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

22/03/2016
Nature Communications
7 (11071)

Type

Article dans des revues

Auteurs

Cornejo-Castillo Francisco M.
Cabello Ana M
Salazar Guillem
Sànchez-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.

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 prym- nesiophyte partners acted as a barrier driving an allopatric speciation of extant UCYN-A lineages.