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, 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
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: eScholarship, University of California 2016
• Subjects: Phytoplankton; Cyanobacteria; Genomics; Seawater; Nitrogen Fixation; Symbiosis; Atlantic Ocean; Biological Evolution; Haptophyta; Genetics; Human Genome; South Atlantic Ocean
• Online Access: https://escholarship.org/uc/item/2q17p2k9

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.