Ascoviruses, family Ascoviridae, are large, enveloped ds DNA viruses that mainly attack lepidopteran larvae of the family Noctuidae causing a chronic, but fatal disease. Their cytopathology differs from other viruses by destroying the nucleus, some species using a caspase, followed by cleavage of the cell into numerous viral vesicles, where most virions are produced. Cell cleavage resembles apoptosis, but differs markedly because mitochondria are not destroyed but rescued by ascoviruses, providing energy for replication and membrane synthesis for viral vesicles and virion envelopes. Ascovirus transmission is also unique in that virions are vectored to hosts on the ovipositor of parasitic wasps.
In my research, I used molecular techniques to determine genomic expression patterns underlying ascovirus cytopathology in larvae for two ascoviruses, the Spodoptera frugiperda ascovirus (SfAV) and Trichoplusia ni ascovirus (TnAV). Specifically, through in vivo transcriptome studies, including RNA-Sequencing, qRT-PCR, manual genome curating and bioinformatic analysis, I determined three temporal classes for SfAV and TnAV genes; early, late, and very late. In SfAV, three proteins that inhibit apoptosis were synthesized early, whereas the caspase was synthesized very late, which correlates with apoptotic-like events resulting in vesicle formation. I identified 15 SfAV bicistronic and tricistronic messages, and similar transcripts in TnAV, indicating multi-cistronic transcripts are common in ascoviruses, and may regulate transcription or translation. Analyses of viral vesicle transcripts from Trichoplusia ni hemolymph revealed much higher viral genome expression than in the fat body, epidermis and tracheal matrix, showing larval blood is important for virus reproduction. Host responses to SfAV infection in Spodoptera frugiperda larvae showed mitochondrial gene expression was similar to controls or upregulated slightly during vesicle formation. Interestingly, few cytoskeleton genes known to encode motor proteins were upregulated despite mitochondrial involvement in vesicle formation. Analysis of antimicrobial peptides genes showed several where highly upregulated, as much as 32-fold, for example, moricin and gloverin, whereas innate immunity genes of the Toll, melanization, and phagocytosis pathways were activated only moderately by infection. Phenoloxidase and Toll negative regulators increased 15-fold and 3-fold, respectively. Therefore, conserving mitochondria while balancing immunity gene inducers and repressors may explain the chronic nature of ascovirus diseases.
