Characterization of new baculovirus genotypes arising from inoculation of Pieris brassicae with granulosis viruses.

Previous studies have shown that of 15 Artogeia (Pieris) rapae granulosis virus isolates (ArGV1 to ArGV15) only two, ArGV1 and ArGV2, gave a normal dose-mortality response in larvae from an established colony of Pieris brassicae. We report here that at extremely high doses, approaching 10000 times the LD50 for ArGV1 and ArGV2, three other ArGV isolates caused low and irregular levels of mortality in P. brassicae. At similar doses Agrotis segetum GV caused 43% mortality in one infection, but no deaths ensued from other inoculations with this virus. Restriction endonuclease analysis of viral DNA recovered from individual larval cadavers revealed that, in most cases, progeny virus differed from the inoculum and consisted either of ArGV1 or of novel genotypes explicable as recombinants between genomes of the inoculum and of ArGV1. Field-collected P. brassicae inoculated with ArGV8 yielded a similar range of progeny genotypes. Physical maps were constructed for two such recombinants, based on comparative restriction analysis with reference to the published map of ArGV1 and to those of ArGV5 and ArGV8, which are presented. Replication of the inoculum genotype was observed in only two infections. The origin of ArGV1 DNA appearing among progeny from these infections and the relevance of our results to identifying ArGV DNA sequences that modulate pathogenicity for P. brassicae are discussed.