The development of larval resistance to a nucleopolyhedrovirus is not accompanied by an increased virulence in the virus

Partial Loss of Inheritable Type I Resistance of Codling Moth to Cydia pomonella qranulovirus

Current knowledge of the field resistance of codling moth (CM, Cydia pomonella, L) against Cydia pomonella granulovirus (CpGV) is based mainly on the interaction between the Mexican isolate CpGV-M and CpRR1, a genetically homogeneous CM inbreed line carrying type I resistance. The resistance level of laboratory-reared CpRR1 to CpGV-M was recently found to have decreased considerably, compared to the initially high resistance. To understand the background of this phenomenon, CpRR1 larvae were exposed over several generations to CpGV-M for re-selection of the original resistance level. After five and seven generations of selection, new CpRR1_F5 and CpRR1_F7 lines were established. The resistance ratio of these selected lines was determined by full range bioassays. The CpRR1_F5 strain regained a higher level of resistance against CpGV up to 104-fold based on LC50 values compared to susceptible larvae (CpS), which indicated that the absence of virus selection had resulted in a reduction of resistance under laboratory rearing conditions. In addition, some fitness costs of fecundity were observed in CpRR1_F5. Single-pair crossings between CpRR1_F5 or CpRR1_F7 with susceptible CpS moths revealed a dominant but not fully sex-linked inheritance, which suggests a partial loss of previous resistance traits in CpRR1.

Using Next Generation Sequencing to Identify and Quantify the Genetic Composition of Resistance-Breaking Commercial Isolates of Cydia pomonella Granulovirus

The use of Cydia pomonella granulovirus (CpGV) isolates as biological control agents of codling moth (CM) larvae is important in organic and integrated pome fruit production worldwide. The commercially available isolates CpGV-0006, CpGV-R5, and CpGV-V15 have been selected for the control of CpGV resistant CM populations in Europe. In infection experiments, CpGV-0006 and CpGV-R5 were able to break type I resistance and to a lower extent also type III resistance, whereas CpGV-V15 overcame type I and the rarely occurring type II and type III resistance. The genetic background of the three isolates was investigated with next generation sequencing (NGS) tools by comparing their nucleotide compositions to whole genome alignments of five CpGV isolates representing the known genetic diversity of the CpGV genome groups A to E. Based on the distribution of single nucleotide polymorphisms (SNPs) in Illumina sequencing reads, we found that the two isolates CpGV-0006 and CpGV-R5 have highly similar genome group compositions, consisting of about two thirds of the CpGV genome group E and one third of genome group A. In contrast, CpGV-V15 is composed of equal parts of CpGV genome group B and E. According to the identified genetic composition of these isolates, their efficacy towards different resistance types can be explained and predictions on the success of resistance management strategies in resistant CM populations can be made.

A Third Type of Resistance to Cydia pomonella Granulovirus in Codling Moths Shows a Mixed Z-Linked and Autosomal Inheritance Pattern

Different isolates of Cydia pomonella granulovirus (CpGV) are used worldwide to control codling moth larvae (Cydia pomonella) in pome fruit production. Two types of dominantly inherited field resistance of C. pomonella to CpGV have been recently identified: Z-chromosomal type I resistance and autosomal type II resistance. In the present study, a CpGV-resistant C. pomonella field population (termed SA-GO) from northeastern Germany was investigated. SA-GO individuals showed cross-resistance to CpGV isolates of genome group A (CpGV-M) and genome group E (CpGV-S), whereas genome group B (CpGV-E2) was still infective. Crossing experiments between individuals of SA-GO and the susceptible C. pomonella strain CpS indicated the presence of a dominant autosomal inheritance factor. By single-pair inbreeding of SA-GO individuals for two generations, the genetically more homogenous strain CpRGO was generated. Resistance testing of CpRGO neonates with different CpGV isolates revealed that isolate CpGV-E2 and isolates CpGV-I07 and -I12 were resistance breaking. When progeny of hybrid crosses and backcrosses between individuals of resistant strain CpRGO and susceptible strain CpS were infected with CpGV-M and CpGV-S, resistance to CpGV-S appeared to be autosomal and dominant for larval survivorship but recessive when success of pupation of the hybrids was considered. Inheritance of resistance to CpGV-M, however, is proposed to be both autosomal and Z linked, since Z linkage of resistance was needed for pupation. Hence, we propose a further type III resistance to CpGV in C. pomonella, which differs from type I and type II resistance in its mode of inheritance and response to CpGV isolates from different genome groups.IMPORTANCE The baculovirus Cydia pomonella granulovirus (CpGV) is registered and applied as a biocontrol agent in nearly all pome fruit-growing countries worldwide to control codling moth caterpillars in an environmentally friendly manner. It is therefore the most widely used commercial baculovirus biocontrol agent. Since 2005, field resistance of codling moth to CpGV products has been observed in more than 40 field plantations in Europe, threatening organic and integrated apple production. Knowledge of the inheritance and mechanism(s) of resistance is indispensable for the understanding of host response to baculovirus infection on the population level and the coevolutionary arms race between virus and host, as well as for the development of appropriate resistance management strategies. Here, we report a codling moth field population with a new type of resistance, which appears to follow a highly complex inheritance in regard to different CpGV isolates.

Midgut-based resistance to oral infection by a nucleopolyhedrovirus in the laboratory-selected strain of the smaller tea tortrix, Adoxophyes honmai (Lepidoptera: Tortricidae)

A strain of Adoxophyes honmai resistant to Adoxophyes honmai nucleopolyhedrovirus (AdhoNPV) was established from a field-collected colony by repeated selection. Fifth-instar larvae of this resistant strain (R-strain) had over 66 666-fold greater resistance in terms of 50 % lethal concentration values to oral infection of AdhoNPV than non-selected strain larvae (susceptible for AdhoNPV; S2-strain). In this study, the mechanism of resistance to AdhoNPV was determined in R-strain larvae. An assessment of viral genome replication in AdhoNPV-infected S2- and R-strain larvae by quantitative PCR showed no viral genome replication occurring in R-strain larvae. Transcription of AdhoNPV ie-1, vp39 and polyhedrin genes was also not detected in R-strain midgut cells. Besides, a fluorescent brightener had no effect on AdhoNPV infection in either S2- or R-strain. However, binding and fusion of occlusion-derived virus with R-strain were significantly lower than those of S2-strain. These findings suggest that R-strain Adoxophyeshonmai larvae possess a midgut-based resistance to oral infection by AdhoNPV in which midgut epithelial cells are infected less efficiently.

Acquired resistance to a nucleopolyhedrovirus in the smaller tea tortrix Adoxophyes honmai (Lepidoptera: Tortricidae) after selection by serial viral administration

A laboratory colony of Adoxophyes honmai was selected for resistance over 156 generations by feeding neonate larvae of every generation with the LC₆₀ or LC₇₀ of its nucleopolyhedrovirus, Adoxophyes honmai nucleopolyhedrovirus (AdhoNPV). A significant difference in LC₅₀ values between the selected (R-strain) and unselected (S1- and S2-strain) strains was first observed after three generations of selection, and the resistance level then increased continuously. The highest degree of acquired resistance, based on the ratio of the LC₅₀ values of R- and S1-strains, was more than 400,000-fold. After selection was stopped at either the 21st or the 149th generation, LC₅₀ values did not decrease significantly, suggesting that resistance of the R-strain to AdhoNPV was stable. To assess which of the two routes of baculovirus infection is affected by resistance to AdhoNPV, 5th instar larvae of the R-strain were inoculated orally and intrahemocoelically with AdhoNPV and their susceptibility was compared to that of S-strain. The ratio of the LC₂₅ values of selected and unselected strains was 91-fold when budded viruses were injected into 5th instar larvae, but was 107,000-fold after oral inoculation. These results indicate that the resistance mechanism of the R-strain of A. honmai disrupts both midgut primary infection and hemocoelic secondary infection.

Evolution of Drosophila resistance against different pathogens and infection routes entails no detectable maintenance costs

Pathogens exert a strong selective pressure on hosts, entailing host adaptation to infection. This adaptation often affects negatively other fitness-related traits. Such trade-offs may underlie the maintenance of genetic diversity for pathogen resistance. Trade-offs can be tested with experimental evolution of host populations adapting to parasites, using two approaches: (1) measuring changes in immunocompetence in relaxed-selection lines and (2) comparing life-history traits of evolved and control lines in pathogen-free environments. Here, we used both approaches to examine trade-offs in Drosophila melanogaster populations evolving for over 30 generations under infection with Drosophila C Virus or the bacterium Pseudomonas entomophila, the latter through different routes. We find that resistance is maintained after up to 30 generations of relaxed selection. Moreover, no differences in several classical life-history traits between control and evolved populations were found in pathogen-free environments, even under stresses such as desiccation, nutrient limitation, and high densities. Hence, we did not detect any maintenance costs associated with resistance to pathogens. We hypothesize that extremely high selection pressures commonly used lead to the disproportionate expression of costs relative to their actual occurrence in natural systems. Still, the maintenance of genetic variation for pathogen resistance calls for an explanation.

Evolution and the microbial control of insects

Insect pathogens can be utilized in a variety of pest management approaches, from inundative release to augmentation and classical biological control, and microevolution and the consideration of evolutionary principles can potentially influence the success of all these strategies. Considerable diversity exists in natural entomopathogen populations and this diversity can be either beneficial or detrimental for pest suppression, depending on the pathogen and its mode of competition, and this should be considered in the selection of isolates for biological control. Target hosts can exhibit considerable variation in their susceptibility to entomopathogens, and cases of field-evolved resistance have been documented for Bacillus thuringiensis and baculoviruses. Strong selection, limited pathogen diversity, reduced gene flow, and host plant chemistry are linked to cases of resistance and should be considered when developing resistance management strategies. Pre- and post-release monitoring of microbial control programs have received little attention; however, to date there have been no reports of host-range evolution or long-term negative effects on nontarget hosts. Comparative analyses of pathogen population structure, virulence, and host resistance over time are required to elucidate the evolutionary dynamics of microbial control systems.