The first life cycle-wide characterization of the Platynereis dumerilii microbiome

The marine annelid Platynereis dumerilii is emerging as key model for evolutionary developmental biology, yet its associated microbiome remains largely unexplored. Understanding host-symbionts interactions in this system is important both for interpreting developmental processes and for assessing the reproducibility of laboratory cultures used across research facilities. Here we present the first longitudinal characterization of the microbiome of laboratory- cultured P. dumerilii across its entire life cycle and across major culture subenvironments. Using 16S rRNA gene sequencing, we analysed microbial communities associated with worms, microbial mats, and seawater. We identify a persistent host-associated microbiome characterized by temporal continuity of specific taxa, together with strong habitat-specific microbial communities within the culture system. Microbiome diversity in worms showed pronounced developmental dynamics, with transient enrichment of environmentally derived taxa during feeding stages. A major restructuring event occurred following a change in culture conditions, leading to extensive taxonomic turnover and reorganization of microbial interaction networks. Source-tracking analyses indicate that the worm microbiome is primarily maintained through temporal persistence, with transient contributions from the microbial mat and embryonic jelly, while food represents a negligible source of colonizing bacteria. Comparative analyses with other Platynereis datasets highlight substantial divergence between cultures, indicating a strong influence of laboratory conditions on microbiome assembly. Together, our results position P. dumerilii as a tractable holobiont model and provide a first framework for integrating host-microbiome interactions into evolutionary and developmental studies using marine annelids.