Polyploïdie et tolérance thermique chez les microcrustacés du genre Daphnia

RÉSUMÉ: Il est depuis longtemps reconnu que les organismes asexués et polyploïdes présentent des distributions différentes des espèces parentes sexuées et diploïdes. Ce phénomène, nommé « parthénogenèse géographique », se retrouve à la fois chez les plantes et les animaux. Les causes de ce patron de distribution sont encore mal comprises aujourd'hui, même si la parthénogenèse géographique est étudiée chez diverses espèces depuis plusieurs décennies. De récentes études sur des plantes alpines suggèrent que la température est un facteur important pour expliquer la distribution de la polyploïdie. Chez les animaux, la température n'a pas encore été directement liée à la parthénogenèse géographique. Les hautes latitudes, où les températures locales sont plus basses, sont des environnements de prédominance pour les polyploïdes. Le rôle de la tolérance thermique et d'autres traits associés à la température méritent d'être examinés pour comprendre ce phénomène. Cette étude a pour but principal de vérifier si des différences de tolérance thermique pourrait expliquer la distribution différentielle des espèces polyploïdes. Les objectifs spécifiques sont de vérifier : 1) si les clones diploïdes et polyploïdes ont des différences de tolérance thermique, 2) si la taille corporelle et la taille cellulaire est liée à la tolérance thermique et 3) si l'expression de deux gènes clés, Hsp70 et la catalse, sont associés à la tolérance thermique. Dans une première étude, des daphnies Daphnia tenebrosa diploïdes et triploïdes sont échantillonnées et élevées jusqu'à l'âge de trois jours. La taille corporelle et la tolérance thermique, par la mesure du Ctmax, sont évaluées. Des différences significatives de taille et de tolérance thermique entre les daphnies diploïdes et triploïdes sont relevées. Une deuxième étude a été menée avec des Daphnia pulex diploïdes et triploïdes de différentes régions climatiques et en ajoutant des mesures de tailles cellulaires et d'expression génique de Hsp70 et de la catalase. Les triploïdes ont une tolérance thermique plus faible et des cellules plus grosses que les diploïdes. Ce projet est le premier à démontrer des différences de tolérance thermique en fonction de la ploïdie chez les daphnies. Les résultats suggèrent également que la taille de cellules est le mécanisme qui limite la tolérance thermique plutôt que les autres déterminants étudiés. -- Mot(s) clé(s) en français : Ctmax, tolérance thermique, Daphnia pulex, polyploïdie, taille cellulaire, parthénogenèse géographique. -- ABSTRACT: It has long been recognized that asexuals and polyploids have different geographic distributions compared to their sexual and diploid parent species. The phenomenon, named "geographical parthenogenesis", is found in both plant and animal species. Causes for this distribution pattern are still unclear today, even though geographical parthenogenesis has been reported decades ago. Recent studies on alpine plant species suggest that temperature could be an important factor of polyploid distribution. Temperature has yet to be directly tested in the context of geographical parthenogenesis in animals. High latitudes, with lower local temperatures than temperate regions, are environments with high frequencies of polyploids. Temperature and traits related to thermal tolerance should be investigated as causal factors of geographical parthenogenesis. In this study, we investigated geographic distribution of polyploidy as a function of thermal tolerance in relation to: body size, cell size, gene expression of heat-shock protein Hsp70 and/or gene expression of antioxidant enzyme catalase in diploid and polyploid clones of the microcrustacean Daphnia. In a preliminary study, triploid and diploid Daphnia tenebrosa were sampled and reared until three days old. Body size and Ctmax, a measure of thermal tolerance, were measured. Results show significant differences of body size and thermal tolerance between ploidy levels. A laboratory study was concluded, using triploid and diploid clones of Daphnia pulex from different climatic regions along with measures of cell sizes and gene expression of Hsp70 and catalase. Our results showed that triploid daphnia had significantly lower thermal tolerance and larger cells. This study is the first to show thermal tolerance differences between polyploid and diploid Daphnia. Our results imply that cell size is the mechanism setting thermal tolerance limits. -- Mot(s) clé(s) en anglais : Ctmax, thermal tolerance, Daphnia pulex, polyploidy, cell size, geographical parthenogenesis

Microbiota modification of Mytilus edulis larvae in response to the use of a new probiotic, the marennine, in aquaculture

In bivalve hatcheries, opportunistic pathogens have been associated with important mass mortality events of larvae and important economic lost for producers. New alternatives to the use of antibiotics, such as probiotics, have been proposed to limit the occurrence of such events in bivalve hatcheries and to stabilize bivalve production. Probiotics are microorganisms, or natural molecules, that are associated with beneficial effects for the host at different levels, especially to enhance their resistance to external stressors such as bacterial pathogens. It is know recognized that the composition of the host microbiota influences host health status and could be a target of probiotics. The aim of this study is to highlight the protective effect of a new probiotic, the marennine, on Mytilus edulis larvae during bacterial challenges in relation with a potential modification of the microbiota of the marennine-treated larvae. The main hypothesis is that the addition of marennine during larvae rearing processes could modify the conditions prevailing in the rearing medium and, as a consequence, the composition of the larvae microbiota, leading to a better resistance to bacterial infections. The marennine is a blue pigment originating from the diatom Haslea ostrearia, that have demonstrated a positive effect on larvae survival at a final concentration of 500 µg L-1. In this study, D-larvae (9 days old) and post-larvae (29 days old) were exposed for 96h to Vibrio splendidus with and without mareninne at a final concentration of 500 µg L-1. Our results demonstrated that at this concentration, the marennine have no direct antimicrobial effect on Vibrio splendidus growth kinetics. In addition, the presence of marennine did not modify the abundance of bacteria in the rearing medium, suggesting no direct antimicrobial effect of marennine on the bacterial load to which larvae were exposed during the experiments. Nevertheless, the presence of marennine increased the survival of D-larvae exposed to the pathogen but have no effect on post-larvae survival. The undergoing molecular analyses and the future metagenomics analyses of the larvae microbiota diversity will allow us to demonstrate if a modification in the larval microbiota’s richness might explain the increase of the survival rate during the production of blue mussels in aquaculture. Ultimately, our work will enable us to shed light on the importance of the larval microbiota in pathogen-resistance during bivalves rearing process

Effect of marennine on the rearing medium and microbiota of <i>Mytilus edulis</i> larvae and its protective effect after exposure to the pathogenic <i>Vibrio splendidus</i>

Opportunistic pathogens have been associated with yield-limiting factors in bivalve hatcheries. Numerous natural compounds are being investigated for their beneficial effects and potential to enhance larval resistance without requiring antibiotics. One of those is the use of marennine, a blue pigment, originating from the diatom Haslea ostrearia, which has demonstrated a positive effect on larvae survival. The aim of this study was to highlight the protective effect of marennine on Mytilus edulis larvae during bacterial challenges in relation to a potential modification of the marennine-treated larvae microbiota. D-larvae and post-larvae were exposed for 96 h to Vibrio splendidus (106 cell mL−1) with and without mareninne (500 µg L−1). The presence of marennine increased the survival rate of D-larvae exposed to the pathogen. The molecular analysis of the larvae microbiota diversity indicated a modification in the D-larval microbiota’s richness related to survival rates of larvae. Ultimately, our study sheds light on the importance of the larval microbiota in pathogen resistance during the bivalve rearing process.</p