Comparative transcriptome analysis at the onset of speciation in a mimetic butterfly, the Ithomiini Melinaea marsaeus.

Ecological speciation entails divergent selection on specific traits, and ultimately on the developmental pathways responsible for these traits. Selection can act on gene sequences, but also on regulatory regions responsible for gene expression. Mimetic butterflies are a relevant system for speciation studies because wing color pattern (WCP) often diverges between closely related taxa, and is thought to drive speciation through assortative mating and increased predation on hybrids. Here we generate the first transcriptomic resources for a mimetic butterfly of the tribe Ithomiini, Melinaea marsaeus, to examine patterns of differential expression between two subspecies and between tissues that express traits that likely drive reproductive isolation; WCP and chemosensory genes. We sequenced whole transcriptomes of three life stages to cover a large catalogue of transcripts and we investigated differential expression between subspecies in pupal wing discs and antennae. Eighteen known WCP genes were expressed in wing discs and 115 chemosensory genes were expressed in antennae, with a remarkable diversity of chemosensory protein genes. Many transcripts were differentially expressed between subspecies, including two WCP genes and one odorant receptor. Our results suggest that in M. marsaeus the same genes as in other mimetic butterflies are involved in traits causing reproductive isolation, and point at possible candidates for the differences in those traits between subspecies. Differential expression analyses of other developmental stages and body organs and functional studies are needed to confirm and expand these results. Our work provides key resources for comparative genomics in mimetic butterflies, and more generally in Lepidoptera.