Shrimp draft genome contains independent clusters of ancient and current endogenous viral elements (EVE) of the parvovirus IHHNV

BackgroundShrimp have the ability to accommodate viruses in long term, persistent infections without signs of disease. Endogenous viral elements (EVE) play a role in this process probably via production of negative-sense Piwi-interacting RNA (piRNA)-like fragments. These bind with Piwi proteins to dampen viral replication via the RNA interference (RNAi) pathway. We searched a draft genome of the giant tiger shrimp (Penaeus monodon)(GenBank record JABERT000000000) for the presence of EVE related to a shrimp parvovirus originally named infectious hypodermal and hematopoietic necrosis virus (IHHNV). ResultsThe shrimp draft genome contained 3 piRNA-like gene clusters containing scrambled IHHNV EVE. Two clusters were located distant from one another in linkage group 35 (LG35). Both LG35 clusters contained multiple DNA fragments with high homology (99%) to GenBank records DQ228358 and EU675312 that were both called " non-infectious IHHNV Type A" (IHHNV-A) when originally discovered. However, our results and those from a recent Australian P. monodon genome assembly indicate that the relevant GenBank records for IHHNV-A are sequence-assembly artifacts derived from scrambled and fragmental IHHNV-EVE. Although the EVE in the two LG35 clusters showed high homology only to IHHNV-A, the clusters were separate and distinct with respect to the arrangement (i.e., order and reading direction) and proportional content of the IHHNV-A GenBank records. We conjecture that these 2 clusters may constitute independent allele-like clusters on a pair of homologous chromosomes. The third EVE cluster was found in linkage group 7 (LG7). It contained EVE with high homology (99%) only to GenBank record AF218266 with the potential to protect shrimp against infectious IHHNV. ConclusionsOur results suggested the possibility of viral-type specificity in EVE clusters. Specificity is important because whole EVE clusters for one viral type would be transmitted to offspring as collective hereditary units. This would be advantageous if one or more of the EVE within the cluster were protective against disease caused by the cognate virus. It would also facilitate gene editing for removal of non-protective EVE clusters or for transfer of protective EVE clusters to genetically improve existing shrimp breeding stocks that might lack them.