Endosymbiont diversity within host communities and insights into their role in species coexistence : oak weevil species (Curculio spp.) as a case study

Parmi les théories explicatives de la biodiversité, la théorie des niches propose que des espèces en compétition puissent coexister durablement à la condition qu'elles expriment des différences de caractères phénotypiques conduisant à un partitionnement de leurs niches écologiques. Par ailleurs, l'acquisition de bactéries endosymbiotiques peut être assimilée à une extension du phénotype de l'hôte, susceptible de modifier sa niche écologique. Cependant, l'implication de ces micro-organismes endosymbiotiques dans la structure des communautés d'espèces d'insectes en compétition n'a encore jamais été envisagée. Mon hypothèse est que les communautés de bactéries endosymbiotiques hébergées par différentes espèces-hôtes en compétition pourraient impacter la dynamique des communautés d'hôtes en facilitant la différentiation de leurs traits et ainsi stabiliser leur coexistence. Réciproquement, les interactions de nature compétitive entre espèces-hôtes pourraient influencer la dynamique évolutive des cortèges endosymbiotiques de chacune de ces espèces en favorisant notamment les occasions de transfert horizontal de bactéries endosymbiotiques. Durant ma thèse, j'ai étudié les relations endosymbiotiques au sein des communautés composées de quatre espèces d'insectes phytophages spécialistes du chêne (Curculio spp., Coléoptères Curculionidés), qui sont en compétition pour les glands qui représentent le siège du développement larvaire. Afin d'envisager l'implication des endosymbiotes dans le partitionnement des niches écologiques de ces espèces, j'ai développé ma thèse autour de deux objectifs principaux visant à (i) décrire et comparer les cortèges de bactéries endosymbiotiques hébergés par les quatre espèces, par une caractérisation moléculaire des souches bactériennes et une mesure de leur prévalence dans les populations d'hôtes et (ii) identifier les cas de transferts horizontaux interspécifiques d'endosymbiotes à l'aide d'une approche multi-gènes couplée à l'estimation du taux de multi-infestation plurispécifique à l'échelle du gland. J'ai pu montrer que ces espèces d'insectes hébergent des cortèges endosymbiotiques distincts, trois des quatre espèces hébergeant leur propre endosymbiote secondaire majoritaire. Une telle différentiation pourrait contribuer au partitionnement de niches écologiques de ces quatre espèces, notamment au vu des différences, mises en évidence au cours de ma thèse, dans la période d'exploitation des glands au cours de la saison de reproduction. Par ailleurs, le fait que chaque espèce-hôte possède, en prévalence résiduelle, la plupart des symbiotes majoritaires chez d'autres espèces laisse suspecter des transferts horizontaux au sein de cette communauté. Cette hypothèse semble confirmée par la comparaison approfondie des souches de Wolbachia présentes dans ces communautés d'hôtes et par la proportion non négligeable de glands infestés simultanément par plusieurs espèces d'insectes. Les résultats obtenus au cours de ma thèse corroborent l'hypothèse d'un impact possible des communautés endosymbiotiques sur la dynamique des communautés d'hôtes en compétition et soulignent l'importance de prendre en compte l'échelle des communautés d'hôtes pour comprendre la dynamique des communautés bactériennes au sein des populations de l'espèce-hôteAmong biodiversity theories, the niche theory states that competing species can durably coexist whenever they exhibit differences in their traits leading to partitioning of their ecological niches. Moreover, housing symbiotic bacteria is now recognized as expanding the host phenotype and is thus likely to interfere with its ecological niche. Intriguingly, the question of whether endosymbionts are likely to shape communities of competing insect species has not yet been addressed in natural systems. My hypothesis is that endosymbiosis can be determinant in driving the dynamics of communities composed of competing host species by amplifying differences in theirs traits and thus, stabilizing their coexistence. Reciprocally, ecological interactions among these host species could have a decisive role in the evolutionary dynamic of endosymbiotic cortege of each species by providing opportunities of horizontal transfer of bacterial endosymbionts. During my PhD, I studied endosymbiotic relationships among communities consisting of four species of specialist oak weevil insects (Curculio spp., Coleoptera, Curculionidae) that are competing for egg-laying into acorns where the larva grow. The two main objectives of my thesis were (i) to describe and compare the endosymbiotic corteges housed by the four species, by means of molecular screening of the bacterial diversity in female ovaries and measuring their prevalence in males and females of host populations, and (ii) to check the existence horizontal transfers of endosymbionts between host species with a multigenic approach combined to estimating the probability for one acorn to be infested by larva belonging to distinct Curculio species. I found that the four weevil species overall harboured distinct endosymbiotic communities, and notably that three of the four species had their own major secondary symbiont. Additionally, I found that the four species had a clearly distinct period of larval development during the reproductive season. Such diversification of endosymbionts across species competing with each other might contribute to their temporal partitioning in the acorn use. Otherwise, the fact that each host species also hosts, with residual prevalence, most of endosymbionts found in the other species, argues for the existence of horizontal transfer among this community. This hypothesis seems to be confirmed by a fine-tuned comparison of the Wolbachia strains detected in these host communities and by the non negligible proportion of acorns which are simultaneously infested by two or more weevil species. These results support the hypothesis of an impact of endosymbiotic communities on the dynamics of competing host communities and point out the need to consider the host community scale in order to understand the dynamics of bacterial communities within host species population

Endosymbiont diversity among sibling weevil species competing for the same resource

International audienceBACKGROUND: Whereas the impact of endosymbionts on the ecology of their hosts is well known in some insect species, the question of whether host communities are influenced by endosymbionts remains largely unanswered. Notably, the coexistence of host species competing with each other, which is expected to be stabilized by their ecological differences, could be facilitated by differences in their endosymbionts. Yet, the composition of endosymbiotic communities housed by natural communities of competing host species is still almost unknown. In this study, we started filling this gap by describing and comparing the bacterial endosymbiotic communities of four sibling weevil species (Curculio spp.) that compete with each other to lay eggs into oak acorns (Quercus spp.) and exhibit marked ecological differences. RESULTS: All four species housed the primary endosymbiont Candidatus Curculioniphilus buchneri, yet each of these had a clearly distinct community of secondary endosymbionts, including Rickettsia, Spiroplasma, and two Wolbachia strains. Notably, three weevil species harbored their own predominant facultative endosymbiont and possessed the remaining symbionts at a residual infection level. CONCLUSIONS: The four competing species clearly harbor distinct endosymbiotic communities. We discuss how such endosymbiotic communities could spread and keep distinct in the four insect species, and how these symbionts might affect the organization and species richness of host communities

Influence of Emotional Content and Context on Memory in Mild Alzheimer's Disease

International audienceHealthy subjects remember emotional stimuli better than neutral, as well as stimuli embedded in an emotional context. This better memory of emotional messages is linked to an amygdalo-hippocampal cooperation taking place in a larger fronto-temporal network particularly sensitive to pathological aging. Amygdala is mainly involved in gist memory of emotional messages. Whether emotional content or context enhances memory in mild Alzheimer's disease (AD) patients is still debated. The aim of the present study is to examine the influence of emotional content and emotional context on the memory in mild AD, and whether this influence is linked to amygdala volume. Fifteen patients affected by mild AD and 15 age-matched controls were submitted to a series of negative, positive, and neutral pictures. Each series was embedded in an emotional or neutral sound context. At the end of each series, participants had to freely recall pictures, and answer questions about each picture. Amygdala volumes were measured on patient 3D-MRI scans. In the present study, emotional content significantly favored memory of gist but not of details in healthy elderly and in AD patients. Patients' amygdala volume was positively correlated to emotional content memory effect, implying a reduced memory benefit from emotional content when amygdala was atrophied. A positive context enhanced memory of pictures in healthy elderly, but not in AD, corroborating early fronto-temporal dysfunction and early working memory limitation in this disease