Comparative transcriptomics of albino and warningly coloured caterpillars

Colouration is perhaps one of the most prominent adaptations for survival and reproduction of most taxa. Colouration is of particular importance for aposematic species, which rely on their colouring and patterning to act as a warning signal against predators. Most research has focused on the evolution of warning colouration by natural selection. However, little information is available for colour mutants of aposematic species, particularly at the genomic level. Here I compare the transcriptomes of albino mutant caterpillars of the wood tiger moth (Arctia plantaginis) to those of their full-sibs having their distinctive orange-black warning colouration. The results showed >300 differentially expressed genes transcriptome-wide. Genes involved in the immune system, structural constituents of cuticle, and peptidase activity were mostly down-regulated in the albino larvae. Surprisingly, higher expression was observed in core melanin genes from albino larvae, suggesting that melanin synthesis may be disrupted in terminal ends of the pathway during its final conversion. I further identified 25 novel genes uniquely expressed in the albino larvae. Functional annotation showed that these genes are involved in nucleotide biosynthesis, copper ion transmembrane transport, and nucleic acid binding. Taken together, these results suggest that caterpillar albinism may not be due to a depletion of melanin precursor genes. In contrast, the albino condition may result from the combination of faulty melanin conversion late in its synthesis and structural deficiencies in the cuticle preventing its deposition.