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

• Main Authors: Cornejo-Castillo, Francisco M., Cabello, Ana M., Salazar Guiral, Guillem, id_orcid:0 000-0002-9786-1493, Sánchez-Baracaldo, Patricia, Lima-Mendez, Gipsi, Hingamp, Pascal, Alberti, Adriana, Sunagawa, Shinichi, id_orcid:0 000-0003-3065-0314, Bork, Peer, de Vargas, Colomban, Raes, Jeroen, Bowler, Chris, Wincker, Patrick, Zehr, Jonathan P., Gasol, Josep M., Massana, Ramon, Acinas, Silvia G.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Nature 2016
• Subjects: South Atlantic Ocean
• Online Access: https://hdl.handle.net/20.500.11850/371230; https://doi.org/10.3929/ethz-b-000371230

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. ISSN:2041-1723