The Biological Property Of Macrobrachium Rosenbergii Nodavirus (MrNV) And Its Satellite Virus, Extra Small Virus (XSV)

Macrobrachium rosenbergii Nodavirus (MrNV) and Extra Small Virus (XSV)were recently isolated from Macrobrachium rosenbergii post-larvae suffering fromwhite tail disease (WTD). Previously studies showed that MrNV and XSV, or one ofthem, are the pathogens of WTD, but the role of these two viruses remains unclear.Due to its small size of 15 nm in diameter, XSV was believed to be a new satellitevirus.In this study, the sequence of XSV (Chinese strain, XSV-CHN), 874 nt in length,was obtained by random eDNA library and various method of rapid amplification ofeDNA ends (RACE). It is 26 and 50 nt longer, respectively, at the 5' and 3' termini ofXSV-CHN when compared with XSV-FRA (French strain). And the termini ofXSV-CHN, not XSV-FRA, can fold into the secondary structures similar with thoseof plant satellite viruses. The data will be helpful for determining the taxonomy ofXSV.Taqman probes based real time RT-PCR was developed for the detection ofMrNV and XSV. The method could detect as few as 50 copies of MrNV or XSV perreaction, with a very good reproducibility. This method was applied to detect viralload in infected post-larvae. Real time RT-PCR was 1000-fold more sensitive thanmultiplex RT-PCR and can be available for the prevention and early diagnosis ofWTD.The potential role of MrNV and XSV and their relationship were investigated.MrNV and XSV were purified from diseased prawns and used to infect healthy post-larvae by an immersion method. Three groups of prawns were challenged withdifferent concentrations of MrNV and XSV. Clinical signs of WTD were observed inGroup 1 and 2 that were challenged with a combination of higher dose of MrNV andlower dose of XSV. By contrast, there are few WTD post-larvae in Group 3 that waschallenged with a higher dose of XSV and lower dose of MrNV compared with theGroup 1 and 2. The quantification of viral genome copies by real time RT-PCR furtherdemonstrated that genomic copies of MrNV were much higher in Group 1 and 2 thanthat in Group 3. These results indicated that MrNV plays a key role in WTD of M.rosenbergii. Moreover, statistic analysis suggested that there was a strong correlationbetween MrNV and XSV genome copies in infected prawns, demonstrating that XSVis a satellite virus, dependent on MrNV.MrNV subgenomic RNA3 was sequenced and located on RNA1 by 5'-RACE.With RNAi technique in mammalian cells, we proved that the protein B2 encoded byRNA3 is an inhibitor of host's RNAi in Hela cells, suggesting that the B2 of MrNVmay be functional in crustaceans and can be used as a target for studying the antiviraldefense in crustaceans.