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De la génétique des populations à l’écologie des communautés : le cas des mollusques d’eau douce aux Antilles françaises / From population genetics to community ecology : the case of the freshwater snail metacommunity in the French West IndiesLamy, Thomas 07 December 2011 (has links)
Malgré des ressemblances frappantes dans les processus qui les contrôlent, la diversité génétique et la diversité spécifique ont été majoritairement étudiées de manière disjointe, d'une part par la génétique des populations, d'autre part par l'écologie des communautés. L'objectif de cette thèse est de quantifier les processus qui contrôlent la diversité génétique au sein des espèces et la diversité spécifique au sein des communautés. Mon étude porte sur la métacommunauté de mollusques d'eau douce présente sur l'île de Grande-Terre (800 km²) en Guadeloupe. Celle-ci se compose de 29 espèces qui vivent dans une mosaïque de milieux d'eau douce soumis à une forte instabilité temporelle (à-secs fréquents), supposée induire une dynamique en extinction-colonisation des espèces. Cependant, tout à-sec n'entraîne pas nécessairement une extinction, car certaines espèces ont la capacité de rester en vie ralentie dans le sol (estivation). Je me suis tout d'abord focalisé sur le gastéropode pulmoné Drepanotrema depressissimum en combinant trois approches complémentaires : (1) une étude de la distribution spatiale et temporelle de la diversité génétique, (2) un modèle de dynamique de l'occupation des sites qui prend en compte les formes de résistance (estivation) et (3) une expérience en laboratoire pour évaluer si cette espèce peut utiliser l'autofécondation lors de la recolonisation de sites vides. Ces approches révèlent que les extinctions ne sont pas aussi fréquentes que présumées au départ, et qu'elles ne sont pas liées aux à-secs. Au contraire, les sites instables sont les moins touchés. En effet, D. depressissimum surmonte très efficacement les périodes d'à-sec en estivant, et recourt très peu à l'autofécondation. Les extinctions sont en revanche plus importantes dans les sites stables qui correspondent aussi aux communautés les plus riches en espèces. Enfin, à l'échelle d'étude, la diversité génétique dépend des variations de taille et de connectivité des sites plutôt que de l'âge supposé de la population, suggérant un impact relativement mineur de la dynamique d'extinction-colonisation dans ce système. Dans la dernière partie de ma thèse, je mesure l'impact des processus neutres sur l'assemblage des communautés locales. Pour ce faire, j'analyse la corrélation entre la diversité moléculaire (microsatellites) des deux espèces les plus fréquentes, diversité considérée comme une référence reflétant uniquement les processus neutres d'échantillonnage, avec la diversité spécifique des communautés locales. Cette corrélation se révèle très significativement positive, et résulte de l'action parallèle d'une même variable (la connectivité des sites au réseau hydrographique lors de la saison des pluies) sur la migration et la colonisation à la fois des allèles et des espèces, révélant ainsi un rôle important de processus neutres dans la régulation de la diversité spécifique locale. / Striking similarities underlies those processes involved in the dynamics of genetic diversity and species diversity. However these diversities have been considered separately as part of the agenda of population genetics and community ecology respectively. My work aims at quantifying processes that govern genetic diversity within species and species diversity within communities. To address this question I focus on the freshwater snail metacommunity from Grande-Terre Island (800 km²) in Guadeloupe. Twenty-nine snail species inhabit a network of highly unstable freshwater habitats that frequently dry out. This instability probably triggers extinction-colonization cycles of local populations. However, drying-out periods do not necessarily lead to extinction as some species are able to aestivate. First, I focused on the pulmonate gastropod Drepanotrema depressissimum. I used three complementary approaches: (1) an analysis of the spatial and temporal distribution of genetic diversity, (2) a patch occupancy model that take into account the presence of resistance forms such as aestivating snails and (3) a laboratory experiment in order to infer whether this species can rely on selfing to recolonize empty sites. These analyses reveal that extinction is not as frequent as previously thought and is not positively related to instability. Indeed, unstable sites are less prone to extinction. D. depressissimum overcome efficiently drying-out events aestivating in the ground and do not rely on selfing. Besides, extinction is more frequent in stable environments that encompass more speciose communities. Finally, at the scale of investigation, genetic diversity depends much more on patch size and connectivity than on apparent population age, suggesting that extinction-colonization cycles play a minor role in the species dynamics. In the last chapter, I quantify the impact of neutral processes on community assemblage. To this aim, I analyze the correlation between genetic diversity (microsatellites) of the two most-commonly encountered species – this diversity reflects neutral sampling process, with species diversity of local communities. This correlation is highly positive and relies on the parallel effect of a single site characteristic (connection to the local hydrographic network during the rainy season) on migration and colonization of both alleles and species. This suggests that neutral processes play an important role on the regulation of both genetic diversity and local species occurrence.
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Microbotryum violaceum on Silene dioica : understanding traits that influence plant-pathogen interactionsGranberg, Åsa January 2007 (has links)
The dynamics of a plant-pathogen interaction vary both within and among species. Both spatial structuring and specific genetic and life-history characteristics will affect the interaction and the outcome of a potential co-evolution between the two organisms. In this thesis I have studied the interaction between the wild perennial herb Silene dioica and its automictic, obligate anther smut Microbotryum violaceum MvSd. From the plant perspective, I have examined different aspects of biochemical resistance in S. dioica to M. violaceum MvSd. From the pathogen perspective, I have focused on the breeding system of M. violaceum MvSd and its connection to fitness and distribution of genetic diversity. I have used varying methods; glasshouse trails involving inoculation of plants with the pathogen, classical Mendelian analysis involving controlled crosses between plants, microscopic studies of spores and molecular DNA-analysis. With the results I demonstrate that resistance to M. violaceum MvSd in S. dioica can be specific to the attacking pathogen strain and also spatially highly diverse both within and among populations within a metapopulation. Together, these factors are likely to delay the establishment of the disease within host populations and reduce the spread and amount of disease, once it has been established. The results also suggest that the specific resistance expressed against two different M. violaceum MvSd strains were determined by separate gene systems and that, in both cases, the resistance was simply inherited. This implies a potential for relatively rapid response to M. violaceum-induced selection in S. dioica populations variable for resistance. My results also show that automixis clearly is the predominating breeding system of M. violaceum MvSd, similarly to what earlier has been shown for M. violaceum MvSl. Furthermore, I found lower levels of neutral genetic diversity in M. violaceum MvSd in the northern parts of Sweden, compared to what has been found in populations in more southern Europe. This result is consistent with predictions that populations in the outer regions of a species distribution have lower levels of genetic variation. Moreover, populations were highly differentiated in northern Sweden, which could have been generated by high selfing rates, genetic drift and high population turnover rates, all factors that coincide with life-history and ecology of M. violaceum MvSd. However, despite the general low variability in neutral genetic markers, I did find variation in fitness related traits, both within and among populations, as well as differences in infection ability between strains, suggesting there is a potential for co-evolution between S. dioica and M. violaceum MvSd in the area. To summarize, this thesis reflect a plant-pathogen system that is highly influenced by constant colonisation-extinction dynamics, which is likely to have influenced both the genetics of resistance in the plant and the breeding system of the pathogen and thus also the interaction between the two organisms.
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