Evidences That Host Genetic Background More Than the Environment Shapes the Microbiota of the Snail <i>Bulinus truncatus</i> , an Intermediate Host of <i>Schistosoma</i> Species

Microbiota have emerged as fundamental regulators of host physiology, shaping both ecological interactions and evolutionary trajectories. Yet, the determinants of microbiota diversity and structure in wild populations-particularly the respective roles of host genetics and environmental context-are still poorly understood. In this study, we investigated these influences in the freshwater snail Bulinus truncatus, a key intermediate host for human and animal Schistosoma parasites, using a multifactorial approach. We developed 31 new microsatellite markers to resolve population genetic structure across nine sites in Senegal. Metabarcoding methods were then employed to profile the bacterial microbiota of individual snails and to characterize environmental bacterial assemblages from each location via environmental DNA. Shell measurements and molecular diagnostics for trematode infection status were included to assess additional potential contributors. Employing multiple regression on distance matrices (MRM), we quantified how snail population genetics, site-specific environmental bacterial communities, spatial patterns, and infection status shape microbiota composition. Our analyses reveal that snail geographic distribution and population genetic structure drive the composition of Bulinus truncatus microbiota, with environmental bacterial communities exerting a weaker but still significant effect. In contrast, neither shell size nor trematode infection status impacted microbiota structure significantly. Notably, a considerable fraction of variation remains unexplained, indicating the likely involvement of other ecological or intrinsic factors. These results advance understanding of microbiota determinants in natural populations and underscore the intricate interplay between host genetics, environment, and microbial communities.