Samuele Tecchio

Dynamics of DNA methylomes underlie oyster development

DNA methylation is a critical epigenetic regulator of development in mammals and social insects, but its significance in development outside these groups is not understood. Here we investigated the genome-wide dynamics of DNA methylation in a mollusc model, the oyster Crassostrea gigas, from the egg to the completion of organogenesis. Large-scale methylation maps reveal that the oyster genome displays a succession of methylated and non methylated regions, which persist throughout development. Differentially methylated regions (DMRs) are strongly regulated during cleavage and metamorphosis. The distribution and levels of methylated DNA within genomic features (exons, introns, promoters, repeats and transposons) show different developmental lansdscapes marked by a strong increase in the methylation of exons against introns after metamorphosis. Kinetics of methylation in gene-bodies correlate to their transcription regulation and to distinct functional gene clusters, and DMRs at cleavage and metamorphosis bear the genes functionally related to these steps, respectively. This study shows that DNA methylome dynamics underlie development through transcription regulation in the oyster, a lophotrochozoan species. To our knowledge, this is the first demonstration of such epigenetic regulation outside vertebrates and ecdysozoan models, bringing new insights into the evolution and the epigenetic regulation of developmental processes.

Microbial parasites make cyanobacteria blooms less of a trophic dead end than commonly assumed

Parasites exist in every ecosystem and can have large influence on food web structure and function, yet, we know little about parasites’ effect on food web dynamics. Here we investigate the role of microbial parasitism (viruses of bacteria, phytoplankton and cyanobacteria, and parasitic chytrids on cyanobacteria) on the dynamics of trophic pathways and food web functioning during a cyanobacteria bloom, using linear inverse food web modeling parameterized with a 2-month long data set (biomasses, infection parameters, etc.). We show the importance of grazing on heterotrophic bacteria (the microbial pathway: DOC → bacteria → consumer) and how consumers depended on bacteria during peak-cyanobacteria bloom, which abundance was partly driven by the viral activity. As bacteria become the main energy pathway to the consumers, the system takes a more web-like structure through increased omnivory, and may thereby facilitate the system’s persistence to the cyanobacteria outbreak. We also showed how the killing of cyanobacteria host cells by chytrids had important impact on the food web dynamics by facilitating grazing on the cyanobacteria, and by offering alternative pathways to the consumers. This seemed to increase the system’s ability to return to a mix of trophic pathways, which theoretically increases the stability of the system.

Global Changes Jeopardize the Trophic Carrying Capacity and Functioning of Estuarine Ecosystems

AbstractnAt the interface between terrestrial and marine biomes, estuaries display high ecological productivity and provide goods and services to humans. Associated with many ecological functions, they are nursery, refuge, and growing areas for many species fish. These ecological functions and services depend on both their ecological production and trophic carrying capacity and the durability of food web functioning. These transitional key habitats undergo both strong anthropogenic pressures and climatic influences that impact the structure and dynamics of estuarine biodiversity. In this context, we explore, here, three decades of the Gironde estuary ecosystem history to detect the food web’s response to global changes-induced effect on biodiversity. At least two Ecological Abrupt Shifts associated with deep modifications in the biodiversity at most trophic levels have been documented for this particular ecosystem. Three food web models were thus calibrated, one for each of the three periods discriminated by the two shifts that occurred at the end of the 1980s and the beginning of the 2000s. Results highlighted that the ecotrophic efficiency estimate for subtidal macrofauna and shrimps reached the maximum possible values during the last period. This could mean that the Gironde estuary fully reached its trophic carrying capacity due to a food limitation especially for benthos demersal fish. We also observed a significant decrease in some food web indicators (such as Average Mutual Information, System Omnivory Index, and Average Path Length) usually associated with ecosystem stress, suggesting a significant impact of global change on the Gironde estuary ecosystem health and questioning the sustainability of the ecological functions associated with this ecosystem.