Abstract
Eukaryotes are habitats for bacterial organisms where the host colonization and dispersal among individual hosts have consequences for the bacterial ecology and evolution. Vertical symbiont transmission leads to geographic isolation of the microbial population and consequently to genetic isolation of microbiotas from individual hosts. In contrast, the extent of geographic and genetic isolation of horizontally transmitted microbiota is poorly characterized. Here we show that chemosynthetic symbionts of individual Bathymodiolus brooksi mussels constitute genetically isolated subpopulations. The reconstruction of core genome-wide strains from high-resolution metagenomes revealed distinct phylogenetic clades. Nucleotide diversity and strain composition vary along the mussel life span and individual hosts show a high degree of genetic isolation. Our results suggest that the uptake of environmental bacteria is a restricted process in B. brooksi, where self-infection of the gill tissue results in serial founder effects during symbiont evolution. We conclude that bacterial colonization dynamics over the host life cycle is thus an important determinant of population structure and genome evolution of horizontally transmitted symbionts.
Highlights
Bacteria inhabit most eukaryotes where their presence has consequences for key aspects of the host biology [1], such as host development [2], nutrition [3], or behavior [4]
To investigate the degree of genetic cohesion within strain clades in the population, we studied the allele frequency spectrum (AFS) of each mussel
To study the evolution of SOX and MOX strains in Bathymodiolus, we examined the selection regimes that have been involved in the formation of cohesive genetic SOX and MOX units
Summary
Bacteria inhabit most eukaryotes where their presence has consequences for key aspects of the host biology [1], such as host development [2], nutrition [3], or behavior [4]. Animals constitute an ecological niche, where microbial communities utilize the resources of their host habitat [5]. The microbiota biodiversity over the host life cycle is determined by bacteria colonization
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