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

Abstract

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.

The transcription factor Apontic-like controls diverse colouration pattern in caterpillars

“…We recently found a specialized cuticular structure designated 'bulge' in the crescent marking area, in which Wnt1 was ectopically expressed and that Wnt1 expression in the larval epidermis had the potential to cause twin spot markings 32 . In this study, using in vivo electroporation, we observed that ectopically expressed Wnt1 induced apt-like expression and melanin pigmentation in þ p epidermis but not in p epidermis (Fig.…”

“…Notably, larval markings should be repeatedly redrawn at each larval moult, which is different from adult pigmentation that occurs only once during metamorphosis. We recently showed that the moulting hormone ecdysteroid possibly regulated larval pigmentation through the function of Wnt1 in lepidopteran spot markings 32 or E75 in eye spots of the swallowtail butterfly 52 . We speculate that in contrast to adult pigmentation, module-type control of gene expression of the melanin synthesis genes is necessary for repeated pigmentation in the larval epidermis.…”

“…We previously found that the melanin synthesis genes yellow and ebony were responsible for the silkworm colour mutants chocolate (ch) and sooty (so) loci, respectively 31 . In addition, we recently found that L resulted from the ectopic expression of Wnt1 in the epidermis 32 . These studies demonstrated that silkworm mutants are useful resources for understanding colour pattern formation.…”

“…We previously showed that the ectopic expression of Wnt1 caused melanin pigmentation in the larval epidermis of the þ p phenotype 32 . Here we attempted to determine the genetic pathways among apt-like, Wnt1 and Distal-less (Dll), using in vivo electroporation as described above.…”

Morphological Evolution Repeatedly Caused by Mutations in Signaling Ligand Genes

“…While these studies lack the phylogenetic resolution and replication observed in butterflies, they provide one of the most detailed mechanistic accounts of truly novel traits, where the deployment of Wnt expression in three different body regions is driven by independent cis-regulatory changes. Of note, wg is also associated to color patterns and wing contours in both flies and butterflies (Macdonald et al 2010;Martin and Reed 2010;Koshikawa et al 2015), and a redeployment of this gene to new body regions is likely to drive the evolution of new patterns as well, as it seemed to have occurred during the evolution of larval cuticle patterns in Lepidoptera (Yamaguchi et al 2013). We note that while Koshikawa et al did not detect any pattern-related Wnt6 and Wnt10 expression in D. guttifera developing wings ; S. Koshikawa, personal communication), these two paralogs are co-deployed with wg in butterflies where they may underlie a more complex architecture, with partially redundant ligand activities (Martin and Reed 2014).…”