Larval transcriptomes reflect the evolutionary history of plant-insect associations

We performed a phylogenetically informed study comparing the transcriptomes of four nymphalid butterfly species, three in Polygonia and one in the closely related genus Nymphalis. Larvae fed on Urtica dioica, Salix spp. and Ribes spp. Plant-specific gene expression was found to be similar across butterfly species, even if the plant is no longer a host. These results suggest that plant-specific transcriptomes can be robust over evolutionary time, and we propose that they can be understood as a set of modules of co-expressed genes. Such conserved modules may provide a partial explanation for phylogenetic conservatism of host use in phytophagous insects, including re-colonization of ancestral hosts that have been lost from the repertoire.