Extended in vivo transcriptomes of two ascoviruses with different tissue tropisms reveal alternative mechanisms for enhancing virus reproduction in hemolymph

Abstract

Ascoviruses are large dsDNA viruses characterized by the extraordinary changes they induce in cellular pathogenesis and architecture whereby after nuclear lysis and extensive hypertrophy, each cell is cleaved into numerous vesicles for virion reproduction. However, the level of viral replication and transcription in vesicles compared to other host tissues remains uncertain. Therefore, we applied RNA-Sequencing to compare the temporal transcriptome of Spodoptera frugiperda ascovirus (SfAV) and Trichoplusia ni ascovirus (TnAV) at 7, 14, and 21 days post-infection (dpi). We found most transcription occurred in viral vesicles, not in initial tissues infected, a remarkably novel reproduction mechanism compared to all other viruses and most other intracellular pathogens. Specifically, the highest level of viral gene expression occurred in hemolymph, for TnAV at 7 dpi, and SfAV at 14 dpi. Moreover, we found that host immune genes were partially down-regulated in hemolymph, where most viral replication occurred in highly dense accumulations of vesicles.

Early in vivo transcriptome of Trichoplusia ni ascovirus core genes

“…Interestingly, a recent study reported that the host immune cascade of melanization promoted viral replication of HvAV-3h [35]. In addition to our results presented above, a variety of evidence suggests that virions produced in circulating viral vesicles are shed into the haemolymph [10,13,15], where they can also be acquired on the ovipositor of female wasps. Phylogenetic analysis indicates that ascoviruses evolved iridoviruses [36].…”

“…for TnAV, and 14 days p.i. for SfAV [13]. This study and others [15,16] demonstrated that ascoviruses have a unique reproduction strategy that distinguishes them from all other viruses and most intracellular pathogens.…”

“…In a recent comparative study of weekly gene expression during the first 3 weeks of infection it was shown that by day 7 p.i. TnAV transcription of core genes in haemolymph was significantly higher than that of SfAV, where high levels did not occur until the second week [13]. Thus, a major focus of the present study was to determine when during the first week of TnAV infection gene expression began to accelerate in the haemolymph.…”

“…The most unique property of most ascoviruses compared to all viruses of eukaryotes is their ability to radically reorganize the infected cell cytoskeleton, cleaving the cell into numerous viral vesicles [2,10,12,13]. After formation, these vesicles accumulate and circulate in the haemolymph where replication increases, yielding many more progeny virions than the initial cells infected [14,15].…”

“…However, given that the HvAV caspase-like protein was unable to induce the apoptosis in Sf9 cells [38], due to the lack of cysteine residue in the ‘QACXG’ catalytic domain (conserved in all functional caspase proteins and also present in the SfAV caspase), and the absence of p20 (large unit) and p10 (small unit) potential cleavage site, we can conclude that these caspase-like enzymes may be associated with other functions during the infection. In HvAV the RNAi silencing of the caspase-like gene demonstrated its essential role in viral replication, which would fit more with the proposed function for the vesicles as anucleate virus factories [13]. In general, caspases as intracellular cysteine proteases are associated with many functions other than cell killing, for example, signal transduction, cell differentiation and cytoskeleton remodelling [39].…”