Genetic and phenotypic variability in Spodoptera exigua nucleopolyhedrovirus isolates from greenhouse soils in southern Spain

Genotypic and phenotypic variation of SeMNPV was examined in seven isolates of SeMNPV originating from occlusion body (OB) populations in the soil of greenhouses in Spain. Semi-quantitative PCR indicated that some of the isolates were composed of a single dominant genotype, whereas other isolates were composed of two or three genotypes in equal proportions. The coexistence of genotypes could be explained by trade-offs among the three phenotypic traits analyzed, namely pathogenicity (LD50), speed of kill (mean time to death), and OB yields, so that increases in one trait were accompanied by decreases in another. Mixed genotype isolates tended to behave differently to single genotype isolates. Two of the genotypic mixtures were significantly more pathogenic (lower LD50 values) for Spodoptera exigua (Hübner) larvae than the single genotypes that they comprised. OB yield/insect was greater for single genotype compared to mixed genotype isolates, despite genotype-specific differences in mean times to death. Total OB production/insect was positively correlated with time to death. Two out of three of the mixed genotype isolates had lower OB yield/mg insect weight at death compared to single genotype isolates. Each genotypic combination appeared to interact to produce a unique phenotype. This suggests the existence of trade-offs between traits leading to the coexistence of distinct genotypes and genotypic mixtures with similar transmissibility.