Species interactions represent a central topic in biological research. Due to their high ecological and economical significance (e.g. pests) a lot of effort has been put into furthering our understanding of adaptations between insects and their host plants. These adaptations are crucial for a species’ evolutionary fate as they can affect the exploitation of a specific resource (e.g. metabolization of plant tissue) and, thus, the fitness of multiple generations. Plants consist of a diverse and complex composition of various structural and chemical compounds with only a small portion being involved in defensive mechanisms against herbivores. This variation can represent a challenge for polyphagous insects since it requires a flexible set of adaptations to cope with the diverse properties of different plants. The comma butterfly, Polygonia c-album, covers a wide range of different host species. Expressing all genes necessary to feed and survive on all potential hosts would probably be detrimental and costly. As opposed to this, previous studies showed that different sets of genes are activated in response to a particular host. To further investigate these patterns, host switch experiments were performed. The combination of RNASeq-data from two experiments as well as performance assays, showed a high transcriptional and phenotypic plasticity in the larval response to different host plants. In a “selection experiment” , gene expression profiles were monitored over multiple generations of artificial selection for increased performance on a particular host. In combination with further host switch experiments, this data gave important insights about the stability of host specific adaptations over time with a special focus on the role of (transcriptional and performance) trade-offs for the evolution of host-specific adaptations. To further investigate the mechanisms underlying specialization to a particular host plant, two populations of Polygonia c-album, that differ in the degree of specialization to Urtica dioica, were compared using population genomics and RNA-Seq (“cross experiment”). Furthermore, hybrids between these populations were included to investigate proposed sex-linked effects on larval responses to different diets. First results already identified some strong candidates for a sex-linked pattern and a potential connection between oviposition preference and larval performance patterns. Looking deeper into allele specific expression differences will give further insight into the basis of altered regulation of gene expression in connection to host use. Additional comparisons using different references will further help to consider the relevance of local reference genomes (SWE UK) for such studies. In a pilot project RNA from single cells was collected from larval midguts after feeding on different hosts ("single cell experiment"). First comparisons within the species Pieris napi suggested cell type specific expression for certain detoxification genes. Similar comparisons shall now be made for P. c-album. This allows to study the components of host specific responses at a cellular level. This project will significantly contribute to a further understanding of the mechanisms and genetic basis underlying adaptations to specific environments (i.e. hosts) and, thus, will increase our knowledge and predictability of species associations and interaction dynamics.