Phytophagous insects depend on plants as a food resource during at least one of their developmental stages. While the majority of these insects are specialists with a narrow host range, generalists that can utilize multiple and diverse host plants are still common. The mechanisms underlying host use and driving the expansion or restriction of a host repertoire are, however, still not fully understood. Using the polyphagous comma butterfly (Polygonia c-album), this thesis combines gene expression analyses, genome comparisons, physiological measurements, and performance assays to improve our understanding of the basis and limits of host plant adaptation and host repertoire evolution.

Chapter I investigated how polyphagous species can feed on a variety of different host plants. Larvae that were transferred to different hosts during their development showed a quick adjustment to the new diet. Although a stronger transcriptional response to some host plants implied that they were more challenging to process, there was no effect on larval performance. This suggests transcriptional and phenotypic plasticity as a key mechanism for the utilization of multiple and chemically diverse hosts.

                      Chapter II tested the effect of artificial selection on host plant use. Despite differences in host-specific transcriptional responses, no evidence for genetically based trade-offs restricting the diet repertoire was found. These findings support that host-specific adaptations are largely maintained despite selective pressure and also suggest that high plasticity is not only important for the utilization but also for the maintenance of a broad host repertoire.

Despite this overall plasticity, Chapter III showed that energetic trade-offs between digestion and growth may, at least partly, explain performance differences and could promote specialization on chemically less challenging host plants.

Chapter IV focused on genomic and transcriptional patterns underlying host plant specialization by comparing British and Swedish populations and their reciprocal hybrids. The populations differ in their degree of specialization on U. dioica, a pattern likely linked to differences in seasonal length and voltinism. Although populations differed in genomic regions associated with circadian rhythm, voltinism and diapause, these differences were not reflected in the transcriptional response to different host plants. However, we found parental effects and indications that seasonal length could indirectly affect specialization through influencing larval growth and development.

Overall, this thesis provides new insights into the physiological, molecular, and evolutionary mechanisms underlying host plant utilization and how they might constrain the evolution of host repertoires in polyphagous species.