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Sammanfattning

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 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 genome evolution. Our results show an enrichment of short-length transposases and pseudogenes in periplasmic symbiont genomes, suggesting an active and transitionary period in genome evolution. 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 N-2-fixing cyanobacteria revealed that Richelia endobionts have similar patterns of metabolic loss but are distinguished by the absence of specific pathways (e.g., cytochrome 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 N-2-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.

Publicerad i

Current Biology
2025, volym: 35, nummer: 18, sidor: 4479-4493
Utgivare: CELL PRESS

SLU författare

UKÄ forskningsämne

Evolutionsbiologi
Genetik och genomik

Publikationens identifierare

  • DOI: https://doi.org/10.1016/j.cub.2025.08.003

Permanent länk till denna sida (URI)

https://res.slu.se/id/publ/143966