Recognizing the life cycle of an organism is key to understanding its biology and ecological impact. Emiliania huxleyi is a cosmopolitan marine microalga, which displays a poorly understood biphasic sexual life cycle comprised of a calcified diploid phase and a morphologically distinct biflagellate haploid phase. Diploid cells (2N) form large-scale blooms in the oceans, which are routinely terminated by specific lytic viruses (EhV). In contrast, haploid cells (1N) are resistant to EhV. Further evidence indicates that 1N cells may be produced during viral infection. A shift in morphology, driven by meiosis, could therefore constitute a mechanism for E. huxleyi cells to escape from EhV during blooms. This process has been metaphorically coined the ‘Cheshire Cat’ (CC) strategy. We tested this model in two E. huxleyi strains using a detailed assessment of morphological and ploidy-level variations as well as expression of gene markers for meiosis and the flagellate phenotype. We showed that following the CC model, production of resistant cells was triggered during infection. This led to the rise of a new subpopulation of cells in the two strains that morphologically resembled haploid cells and were resistant to EhV. However, ploidy-level analyses indicated that the new resistant cells were diploid or aneuploid. Thus, the CC strategy in E. huxleyi appears to be a life-phase switch mechanism involving morphological remodeling that is decoupled from meiosis. Our results highlight the adaptive significance of morphological plasticity mediating complex host–virus interactions in marine phytoplankton.
Bibliographical noteFunding Information:
This research was supported by the European Research Council (ERC) StG (INFOTROPHIC grant # 280991) and CoG (VIROCELLSPHERE grant no. 681715) https://erc.europa.eu/. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. We thank Toot Moran, Uri Sheyn, Guy Schleyer and Shlomit Sharoni for technical assistance, and Daniella Schatz for contributing to the transmission electron microscopy work. We thank the Moskowitz Center for Bio-Nano Imaging at the Weizmann Institute of Science. Finally, we are grateful to the anonymous reviewers who provided valuable feedback and guidance that strengthened the final version of the manuscript. This research was supported by the European Research Council (ERC) StG (INFOTROPHIC grant no. 280991) and CoG (VIROCELLSPHERE grant no. 681715) awarded to A.V.
© 2017 Frada et al.