Periodic Wnt1 expression in response to ecdysteroid generates twin-spot markings on caterpillars

Among various pigmentation patterns on caterpillars, sequential spot markings are often observed and used for aposematic colouration. In contrast to adult wings, caterpillar cuticle markings are repeatedly generated at each moult, but little is known about how the patterns are formed and maintained periodically. Here we focus on a silkworm mutant, multi lunar (L), with twin-spot markings on sequential segments. Positional cloning of L and expression analyses reveal that cis-regulatory change in Wnt1 is responsible for the spot patterning. The periodical upregulation of Wnt1 in response to ecdysteroid is detected only in epidermis within spot marking area. We verify by transgenic expression that the ectopic Wnt1 induces the additional pigmentation. Furthermore, the association of Wnt1 expression with spot markings is observed in the wild Bombyx species and swallowtail butterfly Papilio machaon. Taken together, we anticipate that periodic Wnt1 expression may contribute to natural variations of spot patterning on caterpillar cuticle.