Microbial communities confer multiple beneficial effects to their multicellular hosts. To evaluate the evolutionary and ecological implications of the animal-microbe interactions, it is essential to understand how bacterial colonization is secured and maintained during the transition from one generation to the next. However, the mechanisms of symbiont transmission are poorly studied for many species, especially in marine environments, where the surrounding water constitutes an additional source of microbes. Nematostella vectensis, an estuarine cnidarian, has recently emerged as model organism for studies on host-microbes interactions. Here, we use this model organism to study the transmission of bacterial colonizers, evaluating the contribution of parental and environmental transmission to the establishment of bacterial communities of the offspring. We induced adult male and female polyps of N. vectensis for spawning and used their gametes for five individual fertilization experiments. While the embryos developed into primary polyps, we sampled each developmental stage and its corresponding medium samples. By analyzing the microbial community compositions of all samples through 16S rRNA gene amplicon sequencing, we showed that all host tissues harbor microbiota significantly different from the surrounding medium. Interestingly, oocytes and sperms are associated with distinct bacterial communities, indicating the specific vertical transmission of bacterial colonizers by the gametes. These differences were consistent among all the five families analyzed. By overlapping the identified bacterial OTUs associated with gametes, offspring and parents, we identified specific bacterial OTUs that are vertically transmitted via mothers and fathers. This is the first study about bacteria transmission in N. vectensis, and among few others on marine spawners that do not brood larvae, and sheds light on how distinct maternal and paternal transfer of bacterial symbionts are consistent in aquatic environments among different life stages and generations.
