests.