Shrimp endogenous viral elements (EVE) correlate with survival in white spot syndrome virus (WSSV) challenges

Shrimp and other arthropods are capable of specific, adaptive immune responses to viruses based on viral copy DNA (vcDNA) fragments in the host genome called endogenous viral elements (EVE). These may produce negative sense RNA transcripts leading to an RNA interference (RNAi) defense response against cognate viruses. We first reported high-frequency-read sequences (HFRS) of white spot syndrome virus EVE (named WSSV-EVE 4,6,8) in a WSSV-free breeding stock of whiteleg shrimp (Penaeus vannamei). Here we describe screening for the same HFRS-EVE in a captured giant tiger shrimp (Penaeus monodon) breeding stock, also free of WSSV. WSSV-EVE 4,6,8 was detected in some of the P. monodon stock individuals with positive or negative RNA expression. Eight broodstock individuals were selected for mating in 4 crosses. The offspring from these crosses were grown sufficiently to allow tagging and pleopod sampling for DNA and RNA analysis prior to challenge with WSSV. This allowed for Mendelian analysis of EVE inheritance and for its expression or not in the offspring, together with analysis of their relationships to survival and WSSV infection level after challenge. The results revealed that EVE inheritance was Mendelian, but that their RNA expression or not was independently controlled. In Crosses 1 and 2, all the offspring died and none of them carried 2 or more of the expressed EVE in their parental shrimp. In contrast, 100% of 10 arbitrarily selected surviving shrimp from Cross 3 and 90% from Cross 4 carried and expressed 2 or more of the 3 expressed EVE transmitted from the parental shrimp. These results reveal a potential protocol for development of viral tolerant shrimp stocks.