Baculovirus diversity and its effect on virulence.

Abstract

The baculovirus, SpexNPV is the biggest mortality agent of natural populations of the African armyworm, Spodoptera exempta and in terms of control has displayed considerable potential as a bioinsecticide in aerial spray trials undertaken by Natural Resources International (NRI). This project was charged with assessing the diversity of natural populations of SpexNPV, a subject of direct relevance for the development of SpexNPV as a bioinsecticde and about which little was known. The genetic composition of a natural SpexNPV population was characterized using in vivo cloning techniques and RFLP analysis. Seventeen individual genotypes were isolated from the wild-type population and phylogenetic analysis was carried out to attempt to assess their potential relatedness, but no single conclusion on their shared histories could be reached. The fitness traits (pathogenicity, speed of kill and viral yield of OBs) of eight genotypes and the wild-type virus were assessed in laboratory bioassays. A nine-fold difference in pathogenicity (estimated in terms of LD50) was witnessed between the different genotypes. Genotypes and wild-type virus also varied in their speed of kill and yield and although there was evidence of a trade-off between speed of kill and yield trade-off, for a number of the genotypes, this study failed to demonstrate any statistical evidence for a general trade-off between these fitness traits. SpexNPV epizootics, from out-breaking populations of armyworm in Northern Tanzania, were sampled in 2002 and 2004. RFLP analysis was conducted on individual virus-infected larvae and revealed a high level of heterogeneity. In addition, forty percentage of all isolates analysed were identified as mixed-genotype infections by the presence of sub-molar bands in RE profiles. This genetic diversity appeared to lack any obvious population structure. Controlled mixed-genotype inoculations were carried out to assess any change in phenotype relative to single-genotype infections. Genotypes, which were found to be equally pathogenic in single-genotype infections, were combined in equal ratio. Thirteen different dual inoculations were carried out across a range of doses. Mixedgenotype infections were found to be more pathogenic, and in general, possess longer speeds of kill relative to single genotype infections. The effect on yield varied considerably between mixtures. The inclusion of more than two genotypes within the mixtures (2-, 4-, 8-, and 16-genotype inoculations) revealed a correlation between level of diversity and pathogenicity and, to a certain extent, between level of diversity and speed of kill. This appears to suggest that mixed-infections possess higher levels of fitness than single genotype infections and that genetic diversity of SpexNPV should be maintained when used as a bioinsecticide.