Development of a quantitative real-time PCR for determination of genotype frequencies for studies in baculovirus population biology.

Two bacmid-derived Autographa californica Multiple-capsid Nucleopolyhedrovirus genotypes - that differ only in a short tag sequence for differential PCR recognition - were generated. By electron microscopy, these genotypes were found to have identical polyhedra morphology. Mixtures of quantified polyhedra were made and used to validate a SYBR Green I-based quantitative real-time PCR (qPCR) to determine genotype frequencies in mixed genotype populations. The PCR could accurately quantify genotype ratios over a range of 8 orders of magnitude. Only a small correction of the genotype ratio was necessary to obtain a valid result. Low levels of aspecific background (a fluorescent signal when the template corresponding with the primer set used is not present) were measured in these validation experiments and in a typical laboratory setup. A small fitness difference between the genotypes generated was observed in a median lethal dose bioassay. The bacmid-derived virus genotypes generated and the qPCR assays are valuable tools for studying the population biology of baculoviruses.