The evolution of porcine reproductive and respiratory syndrome virus: Quasispecies and emergence of a virus subpopulation during infection of pigs with VR-2332

GP5, the principal envelope glycoprotein of porcine reproductive and respir atory syndrome virus (PRRSV), contains a hypervariable region within the ec todomain which is responsible for generating diversity in field isolates. T he purpose of this study was to gain insight into the possible origin of th is diversity by following GP5 sequence changes in pigs exposed to PRRSV str ain VR-2332 in utero. A region of the PRRS virus genome containing portions of ORF4 and ORF5 was amplified directly from tissues of infected pigs from birth to 132 days of age. We observed the emergence of a new PRRSV populat ion, identified by a single nucleotide change in the ectodomain. The Asp to Asn change at amino acid 34 was also found as a minor component in pigs th at expressed the wild-type sequence. The results from this study suggest th at the variability in the ectodomain of ORF5 is the result of positive or n egative selection, of which the mechanism remains to be determined. (C) 199 9 Academic Press.