Affichage des résultats 1601 à 1620 sur 1699 au total
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Cyanobacteria are photossintetic procariotes that produce secundary metabolites named cyanotoxins. Cyanotoxins are classified in three main groups: the hepatotoxins, dermatotoxins and the neurotoxins. Many studies have shown that cynobacteria and their toxins exert deleterious effects in several aquatic organisms, such as acute (i.e. reduction in survivorship, feeding inhibition, paralysis) and chronic effects (i.e. reduction in growth and fecundity), besides biochemical and behavioral effects. As the main target of these toxins seem to be herbivorous zooplâncton, many research have foccused this link as the main route of cyanotoxins, showing indeed its potential for bioaccumulation in the food chain. However, the large variability found in results of many studies has lead to equivocal conclusions. Although the unequivocal evidence of deleterious effects of cyanobacteria and their toxins, the resistance of some zooplâncton species and the absence of effect of microcystins on Daphnia in some studies have lead to question the role of this toxin as a chemical defense mechanism against the zooplâncton grazing. Therefore, more studies are needed to clarify the role of cyanotoxins on aquatic organisms.
"Projet ANR "AVC - in silico" : modélisation multi-échelle des mécanismes physiopathologiques mis en jeu dans l'accident vasculaire cérébral ischémique".
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La caractérisation de l'influence des facteurs environnementaux sur la fertilité des couples est un enjeu important de santé publique. Dans une approche épidémiologique, cette caractérisation peut reposer sur différents modes d'échantillonnage des sujets. L'approche la plus couramment utilisée est rétrospective et repose sur le recrutement de femmes ayant déjà eu une grossesse. Cette approche a l'avantage d'être simple du point de vue logistique mais exclut les couples involontairement inféconds. Nous illustrerons les conséquences de cette exclusion en termes de biais et de perte de puissance statistique à partir de simulations et de données réelles, et discuterons les intérêts et limites de trois approches alternatives : 1) l'approche de cohorte incidente (dans laquelle les couples sont recrutés avant le début d'une tentative de grossesse et suivis), 2) l'approche de cohorte prévalente (dans laquelle on recrute les couples au cours de la tentative de grossesse, avec un suivi) et 3) l'approche des durées en cours, qui repose elle aussi sur un échantillon de couples recrutés en cours de tentative de grossesse, mais sans suivi. Dans chaque cas, les modèles de survie adaptés au mode d'échantillonnage seront brièvement présentés et illustrés, notamment à partir de l'Observatoire épidémiologique de la fertilité en France. Références : Keiding, N. (1992) Independent delayed entry (with discussion). In Klein, J. P. and Goel, P. K. (eds) Survival analysis: State of the Art. Kluwer, Dordrecht, pp. 309-326. Keiding, N., Kvist, K., Hartvig, H., Tvede, M. and Juul, S. (2002) Estimating time to pregnancy from current durations in a cross-sectional sample. Biostatistics 3, 565-578. Scheike, T. H. and Keiding, N. (2006) Design and analysis of time-to-pregnancy. Stat Methods Med Res 15, 127-140. Slama, R., Kold-Jensen, T., Scheike, T., Ducot, B., Spira, A. and Keiding, N. (2004) How would a decline in sperm concentration over time influence the probability of pregnancy? Epidemiology 15, 458-465. Slama, R., Ducot, B., Carstensen, L., Lorente, C., de La Rochebrochard, E., Leridon, H., Keiding, N. and Bouyer, J. (2006) Feasibility of the Current-Duration Approach to Studying Human Fecundity. Epidemiology 17, 440-449.
Understanding how groups of species diversified, and how species phenotypes evolved during evolutionary history, is key to our understanding of patterns of biodiversity as we see them around us today. Phylogenetic comparative methods have highlighted that macroevolutionary rates (i.e. rates of diversification and phenotypic evolution) are strikingly heterogeneous in time and across lineages. However, describing and quantifying this heterogeneity, as well as understanding its drivers, remain challenging. I will present recent developments that allow a better consideration of smooth changes in diversification rates when estimating branch-specific rates across phylogenetic trees. I will also present models that allow better understanding and testing the effect of past environmental changes and interspecific interactions on diversification and phenotypic evolution. Empirical applications demonstrate the preponderance of many small (versus few important) diversification rate shifts in clades' evolution and the pervasive effect of past environmental changes on evolutionary rates across diverse clades spanning macro and microorganims.