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

• Published in: Nature Communications
• Main Authors: Cornejo-Castillo, Francisco M., Cabello, Ana M., Salazar, Guillem, Sánchez-Baracaldo, Patricia, Lima Mendez, Gipsi, Hingamp, Pascal, 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.
• Other Authors: UCL - SST/LIBST - Louvain Institute of Biomolecular Science and Technology
• Format: Article in Journal/Newspaper
• Language: English
• Published: Springer Science and Business Media LLC 2016
• Subjects: South Atlantic Ocean
• Online Access: http://hdl.handle.net/2078.1/231557; https://doi.org/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 B91 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.