Tsv-N1: A Novel DNA Algal Virus that Infects Tetraselmis striata

Abstract

Numbering in excess of 10 million per milliliter of water, it is now undisputed that aquatic viruses are one of the major factors shaping the ecology and evolution of Earth's microbial world. Nonetheless, environmental viral diversity and roles remain poorly understood. Here we report the first thorough characterization of a virus (designated TsV) that infects the coastal marine microalga Tetraselmis striata. Unlike previously known microalgae-infecting viruses, TsV is a small (60 nm) DNA virus, with a 31 kb genome. From a range of eight different strains belonging to the Chlamydomonadaceae family, TsV was only able to infect T. striata. Gene expression dynamics revealed an up-regulation of viral transcripts already 1 h post-infection (p.i.). First clear signs of infection were observed 24 h p.i., with the appearance of viral factories inside the nucleus. TsV assembly was exclusively nuclear. TsV-N1 genome revealed very different from previously known algae viruses (Phycodnaviridae). Putative function and/or homology could be resolved for only 9 of the 33 ORFs encoded. Among those was a surprising DNA polymerase type Delta (only found in Eukaryotes), and two genes with closest homology to genes from human parasites of the urogenital tract. These results support the idea that the diversity of microalgae viruses goes far beyond the Phycodnaviridae family and leave the door open for future studies on implications of microalgae viruses for human health.

Keywords: horizontal gene transfer; infection dynamics; microalgae; viral factory; viral genomics; viral taxonomy; viruses.

Figure 1

Host phylogeny and infectivity range of TsV-N1. The tree corresponds to the Maximum Likelihood phylogeny based on 18S sequences of 38 microalgae strains with 1000 bootstrap replicates (bootstrap values indicated above each branch). Bayesian posterior probabilities are indicated below each branch. The susceptible host strain is indicated with a filled symbol, while the other tested/resistant strains have an open symbol. Culture collection reference for the microalgae strains tested for viral infection is indicated between brackets. GenBank accession number is indicated after their name for sequences retrieved from public databases.

Figure 2

Thin section of a T. striata showing TsV-N1 particles inside (IC) and outside (EC) the cell. Black arrow: filled viral particle, white arrow: empty capsid. Note the hexagonal section of the virions.

Figure 3

Ultrastructural changes in T. striata associated with the progression of TsV-N1 infection. (A) longitudinal section of an uninfected cell, with anterior flagellar pit and posterior pyrenoid; (B) nucleus of an uninfected cell, view corresponding to the frame in (A); (C) longitudinal section of an infected cell, first noticeable changes; (D) detail of the nucleus corresponding to the frame in (C); first viral capsids, intact nuclear envelope. (E) Longitudinal section of an infected cell with highly disordered nucleus; (F) enlargement of frame in (E) showing a viral factory (vf) inside the nucleus; (G) longitudinal section of a cell at the final stage of infection; (H) enlargement of frame in (G) showing numerous viral particles. Facing black arrowheads (B,D) nuclear envelope. Black and white arrows (D,F,H) full and empty viral particles. Legend: bb, basal bodies; chl, chloroplast; fp, flagellar pit; gol, golgi apparatus; mit, mitochondrion; no, nucleole; nu, nucleus; pyr, pyrenoid; rp, rhizoplast; st, starch grain; and vf, viral factory.

Figure 4

Relative expression of TsV genes TsV_019 and TsV_014, encoding for putative DNA Polymerase (DNA Pol) and Capsid Decoration Protein (CDP), respectively. RGE, relative gene expression units.

Figure 5

Phylogenies obtained for TsV_01, TsV_18, and TsV_19 genes, respectively, accompanied by homologous sequences retrieved from PFAM. Branches indicate support values obtained with Maximum Likelihood aLRT SH-like algorithm implemented in Phylogeny.fr.