Species interactions represent one of the central topics in biological research. Due to their high ecological and economical significance (e.g., pests) special effort has been put into understanding the adaptations between phytophagous insects and plants. 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. The insects need adaptations that allow them the efficient use and processing of their diet. These adaptations are crucial for a species’ evolutionary fate as they can affect the identification and successful exploitation of a resource and, thus, the fitness of multiple generations. While the majority of phytophagous insects are specialists with a narrow host range, generalists that can feed on several and diverse plants are not uncommon. The mechanisms that underly host use und drive the change of a host repertoire (expansion or restriction) are, however, still not fully understood. The comma butterfly, Polygonia c-album, is characterized by a rather unusual host repertoire for a butterfly as the larvae are specialized to feed on a few specific species within several different orders of plants. It already has been shown that different sets of genes are activated in response to a particular diet (Celorio-Mancera et al. 2023). Moreover, several studies that were conducted with the support of SNIC/NAISS showed that caterpillars can adjust their transcriptional response to a new diet in case of a host switch (“Switch Experiments”), which can provide important indications about the complexity of evolutionary host shifts (Schneider et al. 2024). Additionally, it was shown that the host specific profiles are largely maintained despite strong artificial selective pressure (“selection experiment”). A comparison between two populations of P. c-album from Sweden and the UK, that differ in their degree of specialization to Urtica dioica furthermore showed genomic differences in regions associated with circadian rhythm, voltinism and diapause. These differences were not reflected in the transcriptional response to different host plants (“Cross experiment”). However, hints for indirect parental effects on host use and specialization were found. In the next step, these patterns shall be further investigated by looking deeper into allele specific expression patterns and potential regulatory differences in the context of host use in hybrids between these populations. To also get a better understanding about the spatial distribution of host specific transcriptional responses, a pilot study was run on the expression patterns of single nuclei collected from the midgut tissue of Pieris napi. This pilot already suggested cell specific expression of detoxification genes. This study shall now be extended by adding individuals from P. c-album and Anthocharis cardamines (another Pierid). This will contribute to a better understanding of the role of individual cell types in detoxification and metabolization of different diets, and will show how conserved these patterns are across different butterfly species (“snRNA-Seq”).