Phenotypic variation between naturally co-existing genotypes of a Lepidopteran baculovirus

There is increasing evidence that populations of microparasites vary genotypically at a variety of spatial scales, including within host individuals. Understanding this fine-scale structuring of microparasite populations requires descriptions of the relative fitness of individual genotypes isolated from natural mixed infections. Here we examine whether differences between virus genotypes isolated from a single host individual translate into phenotypic differences, and discuss the processes by which such variation might be maintained. Twenty-five genotypic variants of a nucleopolyhedrovirus (Baculoviridae) have been identified and purified from a single pine beauty moth (Panolis flammea) larva. The phenotypes of four genotypes were compared. Genotypes differed in three phenotypic traits, each predicted to be an important component of fitness: pathogenicity, speed of kill and yield. Variation in pathogenicity was described by seven-fold differences in LD50 and by differences in the slopes of the fitted dose– response curves. Mean speed of kill of the genotypes differed by up to 36 h. Two genotypes produced 65% higher yields, over and above any differences predicted by a significant intra-genotypic relationship between yield and speed of kill. Inter-genotypic trade-offs between virus phenotypic traits, which could promote the co-existence of genotypes, were not found. Mechanisms that may promote the co-existence of competing virus genotypes are discussed.