Distinctive gene expression in the reduced first thoracic legs of a nymphalid butterfly

Nymphalid butterflies have unique leg morphology among Lepidoptera: they are the only family with greatly reduced forelegs (T1) in adults of both sexes, which are not used for walking. Previous studies have suggested that T1 legs may have chemosensory functions. To investigate which genes underpin this biology, we undertook a differential gene expression analysis in female Maniola jurtina. We find that nymphalid T1 legs have a distinct transcriptomic profile to T2 and T3 legs and also distinct from sensory palps. We find over 250 genes commonly expressed in nymphalid T1 legs and palps, but not in T2 legs. Despite this overlap, T1 legs are still more similar to T2 and T3 legs in their gene expression profiles than they are to palps. Genes expressed in common between palps and T1 legs include one encoding a trypsin-domain protein descendent from a nymphalid-specific duplication and several others involved in sensory functions, including genes with putative chemosensory roles. A blue-sensitive opsin gene is specifically expressed in T1 legs. Our findings indicate clear transcriptomic differences between T1 legs and walking legs in M. jurtina, pointing to the functional basis of these differences, including minor acquisition of palp-like gene expression.