Genome-Wide Association Study Confirms Previous Findings of Major Loci Affecting Resistance to Piscine myocarditis virus in Atlantic Salmon (Salmo salar L.).

Cardiomyopathy syndrome is a viral disease of Atlantic salmon, mostly affecting fish during the late stages of production, resulting in significant losses to the industry. It has been shown that resistance to this disease has a strong genetic component, with quantitative trait loci (QTL) on chromosomes 27 (Ssa27) and Ssa12 to explain most of the additive genetic variance. Here, by analysing animals from a different year-class and a different population, we further aimed to confirm and narrow down the locations of these QTL. The data support the existence of the two QTL and suggest that the causative mutation on Ssa27 is most likely within the 10-10.5 Mbp segment of this chromosome. This region contains a cluster of major histocompatibility complex class I (MHC I) genes with the most strongly associated marker mapped to one of these loci. On Ssa12, the data confirmed the previous finding that the location of the causative mutation is within the 61.3 to 61.7 Mbp region. This segment contains several immune-related genes, but of particular interest are genes related to MHC II. Together, these findings highlight the likely key role of MHC genes in Atlantic salmon following infection with Piscine myocarditis virus (PMCV) and their potential impact on influencing the trajectory of this disease.

Monitoring infection with Piscine myocarditis virus and development of cardiomyopathy syndrome in farmed Atlantic salmon (Salmo salar L.) in Norway.

An epidemiological study was carried out in Norway in 2015-2018, investigating the development of infection with Piscine myocarditis virus (PMCV) and development of cardiomyopathy syndrome (CMS) in farmed Atlantic salmon. Cohorts from 12 sites were followed and sampled every month or every other month from sea transfer to slaughter. PMCV was detected at all sites and in all sampled cages, and fish in six sites developed clinical CMS. The initial infection happened between 1 and 7 months post-sea transfer, and the median time from infection with PMCV until outbreak of CMS was 6.5 months. Generally, fish from sites with CMS had higher viral titre and a higher prevalence of PMCV, compared to sites that did not develop clinical CMS. The virus persisted until the point of slaughter at most (11 out of 12) of the sites. The detection of PMCV in all sites suggests that PMCV is more widespread than previously known. Screening for PMCV as a tool to monitor impending outbreaks of CMS must be supported by observations of the health status of the fish and risk factors for development of disease.

Physiological traits of endornavirus-infected and endornavirus-free common bean (Phaseolus vulgaris) cv Black Turtle Soup.

This study evaluated the physiological traits of eight lines of common bean (Phaseolus vulgaris) cv. Black Turtle Soup, four of which were double-infected with Phaseolus vulgaris endornavirus 1 and Phaseolus vulgaris endornavirus 2, and four of which were endornavirus-free. Plants from all eight lines were morphologically similar and did not show statistically significant differences in plant height, wet weight, number of days to flowering and pod formation, pods per plant, pod thickness, seed size, number of seeds per pod, and anthocyanin content. However, the endornavirus-infected lines had faster seed germination, longer radicle, lower chlorophyll content, higher carotene content, longer pods, and higher weight of 100 seeds, all of which were statistically significant. The endornaviruses were not associated with visible pathogenic effects.

Transcriptome profiling of immune responses to cardiomyopathy syndrome (CMS) in Atlantic salmon.

BACKGROUND: Cardiomyopathy syndrome (CMS) is a disease associated with severe myocarditis primarily in adult farmed Atlantic salmon (Salmo salar L.), caused by a double-stranded RNA virus named piscine myocarditis virus (PMCV) with structural similarities to the Totiviridae family. Here we present the first characterisation of host immune responses to CMS assessed by microarray transcriptome profiling. RESULTS: Unvaccinated farmed Atlantic salmon post-smolts were infected by intraperitoneal injection of PMCV and developed cardiac pathology consistent with CMS. From analysis of heart samples at several time points and different tissues at early and clinical stages by oligonucleotide microarrays (SIQ2.0 chip), six gene sets representing a broad range of immune responses were identified, showing significant temporal and spatial regulation. Histopathological examination of cardiac tissue showed myocardial lesions from 6 weeks post infection (wpi) that peaked at 8-9 wpi and was followed by a recovery. Viral RNA was detected in all organs from 4 wpi suggesting a broad tissue tropism. High correlation between viral load and cardiac histopathology score suggested that cytopathic effect of infection was a major determinant of the myocardial changes. Strong and systemic induction of antiviral and IFN-dependent genes from 2 wpi that levelled off during infection, was followed by a biphasic activation of pathways for B cells and MHC antigen presentation, both peaking at clinical pathology. This was preceded by a distinct cardiac activation of complement at 6 wpi, suggesting a complement-dependent activation of humoral Ab-responses. Peak of cardiac pathology and viral load coincided with cardiac-specific upregulation of T cell response genes and splenic induction of complement genes. Preceding the reduction in viral load and pathology, these responses were probably important for viral clearance and recovery. CONCLUSIONS: By comparative analysis of gene expression, histology and viral load, the temporal and spatial regulation of immune responses were characterised and novel immune genes identified, ultimately leading to a more complete understanding of host-virus responses and pathology and protection in Atlantic salmon during CMS.

Cardiomyopathy syndrome of atlantic salmon (Salmo salar L.) is caused by a double-stranded RNA virus of the Totiviridae family.

Cardiomyopathy syndrome (CMS) of farmed and wild Atlantic salmon (Salmo salar L.) is a disease of yet unknown etiology characterized by a necrotizing myocarditis involving the atrium and the spongious part of the heart ventricle. Here, we report the identification of a double-stranded RNA virus likely belonging to the family Totiviridae as the causative agent of the disease. The proposed name of the virus is piscine myocarditis virus (PMCV). On the basis of the RNA-dependent RNA polymerase (RdRp) sequence, PMCV grouped with Giardia lamblia virus and infectious myonecrosis virus of penaeid shrimp. The genome size of PMCV is 6,688 bp, with three open reading frames (ORFs). ORF1 likely encodes the major capsid protein, while ORF2 encodes the RdRp, possibly expressed as a fusion protein with the ORF1 product. ORF3 seems to be translated as a separate protein not described for any previous members of the family Totiviridae. Following experimental challenge with cell culture-grown virus, histopathological changes are observed in heart tissue by 6 weeks postchallenge (p.c.), with peak severity by 9 weeks p.c. Viral genome levels detected by real-time reverse transcription (RT)-PCR peak earlier at 6 to 7 weeks p.c. The virus genome is detected by in situ hybridization in degenerate cardiomyocytes from clinical cases of CMS. Virus genome levels in the hearts from clinical field cases correlate well with the severity of histopathological changes in heart tissue. The identification of the causative agent for CMS is important for improved disease surveillance and disease control and will serve as a basis for vaccine development against the disease.

Purification and characterization of infectious myonecrosis virus of penaeid shrimp.

The causative agent of myonecrosis affecting cultured Penaeus vannamei in Brazil was demonstrated to be a virus after purification of the agent from infected shrimp tissues. Purified viral particles were injected into specific pathogen-free P. vannamei, resulting in a disease that displayed the same characteristics as those found in the original shrimp used for purification. The virus was named infectious myonecrosis virus (IMNV). The viral particles were icosahedral in shape and 40 nm in diameter, with a buoyant density of 1.366 g ml(-1) in caesium chloride. The genome consisted of a single, double-stranded (dsRNA) molecule of 7560 bp. Sequencing of the viral genome revealed two non-overlapping open reading frames (ORFs). The 5' ORF (ORF 1, nt 136-4953) encoded a putative RNA-binding protein and a capsid protein. The coding region of the RNA-binding protein was located in the first half of ORF 1 and contained a dsRNA-binding motif in the first 60 aa. The second half of ORF 1 encoded a capsid protein, as determined by amino acid sequencing, with a molecular mass of 106 kDa. The 3' ORF (ORF 2, nt 5241-7451) encoded a putative RNA-dependent RNA polymerase (RdRp) with motifs characteristic of totiviruses. Phylogenetic analysis based on the RdRp clustered IMNV with Giardia lamblia virus, a member of the family Totiviridae. Based on these findings, IMNV may be a unique member of the Totiviridae or may represent a new dsRNA virus family that infects invertebrate hosts.

Origin, adaptation and evolutionary pathways of fungal viruses.

Fungal viruses or mycoviruses are widespread in fungi and are believed to be of ancient origin. They have evolved in concert with their hosts and are usually associated with symptomless infections. Mycoviruses are transmitted intracellularly during cell division, sporogenesis and cell fusion, and they lack an extracellular phase to their life cycles. Their natural host ranges are limited to individuals within the same or closely related vegetative compatibility groups. Typically, fungal viruses are isometric particles 25-50 nm in diameter, and possess dsRNA genomes. The best characterized of these belong to the family Totiviridae whose members have simple undivided dsRNA genomes comprised of a coat protein (CP) gene and an RNA dependent RNA polymerase (RDRP) gene. A recently characterized totivirus infecting a filamentous fungus was found to be more closely related to protozoan totiviruses than to yeast totiviruses suggesting these viruses existed prior to the divergence of fungi and protozoa. Although the dsRNA viruses at large are polyphyletic, based on RDRP sequence comparisons, the totiviruses are monophyletic. The theory of a cellular self-replicating mRNA as the origin of totiviruses is attractive because of their apparent ancient origin, the close relationships among their RDRPs, genome simplicity and the ability to use host proteins efficiently. Mycoviruses with bipartite genomes (partitiviruses), like the totiviruses, have simple genomes, but the CP and RDRP genes are on separate dsRNA segments. Because of RDRP sequence similarity, the partitiviruses are probably derived from a totivirus ancestor. The mycoviruses with unencapsidated dsRNA-like genomes (hypoviruses) and those with bacilliform (+) strand RNA genomes (barnaviruses) have more complex genomes and appear to have common ancestry with plant (+) strand RNA viruses in supergroup 1 with potyvirus and sobemovirus lineages, respectively. The La France isometric virus (LIV), an unclassified virus with multipartite dsRNA genome, is associated with a severe die-back disease of the cultivated mushroom. LIV appears to be of recent origin since it differs from its host in codon usage.