- Cyanobacterial symbionts diverged in the late Cretaceous towards lineage-specific nitrogen fixation factories in single-celled phytoplankton. doi link

Auteur(s): Cornejo-castillo Francisco m, Cabello Ana m, Salazar Guillem, Sánchez-baracaldo Patricia, Lima-mendez Gipsi, Hingamp P., Alberti Adriana, Sunagawa Shinichi, Bork Peer, De vargas Colomban, Raes Jeroen, Bowler Chris, Wincker Patrick, Zehr Jonathan p, Gasol Josep m, Massana Ramon, Acinas Silvia g

(Article) Publié: Nature Communications, vol. 7 p.11071 (2016)

PMID 27002549
DOI: 10.1038/ncomms11071

The unicellular cyanobacterium UCYN-A, one of the major contributors to nitrogen fixation in the open ocean, lives in symbiosis with single-celled phytoplankton. UCYN-A includes several closely related lineages whose partner fidelity, genome-wide expression and time of evolutionary divergence remain to be resolved. Here we detect and distinguish UCYN-A1 and UCYN-A2 lineages in symbiosis with two distinct prymnesiophyte partners in the South Atlantic Ocean. Both symbiotic systems are lineage specific and differ in the number of UCYN-A cells involved. Our analyses infer a streamlined genome expression towards nitrogen fixation in both UCYN-A lineages. Comparative genomics reveal a strong purifying selection in UCYN-A1 and UCYN-A2 with a diversification process ∼91 Myr ago, in the late Cretaceous, after the low-nutrient regime period occurred during the Jurassic. These findings suggest that UCYN-A diversified in a co-evolutionary process, wherein their prymnesiophyte partners acted as a barrier driving an allopatric speciation of extant UCYN-A lineages.