Ultrastructural and transcriptional changes during a giant virus infection of a green alga

Abstract

The complete genome sequence of the Oceanusvirus kaneohense strain (Tetraselmis virus 1; TetV-1) was previously reported, but little is known about the virus infection cycle. Using a permissive Tetraselmis isolate (UHM1315), we estimated the eclipse period (4-8 hours), latent period (16 hrs), and burst size (800-1000) of the virus and documented ultrastructural and transcriptional changes in the host during infection. Putative viral factories and electron-dense inclusion bodies appeared in the cytoplasm of infected cells by 8 and 16 h post-infection, respectively. The nucleus and chloroplasts appeared to remain intact but reduced in size after 8 h. Transcriptome sequencing suggests that the viral genome codes for 830 transcripts. Those expressed early in infection (eclipse period at 0.25 and 4 hr) were related to the initiation of transcription, DNA synthesis, translation, and host immune repression. During the later, post-eclipse period (8, 12, 16 hr), virus structural genes were expressed. For the algal host, transcripts related to lipid metabolism and endocytosis were upregulated during the early phase, while those for protein modification/ turnover/ transport were downregulated. In the later period, host transcripts associated with basic cellular processes were upregulated, while genes related to morphogenesis/development were downregulated. Many of the most highly expressed virus and host genes were of unknown function, highlighting a need for additional functional studies.