Life cycle modification in open oceans accounts for genome variability in a cosmopolitan phytoplankton

Emiliania huxleyi is the most abundant calcifying plankton in modern oceans with substantial intraspecific genome variability and a biphasic life-cycle involving sexual alternation between calcified 2N and flagellated 1N cells. We show that high genome content variability in Emiliania relates to erosion of 1N-specific genes and loss of the ability to form flagellated cells. Analysis of 185 E. huxleyi strains isolated from world oceans suggests that loss of flagella arose independently in lineages inhabiting oligotrophic open oceans over short evolutionary time scales. The environmentally-linked physio-genomic change suggests life cycling is not advantageous in very large/diluted populations experiencing low biotic pressure and low ecological variability. Gene loss did not appear to reflect pressure for genome-streamlining in oligotrophic oceans previously observed in picoplankton. Life cycle modifications might be common in plankton and cause major functional variability to be hidden from traditional taxonomic or molecular markers.