PHENOTYPIC PLASTICITY UNDERLIES ESCAPE FROM VIRUSES IN A MARINE PHYTOPLANKTON

Emiliania huxleyi is the most abundant calcifying plankton in modern oceans. Like other Prymnesiophyte microalgae it has been described as possessing a sexual, heteromorphic haplodiplontic life cycle composed by a diploid, calcified phase, and a phenotypically distinct haploid, non-calcified biflagellated phase. Diploid cells are commonly found within modern phytoplankton assemblages and form seasonally large-scale blooms of major ecological and biogeochemical significance. In contrast haploid cells are less known, and field observations are rare. Remarkably, while diploid cells are sensitive to specific viruses that drive blooms termination (Emiliania huxleyi virus, EhV), haploid cells are insensitive to EhV. Further evidence indicates that haploid cells are produced during infection as a mechanism to escape viral demise; a process termed the ‘Cheschire cat’ strategy. Here, however, we provide morphological and genetic evidence that viral evasion does not involve meiosis, but rather the production of viral resistance diploid cells, both accumulating gene-transcripts and displaying a phenotype typical of the haploid phase. This phenotypic alternation uncoupling ploidy and life cycle generation appears to be a conserved response across E. huxleyi strains and to be triggered during active viral infection as detected through single-cell gene expression analysis. Likely, a minute fraction of the infected cells is able to halt viral proliferation and endure upon bloom collapse. To our knowledge this is the first time that such mechanism was observed in eukaryotic cells in response to viruses, highlighting the adaptive significance of phenotypic plasticity in marine phytoplankton.