Mendelian inheritance of endogenous viral elements (EVE) of white spot syndrome virus (WSSV) in shrimp

Abstract

Previous work has shown that non-retroviral endogenous viral elements (EVE) are common in crustaceans, including penaeid shrimp. So far, they have been reported for infectious hypodermal and hematopoietic necrosis virus (IHHNV) and white spot syndrome virus (WSSV). For the latter, it was shown that shrimp sperm were positive for an EVE of WSSV called EVE₃₆₆, suggesting that it was heritable, since shrimp sperm (non-motile) do not contain mitochondria. However, to prove this hypothesis that EVE₃₆₆ was heritable and located in chromosomal DNA, it was necessary to carry out mating tests to show that EVE₃₆₆ could be detected in parental shrimp and distributed in their offspring in a Mendelian fashion. To do this, we analyzed two shrimp crosses using polyacrylamide gels with a multiple-allele, microsatellite marker Pmo11 as a quality control for single allele detection. In both crosses, all of the shrimp (parents and siblings) were positive for 2 Pmo11 alleles as expected. In Cross 1, the female was PCR-positive for EVE₃₆₆ while the male was negative, and in Cross 2, both the female and male were PCR-positive for EVE₃₆₆. Individual analysis of the offspring of Cross 1 revealed a distribution of 1:1 for EVE₃₆₆, indicating that the EVE₃₆₆-positive female parent was heterozygous for EVE₃₆₆. In the second cross, the distribution of EVE₃₆₆ in the offspring was 3:1, indicating that both PCR-positive parents were heterozygous for EVE₃₆₆. These results supported the hypothesis that EVE₃₆₆ was present in shrimp chromosomal DNA and was heritable in a Mendelian fashion. This work provides a model to screen for heritable EVE in shrimp and shows that selection of one parent heterozygous for an EVE and the other negative for it can result in approximately half of the siblings positive and half negative for that EVE as expected. Dividing the siblings of such a cross into an EVE positive group and an EVE negative group followed by challenge with the originating lethal virus should reveal whether or not possession of that specific EVE results in any significant protection against disease caused by the homologous virus.

Keywords: EVE; Endogenous viral elements; Mendelian inheritance; WSSV; White spot syndrome virus.