Chemosensory receptor copy number evolution is not associated with host-plant specialization in butterflies

Chemoreception plays an important role in how insects sense their environment and interact with other organisms. One important ecological role for chemosensory receptors is sensing chemical compounds to locate suitable host plants. Host-plant use is therefore expected to influence insect sensory system evolution as host-plant preferences changes and species become more generalized or specialized. Yet it remains unclear if species that are generalists require a larger chemosensory receptor repertoire. Here, we investigated the relationship between host-plant specialization and the evolution of olfactory receptor (OR) and gustatory receptor (GR) gene families across 87 butterfly species spanning six families. We annotated OR and GR repertoires and used phylogenetic comparative analyses to test whether receptor copy-number was associated with specialist versus generalist host-use strategies or with host-plant breadth. In addition, we reconstructed evolutionary transitions between generalist and specialist strategies and tested whether these transitions were associated with expansions or contractions in gene-family size. We found that OR and GR repertoire size did not differ between specialists and generalists, nor was it correlated with the number of host-plant families used by a species. We also did not find associations between gene family expansions or contractions and transitions between generalist and specialist strategies. Overall, we find that host-plant specialization is not associated with chemosensory receptor repertoire size across butterflies. We suggest that host-plant chemosensory adaptation may depend on other changes such as receptor tuning, expression differences, or changes in specific receptor lineages not captured by our analyses.