Transcriptional plasticity in a specialist, non-model insect associated with living in a toxic environment

Herbivorous insects are among the most numerous and diverse group of eukaryotes described, and their intimate relationships with host plants provide unique and powerful models for understanding the molecular mechanisms underlying species interactions. Here, utilizing the long-studied milkweed-herbivore system, I used RNAseq and organismal experiments in the non-model milkweed aphid (Aphis nerii) system to identify alternative mechanisms contributing to the adaptation of insects towards toxic plants and to understand the relationship of host plant adaptations and those involved in the evolution of insecticide resistance. After long- term selection on more toxic host plants, A. nerii demonstrated costs to development and fecundity and differentially expressed a narrow set of genes compared to gene expression on control, lowly toxic host plants. These genes were involved in hydrolase, glucosidase, and oxidoreductase activity, as well as cuticular components. In a separate set of experiments, I used transcriptomics in an experimental evolution framework to explore the shape of transcriptional plasticity as insects adapt to new stressors. A. nerii were exposed to more toxic host plants and to two insecticides. Gene expression and fitness changes were quantified after one and five generations of exposure. A. nerii differentially expressed a greater set of genes after five generations compared to one, and there was a large overlap between gene expression changes when A. nerii were reared on more toxic host plants compared to insecticide exposure. All three stress treatments had complex, overall negative effects on aphid fitness. Gene co-expression network analysis revealed the importance of pathways associated with secondary metabolism and basic processes involved in protein production. Few detoxification related transcripts that were initially identified as associated with long-term exposure of A. nerii to more toxic plants were differentially expressed after either one or five generations of exposure in any of the stress treatments. Overall, these results indicate that gene expression patterns can be highly variable dependent on the time frame of insect exposure to novel host plants or to other stressors.