Extensive transcriptional response associated with seasonal plasticity of butterfly wing patterns

Abstract

In the eastern United States, the buckeye butterfly, Junonia coenia, shows seasonal wing colour plasticity where adults emerging in the spring are tan, while those emerging in the autumn are dark red. This variation can be artificially induced in laboratory colonies, thus making J. coenia a useful model system to examine the mechanistic basis of plasticity. To better understand the developmental basis of seasonal plasticity, we used RNA-seq to quantify transcription profiles associated with development of alternative seasonal wing morphs. Depending on the developmental stage, between 547 and 1420 transfrags were significantly differentially expressed between morphs. These extensive differences in gene expression stand in contrast to the much smaller numbers of differentially expressed transcripts identified in previous studies of genetic wing pattern variation in other species and suggest that environmentally induced phenotypic shifts arise from very broad systemic processes. Analyses of candidate endocrine and pigmentation transcripts revealed notable genes upregulated in the red morph, including several ecdysone-associated genes, and cinnabar, an ommochrome pigmentation gene implicated in colour pattern variation in other butterflies. We also found multiple melanin-related transcripts strongly upregulated in the red morph, including tan and yellow-family genes, leading us to speculate that dark red pigmentation in autumn J. coenia may involve nonommochrome pigments. While we identified several endocrine and pigmentation genes as obvious candidates for seasonal colour morph differentiation, we speculate that the majority of observed expression differences were due to thermal stress response. The buckeye transcriptome provides a basis for further developmental studies of phenotypic plasticity.

Keywords: Junonia coenia; ecdysone; ommochrome; phenotypic plasticity; polyphenism; transcriptome; wing patterns.

Fig. 1

Seasonal variation and wing development in Junonia coenia. (a) Different light and temperature conditions can induce North Carolina J. coenia to display extremely different seasonal morphs. (b) The four developmental stages sampled in this study: (1) late 5th instar hindwing discs, (2) preommochrome pupal hind-wings, (3) early ommochrome pupal hindwings, and (4) late ommochrome pupal hindwings. (c) Comparisons of transfrag similarity between Heliconius melpomene, Danaus plexippus and J. coenia using OrthoMCL from amino acid sequences. Note that the complete genomes have been sequenced for H. melpomene and D. plexippus, while J. coenia is represented only by wing tissue-specific transcripts. (d) A unified heat map illustrating the differential expression of 3145 loci between four developmental stages and two colour morphs of J. coenia. The diagram shows the relative expression levels from all samples compared against background levels. The relative levels of upregulation or downregulation are represented by a coloured field that ranges from yellow for highly upregulated genes to blue for highly downregulated genes.

Fig. 2

Stage- and phenotype-specific expression levels (FPKM) of selected ecdysone-associated transfrags that show significant differences between induced seasonal morphs. Stages are as shown in Fig. 1b. (*P < 0.05; **P < 0.005).

Fig. 3

Stage- and phenotype-specific expression levels (FPKM) of selected ommochrome-associated transfrags that show significant differences between induced seasonal morphs. Stages are as shown in Fig. 1b. (*P < 0.05; **P < 0.005).

Fig. 4

Stage- and phenotype-specific expression levels (FPKM) of melanin-associated transfrags that show significant differences between induced seasonal morphs. Only transfrags showing differences during pupal development are shown. Stages are as shown in Fig. 1b. (*P < 0.05; **P < 0.005).