Diseases in Asian Aquaculture V Genotypic Variations in Tandem Repeat DNA Segments between Ribonucleotide Reductase Subunit Genes of White Spot

White spot syndrome is a viral disease that affects most commercially cultivated marine shrimp species. The disease first emerged in East Asia in 1991 and has since spread throughout most shrimp farming regions of Asia and the Americas. Disease outbreaks usually result in high mortalities in affected ponds. However, shrimp may also be infected chronically with no signs of disease and often obtain the infection in hatcheries from infected broodstock. A wide range of other crustaceans can also act as apparently healthy carriers of infection. In this report, variations in the number of a 54 nucleotide tandem repeat sequence (TRS), located between genes encoding the large (RR1) and small (RR2) subunits of ribonucleotide reductase, were used as a WSSV strain-specific genetic marker. The marker was applied to examine the extent of variation among WSSV isolates from Penaeus monodon hatcheries and farms in different regions of Vietnam and to obtain a better understanding of the progression of infection in ponds during grow-out. Analysis of approximately 157 WSSV isolates showed common variations in the number of repeats, with some broodstock harbouring more than one genotype. In healthy ponds and in health