Abstract
Heliothis virescens ascovirus 3 h (HvAV-3h), a dsDNA insect virus, belonging to the family Ascoviridae, can infect caterpillars of several Noctuidae species by ovipositing parasitoid wasps. In order to provide a comprehensive overview of the interactive responses of host larvae after infection by the ascovirus, a transcriptome analysis of Spodoptera exigua to HvAV-3h was conducted from 6 to 168 hours post infection (hpi). Approximately 101.64 Gb of RNA sequencing (RNA-seq) data obtained from infected and uninfected S. exigua larvae were used to perform a de novo transcriptome assembly, which generated approximately 62,258 S. exigua unigenes. Using differential gene expression analysis, it was determined that the majority of host transcripts were down-regulated beginning at 6 hpi and continuing throughout the infection period, although there was an increase in up-regulated unigene number during the 12 to 72 hpi stage. It is noteworthy that the most abundantly enriched pathways in KEGG annotation were Metabolism terms, indicating that the host larval metabolic mechanisms were highly influenced post HvAV-3h infection. In addition, the host cuticle protein encoding unigenes were highly down-regulated in most of the situations, suggesting that the host larval cuticle synthesis were inhibited by the viral infection.
Highlights
After long periods of evolution, different kinds of complexity battles or cooperative strategies between microorganisms and their hosts present at all times
To analyze the larval response to Heliothis virescens ascovirus 3 h (HvAV-3h) infection, the 3rd instar S. exigua larvae were infected with HvAV-3h, and messenger RNAs were extracted for transcriptome analysis at 6, 12, 72 and 168 hpi
A sequence similarity search identified 38,510 unigenes (61.9%) similarities to existing GenBank entries. These annotated unigenes showed significant similarity to sequences from various insect species, in which 44.1% unigenes were similar to Bombyx mori, 24.46% unigenes were similar to Danaus plexippus, 1.61% unigenes were similar to Papilio xuthus, 1.52% unigenes were similar to Tribolium castaneum, and 28.32% unigenes were similar to other species according to annotations in the UniProt Protein database (Table S3)
Summary
After long periods of evolution, different kinds of complexity battles or cooperative strategies between microorganisms and their hosts present at all times. The well-characterized ecdysteroid UDP-glucosyltransferase (EGT) protein encoded by baculoviruses inactivates ecdysteroids by sugar conjugation in infected host larvae This glycosylation often leads to failure to molt in baculovirus infected insects. They continue to eat and grow during the time that they would normally be in the premolt wandering phase[15,16] This is thought to provide an advantage for the baculovirus, as the larger larvae produce more progeny virus. To defend against viral infections, host larvae activate an abundance of immune responses to attack the viruses and protect their healthy tissues These defensive reactions can be divided into several stages according to the virus infection phase. Because Ascoviridae is still a relatively recently characterized insect virus family, transcriptome analyses have not yet to be reported in investigations involving host reactions during ascovirus infections. The results obtained in this study would provide a global view of molecular changes in S. exigua larvae during HvAV-3h infection, and may provide some reliable evidence for further studies on interactions between HvAV-3h and S. exigua larvae
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