Stepwise genome evolution from a facultative symbiont to an endosymbiont in the N₂-fixing diatom-Richelia symbioses

A few genera of diatoms that form stable partnerships with N₂-fixing filamentous cyanobacteria Richelia spp. are widespread in the open ocean. A unique feature of the diatom-Richelia symbioses is the symbiont cellular location spans a continuum of integration (epibiont, periplasmic, and endobiont) that is reflected in the symbiont genome size and content. In this study, we analyzed genomes derived from cultures and environmental metagenome-assembled genomes of Richelia symbionts, focusing on characters indicative of genome evolution. Our results show an enrichment of short-length transposases and pseudogenes in the periplasmic symbiont genomes, suggesting an active and transitionary period in genome evolution. By contrast, genomes of endobionts exhibited fewer transposases and pseudogenes, reflecting advanced stages of genome reduction. Pangenome analyses identified that endobionts streamline their genomes and retain most genes in the core genome, whereas periplasmic symbionts and epibionts maintain larger flexible genomes, indicating higher genomic plasticity compared with the genomes of endobionts. Functional gene comparisons with other N₂-fixing cyanobacteria revealed that Richelia endobionts have similar patterns of metabolic loss but are distinguished by the absence of specific pathways (e.g., cytochrome bd ubiquinol oxidase and lipid A) that increase both dependency and direct interactions with their respective hosts. In conclusion, our findings underscore the dynamic nature of genome reduction in N₂-fixing cyanobacterial symbionts and demonstrate the diatom-Richelia symbioses as a valuable and rare model to study genome evolution in the transitional stages from a free-living facultative symbiont to a host-dependent endobiont.