A Chrysodeixis chalcites Single-Nucleocapsid Nucleopolyhedrovirus Population from the Canary Islands Is Genotypically Structured To Maximize Survival

Abstract

A Chrysodeixis chalcites single-nucleocapsid nucleopolyhedrovirus wild-type isolate from the Canary Islands, Spain, named ChchSNPV-TF1 (ChchTF1-wt), appears to have great potential as the basis for a biological insecticide for control of the pest. An improved understanding of the genotypic structure of this wild-type strain population should facilitate the selection of genotypes for inclusion in a bioinsecticidal product. Eight genetically distinct genotypes were cloned in vitro: ChchTF1-A to ChchTF1-H. Quantitative real-time PCR (qPCR) analysis confirmed that ChchTF1-A accounted for 36% of the genotypes in the wild-type population. In bioassays, ChchTF1-wt occlusion bodies (OBs) were significantly more pathogenic than any of the component single-genotype OBs, indicating that genotype interactions were likely responsible for the pathogenicity phenotype of wild-type OBs. However, the wild-type population was slower killing and produced higher OB yields than any of the single genotypes alone. These results strongly suggested that the ChchTF1-wt population is structured to maximize its transmission efficiency. Experimental OB mixtures and cooccluded genotype mixtures containing the most abundant and the rarest genotypes, at frequencies similar to those at which they were isolated, revealed a mutualistic interaction that restored the pathogenicity of OBs. In OB and cooccluded mixtures containing only the most abundant genotypes, ChchTF1-ABC, OB pathogenicity was even greater than that of wild-type OBs. The ChchTF1-ABC cooccluded mixture killed larvae 33 h faster than the wild-type population and remained genotypically and biologically stable throughout five successive passages in vivo. In conclusion, the ChchTF1-ABC mixture shows great potential as the active ingredient of a bioinsecticide to control C. chalcites in the Canary Islands.

A Novel Binary Mixture of Helicoverpa armigera Single Nucleopolyhedrovirus Genotypic Variants Has Improved Insecticidal Characteristics for Control of Cotton Bollworms

“…The co-occluded mixture at P 1 was as pathogenic and productive as the OB mixture (P 0 ) but, interestingly, was faster killing. This effect has been observed for ChchSNPV, in which co-occluded mixtures of variants were faster killing than were mixtures of OBs of each variant (20). Bernal et al (20) suggested that the improved speed of kill of the co-occluded mixture of variants in comparison with the mixtures of OBs of each variant could be due to the physical proximity of variants within occlusion-derived virions of the different variants in the co-occluded mixtures.…”

“…However, restriction endonuclease analysis is not the most sensitive method for identifying novel genotypic variants, and additional variants that could not be detected by using this technique may have been present. This was the case with a Chrysodeixis chalcites SNPV (ChchSNPV) isolate from the Canary Islands, in which the wild-type isolate showed the same profile as the genotype that comprised just 36% of the population of genotypes in the isolate (20). Therefore, other genotypes that were not amenable to replication in cell culture may have been present.…”

“…However, as occurs among other genotypes of many alphabaculoviruses, small changes in the genome can result in important differences in biological activity (16,20,34). OBs of HearSP1B were ϳ3-fold more pathogenic than those of HearSP1-wt, although its prevalence in the population was estimated to be 31%.…”

“…The influence of variant interactions on the insecticidal characteristics of mixed-genotype preparations was examined by producing co-occluded mixtures of variants. Co-occlusion of variants involves mixtures of variants within OBs that can be produced according to methods developed by us (20). Briefly, OBs of each genotype were mixed in the desire proportion, and 50 H. armigera fourth-instar insects were orally inoculated with an LC 90 (2.4 ϫ 10 6 OBs/ ml) by the droplet feeding method.…”

“…The molecular heterogeneity of genotypes is often associated with phenotypic differences in pathogenicity, speed of kill, and virus production (8, 9), which are traits of practical importance for the use of these viruses as biological insecticides (10). Infections involving mixtures of genotypes can result in positive (11-14) or negative (15-18) effects on a number of these traits.As natural alphabaculovirus populations in virus-killed insects comprise mixtures of genotypes, the genotypic interactions that modulate the insecticidal characteristics have attracted the attention of insect pathologists (8,(19)(20)(21) and have been used to develop novel products based on unique user-defined genotypic combinations in specific user-defined proportions that result in improved insecticidal properties of the final mixture (22,23). This has resulted in the generation of a novel paradigm for the development of baculovirus-based insecticides, in which isolates are first dissected into their individual genotypes, which are subjected to a phenotype screening process involving insect bioassay techniques, and then assembled into novel mixtures of genotypes and tested for their insecticidal characteristics against the target pest.…”