Viral Decoys: The Only Two Herpesviruses Infecting Invertebrates Evolved Different Transcriptional Strategies to Deflect Post-Transcriptional Editing

The highly versatile group of Herpesviruses cause disease in a wide range of hosts. In invertebrates, only two herpesviruses are known: the malacoherpesviruses HaHV-1 and OsHV-1 infecting gastropods and bivalves, respectively. To understand viral transcript architecture and diversity we first reconstructed full-length viral genomes of HaHV-1 infecting Haliotis diversicolor supertexta and OsHV-1 infecting Scapharca broughtonii by DNA-seq. We then used RNA-seq over the time-course of experimental infections to establish viral transcriptional dynamics, followed by PacBio long-read sequencing of full-length transcripts to untangle viral transcript architectures at two selected time points. Despite similarities in genome structure, in the number of genes and in the diverse transcriptomic architectures, we measured a ten-fold higher transcript variability in HaHV-1, with more extended antisense gene transcription. Transcriptional dynamics also appeared different, both in timing and expression trends. Both viruses were heavily affected by post-transcriptional modifications performed by ADAR1 affecting sense-antisense gene pairs forming dsRNAs. However, OsHV-1 concentrated these modifications in a few genomic hotspots, whereas HaHV-1 diluted ADAR1 impact by elongated and polycistronic transcripts distributed over its whole genome. These transcriptional strategies might thus provide alternative potential roles for sense-antisense transcription in viral transcriptomes to evade the host’s immune response in different virus–host combinations.

Dual Transcriptomic Analysis Reveals a Delayed Antiviral Response of Haliotis diversicolor supertexta against Haliotid Herpesvirus-1

Haliotid herpesvirus-1 (HaHV-1) is the first identified gastropod herpesvirus, causing a highly lethal neurologic disease of abalone species. The genome of HaHV-1 has been sequenced, but the functions of the putative genes and their roles during infection are still poorly understood. In the present study, transcriptomic profiles of Haliotis diversicolor supertexta at 0, 24 and 60 h post injection (hpi) with HaHV-1 were characterized through high-throughput RNA sequencing. A total of 448 M raw reads were obtained and assembled into 2.08 × 105 unigenes with a mean length of 1486 bp and an N50 of 2455 bp. Although we detected increased HaHV-1 DNA loads and active viral expression at 24 hpi, this evidence was not linked to significant changes of host transcriptomic profiles between 0 and 24 hpi, whereas a rich immune-related gene set was over-expressed at 60 hpi. These results indicate that, at least at the beginning of HaHV-1 infection, the virus can replicate with no activation of the host immune response. We propose that HaHV-1 may evolve more effective strategies to modulate the host immune response and hide during replication, so that it could evade the immune surveillance at the early stage of infection.

EXPLORING THE CHRONIC MORTALITY AFFECTING ABALONES IN TAIWAN: DIFFERENTIATION OF ABALONE HERPESVIRUS-ASSOCIATED ACUTE INFECTION FROM CHRONIC MORTALITY BY PCR AND IN SITU HYBRIDIZATION AND HISTOPATHOLOGY

Abalone herpesvirus (AbHV) infection of cultured abalones Haliotis diversicolor supertexta induced acute high mortality in 2003. Years later, sporadic mortality was noted for an extended period of months, resulting in high cumulative mortality. Moribund abalones were analyzed using PCR, in situ hybridization, and histopathology, because thus far no viral particles have been observed by transmission electron microscopy. PCR using 20 primer sets, specifically designed from sequences of acute AbHV infection, failed to amplify any products from abalones suffering from chronic mortality. Subsequently, a 1406-bp sequence was amplified from chronic moribund abalones, and this sequence showed a 92% (553 bp/602 bp) homology with the gene of an AbHV Taiwan isolate (NCBI serial no. KF537536.1), suggestive of an AbHV pathotype. Histopathology of AbHV pathotype infection showed hemocyte infiltration in the lamina propia of the digestive tract, and hemocytes of various stages were evident, as well as the loss of seminal tubules in the gonad. In situ hybridization revealed that in AbHV infection, positive signals were restricted to the neural ganglia, while in AbHV pathotype infection, positive signals were observed only in the hemocytes. It appeared that the tropism of AbHV shifted from mainly neurotropic in AbHV infection to mainly hemocytotropic in abalone suffering from chronic mortality. Abalone shriveling syndrome-associated virus co-infection was detected in some of AbHV pathotype infection events. Further studies are needed to better understand the pathogenesis of AbHV pathotype affecting H. diversicolor in Taiwan.