The "Cheshire Cat" escape strategy of the coccolithophore Emiliania huxleyi in response to viral infection

Abstract

The coccolithophore Emiliania huxleyi is one of the most successful eukaryotes in modern oceans. The two phases in its haplodiploid life cycle exhibit radically different phenotypes. The diploid calcified phase forms extensive blooms, which profoundly impact global biogeochemical equilibria. By contrast, the ecological role of the noncalcified haploid phase has been completely overlooked. Giant phycodnaviruses (Emiliania huxleyi viruses, EhVs) have been shown to infect and lyse diploid-phase cells and to be heavily implicated in the regulation of populations and the termination of blooms. Here, we demonstrate that the haploid phase of E. huxleyi is unrecognizable and therefore resistant to EhVs that kill the diploid phase. We further show that exposure of diploid E. huxleyi to EhVs induces transition to the haploid phase. Thus we have clearly demonstrated a drastic difference in viral susceptibility between life cycle stages with different ploidy levels in a unicellular eukaryote. Resistance of the haploid phase of E. huxleyi provides an escape mechanism that involves separation of meiosis from sexual fusion in time, thus ensuring that genes of dominant diploid clones are passed on to the next generation in a virus-free environment. These "Cheshire Cat" ecological dynamics release host evolution from pathogen pressure and thus can be seen as an opposite force to a classic "Red Queen" coevolutionary arms race. In E. huxleyi, this phenomenon can account for the fact that the selective balance is tilted toward the boom-and-bust scenario of optimization of both growth rates of calcifying E. huxleyi cells and infectivity of EhVs.

Fig. 1.

The impact of EhV on the growth of diploid and haploid E. huxleyi. Growth curves of both life cycle stages of E. huxleyi (strain RCC1216) and the virus EhV201 are shown. The arrow indicates the day of virus addition (multiplicity of infection 0.2). Standard deviation bars are generally too short to be visible.

Fig. 2.

Microscopy and genetic tests for the presence/absence of EhVs inside and/or on infected E. huxleyi haploid and diploid cells. (A) TEM: 1, healthy 2N cell before infection; 2, 2N cell at day 1 after infection, displaying newly formed viral particles (dashed circle); 3, N cell from an infected culture; and 4, detail of the N cell periplast, showing the presence of organic scales attached to the membrane and the absence of coccoliths and viruses. (Scale bars: 1 μm.) (B) PCR amplifications of the viral MCP gene at day 11 after infection (see Fig. 1). Agarose gel lanes: 1, positive control (EhV201 DNA extract); 2, N culture; 3, 2N culture; 4, N culture exposed to viruses; and 5, 2N culture exposed to viruses. Cells were carefully filtered and washed several times to remove free viral particles before DNA extractions.

Fig. 3.

Long-term infection assays of 2N and mixed 2N–N cultures of E. huxleyi. (A) 2N cells infected with EhV201 virus. (B) 2N cells without virus. (C) Mixture (100:1) of 2N and N cells infected with EhV201. (D) Mixture (100:1) of 2N and N cells without virus. 2N naked cells (due to coccolith loss after viral infection) were distinguished from N cells by their permanent immobility. The arrows indicate time of infection. Standard deviation bars are generally too short to be visible.