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

  • Author(s): 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
  • et al.

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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.

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