Plant-phenotypic changes induced by parasitoid ichnoviruses enhance the performance of both unparasitized and parasitized caterpillars

Abstract

There is increasing awareness that interactions between plants and insects can be mediated by microbial symbionts. Nonetheless, evidence showing that symbionts associated with organisms beyond the second trophic level affect plant-insect interactions are restricted to a few cases belonging to parasitoid-associated bracoviruses. Insect parasitoids harbour a wide array of symbionts which, like bracoviruses, can be injected into their herbivorous hosts to manipulate their physiology and behaviour. Yet, the function of these symbionts in plant-based trophic webs remains largely overlooked. Here, we provide the first evidence of a parasitoid-associated symbiont belonging to the group of ichnoviruses which affects the strength of plant-insect interactions. A comparative proteomic analysis shows that, upon parasitoid injection of calyx fluid containing ichnovirus particles, the composition of salivary glands of caterpillars changes both qualitatively (presence of two viral-encoded proteins) and quantitatively (abundance of several caterpillar-resident enzymes, including elicitors such as glucose oxidase). In turn, plant phenotypic changes triggered by the altered composition of caterpillar oral secretions affect the performance of herbivores. Ichnovirus manipulation of plant responses to herbivory leads to benefits for their parasitoid partners in terms of reduced developmental time within the parasitized caterpillar. Interestingly, plant-mediated ichnovirus-induced effects also enhance the performances of unparasitized herbivores which in natural conditions may feed alongside parasitized ones. We discuss these findings in the context of ecological costs imposed to the plant by the viral symbiont of the parasitoid. Our results provide intriguing novel findings about the role played by carnivore-associated symbionts on plant-insect-parasitoid systems and underline the importance of placing mutualistic associations in an ecological perspective.

Keywords: host-parasitoid interaction; parasitoid-associated symbionts; plant-herbivore-microbe interactions; plant-mediated species interactions; polydnaviruses.

FIGURE 1

(a) Principal component analyses (PCA) based on proteins (LFQ = log2 protein intensity) detected in the salivary glands of the different caterpillar treatments. Blue squares: CF = Spodoptera frugiperda caterpillars injected with calix fluid (containing HdIV virions) isolated from the parasitoid Hyposoter didymator; Red squares: PAR = S. frugiperda caterpillars parasitized by H. didymator; Green squares: PBS = S. frugiperda caterpillars injected with phosphate‐buffered saline. (b) Hierarchical clustering based on Pearson correlation (same treatments and colour scheme as above)

FIGURE 2

Scatterplot based on protein abundance ratio (LFQ = log2 protein intensity) detected in the salivary glands of the different Spodoptera frugiperda caterpillar treatments. CF = caterpillars injected with calix fluid (containing HdIV virions) isolated from the parasitoid Hyposoter didymator; PAR = caterpillars parasitized by H. didymator; PBS = caterpillars injected with phosphate‐buffered saline. Each square in the figures represents one protein detected in the salivary glands. The y‐axis of shows the Student's t test difference for each protein based on pairwise comparisons between PAR vs. PBS treatment. The x‐axis of plot shows the same value for CF vs. PBS treatment. Protein from Table 1 are highlighted (see number in first column), colours correspond to different families of proteins

FIGURE 3

(Left) Total number of proteins found in salivary glands of Spodoptera frugiperda caterpillars: bars represent proteins significantly more abundant (red), less abundant (green) or not significantly different (white) in the virus‐infected (CF) and parasitized (PAR) treatments compared with saline‐injected controls (PBS). (Right) direct comparisons of the proteomic changes between CF and PAR treatments in the subset of proteins that displayed a significant increase in abundance (top venn diagram) or a significant decrease in abundance (lower venn diagram) in the previous comparison with PBS. Light grey colour indicates proteins shared in the CF and PAR treatments; medium grey colour indicates unique proteins of CF treatment; dark grey colour unique proteins of the PAR treatment

FIGURE 4

Relative growth rate of unparasitized (a) and parasitized (b) Spodoptera frugiperda caterpillars scored 48 h after feeding on corn leaves either undamaged (UD) or induced with salivary gland extract from: caterpillars injected with phosphate‐buffered saline (PBS); caterpillars parasitized by Hyposoter didymator (PAR); and caterpillars injected with calix fluid (containing virions) isolated from the parasitoid H. didymator (CF). Different letters above bars indicate significant differences among treatments (GLM, p < .05)