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

There is increasing evidence that populations of microparasites vary genoty pically at a variety of spatial scales, including within host individuals. Understanding this fine-scale structuring of microparasite populations requ ires 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 pheno typic differences, and discuss the processes by which such variation might be maintained. Twenty-five genotypic variants of a nucleopolyhedrovirus (Ba culoviridae) have been identified and purified from a single pine beauty mo th (Panolis flammea) larva. The phenotypes of four genotypes were compared. Genotypes differed in three phenotypic traits, each predicted to be an imp ortant component of fitness: pathogenicity, speed of kill and yield. Variat ion in pathogenicity was described by seven-fold differences in LD50 and by differences in the slopes of the fitted dose-response curves. Mean speed o f 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 in tra-genotypic relationship between yield and speed of kill. Inter-genotypic trade-offs between virus phenotypic traits, which could promote the co-exi stence of genotypes, were not found. Mechanisms that may promote the co-exi stence of competing virus genotypes are discussed.