Abstract
HcDNAV (a type species of Genus Dinodnavirus) is a large double-stranded DNA virus, which lytically infects the bloom-forming marine microalga Heterocapsa circularisquama Horiguchi (Dinophyceae). In the present study, detailed observation of the HcDNAV particle and its infection process was conducted via field emission scanning electron microscopy (FE-SEM) and epifluorescence microscopy (EFM). Each five-fold vertex of the icosahedral virion was decorated with a protrusion, which may be related to the entry process of HcDNAV into the host. The transverse groove of host cells is proposed to be the main virus entry site. A visible DAPI-stained region, which is considered to be the viroplasm (virus factory), appeared in close proximity to the host nucleus at 11 h post infection (hpi); the putative viral DAPI signal was remarkably enlarged at 11–30 hpi. It was kidney-shaped at 13–15 hpi, horseshoe-shaped at 20 hpi, doughnut-shaped at 30 hpi, and changed into a three-dimensionally complicated shape at 51–53 hpi, by which time most parts of the host cell were occupied by the putative viral DAPI signal. While the virions were within the viroplasm, they were easily distinguishable by their vertex protrusions by FE-SEM.
Highlights
Dinoflagellates (Dinophyceae) are highly abundant and diverse unicellular eukaryotic microorganisms found in aquatic environments
The resulting field emission scanning electron microscopy (FE-SEM) clearly showed that: the HcDNAV particle was icosahedral in shape as expected by TEM observation [9,10]; each five-fold vertex of the capsid was decorated with a protrusion structure (Figure 1A,B); viral adsorption to host cell surface occurred within at least 5 mpi (Figure 1C,D)
The role of the vertex protrusion was not elucidated, its possible relationship to the process of virus entry into host cells was suggested (Figure 1E). Other viruses, such as bacteriophage phi174, have vertex protrusions on the virion surface, and their function in its infection process has been intensively studied by 3D cryo-electron microscopy [14]
Summary
Dinoflagellates (Dinophyceae) are highly abundant and diverse unicellular eukaryotic microorganisms found in aquatic environments. They constitute a major class of eukaryotes within. Heterocapsa circularisquama Horiguchi, a bloom-forming dinoflagellate, which causes high mortality rates among shellfish such as pearl oysters and mussels, is one of the most intensively studied dinoflagellate species [4,5]. HcDNAV is the sole intensively studied DNA virus isolated from the superphylum, Alveolata [11]. HcDNAV is reported to play a significant role in the disintegration process of H. circularisquama blooms [10,12]. Virions emerge from a specific cytoplasm compartment, Viruses 2018, 10, 554; doi:10.3390/v10100554 www.mdpi.com/journal/viruses
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