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Consequences of the domestication of chili pepper on multitrophic interactions
La domestication des plantes vise à réduire leurs défenses et à sélectionner des variétés ayant un rendement plus élevé que leurs ancêtres sauvages. On pense que ces caractéristiques modifiées ont un impact sur les interactions plantes-insectes. Cependant, dans le cas du piment, ces effets restent peu clair. Par conséquent, cette thèse de doctorat visait à étudier les conséquences de la domestication du piment sur les interactions multitrophiques, en utilisant des variétés de piment et leur ancêtre sauvage Chiltepin comme modèles d'étude. Le premier chapitre se concentre sur l'effet des niveaux de capsaïcine dans les fruits domestiques sur la performance de l'herbivore généraliste S. latifascia et de son ectoparasitoïde E. platyhypenae. Nous avons constaté que la domestication du piment a augmenté et diminué la capsaïcine dans les fruits. Seulement à une concentration élevée de ces métabolites secondaires, la capsaïcine a eu un effet négatif sur les insectes herbivores, et leur ectoparasitoïde. Par conséquent, la réponse de la plante à l'herbivorie dépend du niveau de piquant des fruits. Le chapitre deux explore les insectes nuisibles et leur virus associé sur les piments sauvages et domestiques dans leur environnement naturel. Nos résultats montrent que la domestication du piment a altéré les défenses physiques et chimiques des feuilles. En conséquence, les piments sauvages étaient plus résistants aux insectes broyeurs et aux virus, mais pas à leurs vecteurs, les aleurodes. Le chapitre trois est consacré à l'étude de l'alimentation et de l’oviposition du charançon spécialiste du piment (A. eugenii) sur les piments sauvages et domestiques comme ornement et pour la consommation. Globalement, la domestication a modifié les caractéristiques morphologiques et chimiques (capsaïcine) des fruits avec des conséquences directes sur l'alimentation et l’oviposition du charançon du piment. Ce travail a des implications importantes pour l'agriculture durable qui repose sur une résistance accrue des plantes et une utilisation réduite des pesticides. Enfin, cette étude permet de comprendre l'impact des pressions de sélection à court terme exercées par l'homme sur nos plantes cultivées et les forces de sélection naturelle qui ont façonné les interactions plantes-insectes au cours de l'évolution. ABSTRACT Plant domestication aims to reduce plant defenses and select varieties with a higher yield than their wild ancestors. It is believed that these altered traits have an impact on plant-insect interactions. However, in the case of chili pepper, these effects remain unclear. Therefore, This Ph.D. thesis aimed to study the consequences of domestication of chili pepper on multitrophic interactions, using chili pepper varieties and their wild ancestor Chiltepin as study models. Chapter one focuses on the effect of altered levels of capsaicin in domesticated chili on the performance of the generalist herbivore S. latifascia and its ectoparasitoid E. platyhypenae. We found that chili domestication has increased and decreased capsaicinoids in fruits. Only at a high concentration of these secondary metabolites, capsaicin had a negative effect on insect herbivores, and their ectoparasitoid. Therefore, the response of the plant to herbivory depends on the pungency level in fruits. Chapter two explores the pest insects and their associated virus on wild and domesticated chili peppers in their natural environment. Our results showed chili domestication has altered physical and chemical defenses in leaves. As consequence wild chili were more resistant to chewing insect and viruses’ infection but not to their vector, whiteflies. Chapter three is devoted to studying the feeding and oviposition of the specialist pepper weevil (A. eugenii) on wild and domesticated chiles as ornamental and for consumption. Overall, domestication has altered morphological and chemical (capsaicin) traits in fruits with direct consequences for the feeding and oviposition of the pepper weevil. This work has important implications for sustainable agriculture that relies on enhanced plant resistance and reduced pesticide use. Finally, this study allows understanding the impact of short-term human-mediated selection pressures applied to our crop plants and the natural selection forces that have molded plant-insect interactions over evolutionary time.
Impact of exotic insect herbivores on native tritrophic interactions: a case study of the African cotton leafworm,
1. When exotic herbivores invade new environments, they have the potential to interfere with native predator-prey relationships. This interference can be indirect, via changes induced in native host plants, and may have cascading consequences at the community level. Here we investigate the impact of the presence of an exotic generalist insect herbivore, the African cotton leafworm Spodoptera littoralis, on the outcome of native tritrophic interactions between the plant Brassica rapa, the herbivore Pieris brassicae, and its parasitoid Cotesia glomerata. 2. In olfactometer choice-tests, plants damaged by S. littoralis and plants damaged by both S. littoralis and P. brassicae were consistently less attractive to the parasitoid than plants damaged by P. brassicae alone. Chemical analyses revealed that B. rapa volatiles typically induced by S. littoralis dominated the headspace in case of dual herbivore infestation. 3. In behavioral observations in petri dishes, C. glomerata wasps mistakenly attacked S. littoralis larvae significantly more often when P. brassicae was present, and attacks on both herbivores were comparable in terms of time (attack duration) and energy investment (number of eggs injected). Successful parasitism of S. littoralis was never observed, but larvae exposed to C. glomerata for 24 hours exhibited reduced survivorship, possibly as a result of failed parasitism attempts. 4. In tents with herbivore-infested B. rapa plants, the presence of S. littoralis did not have an effect on the parasitism rates of P. brassicae by C. glomerata, regardless of whether the two species were on the same or on different plants. Field experiments in newly invaded environments are necessary to evaluate the realized impact of S. littoralis. 5. Synthesis. Our study illustrates that exotic herbivores can impact native tritrophic interactions associated with Brassica rapa, even if they cannot be used as prey by native natural enemies. The mechanisms behind such effects, in particular chemical interference with foraging cues via changes in herbivore-induced plant volatiles, have the potential to be quite general, and their long-term effects on native communities should not be underestimated. This article is protected by copyright. All rights reserved.