Abstract
Spine-like or protruding structures, which may be aposematic for predators, are often observed in multiple segments of lepidopteran larvae (caterpillars). For example, the larvae of the Chinese wheel butterfly, Byasa alcinous, display many protrusions on their backs as a warning that they are toxic. Although these protrusions are formed by an integument lined with single-layered epidermal cells, the molecular mechanisms underlying their formation have remained unclear. In this study, we focused on a spontaneous mutant of the silkworm, Bombyx mori, Knobbed, which shows similar protrusions to B. alcinous and demonstrates that Wnt1 plays a crucial role in the formation of protrusion structures. Using both transgene expression and RNAi-based knockdown approaches, we showed that Wnt1 designates the position where epidermal cells excessively proliferate, leading to the generation of knobbed structures. Furthermore, in the B. alcinous larvae, Wnt1 was also specifically expressed in association with the protrusions. Our results suggest that Wnt1 plays a role in the formation of protrusions on the larval body, and is conserved broadly among diverse species in Lepidoptera.
Highlights
IntroductionSome lepidopteran species exhibit aposematic colors and shapes on their larval bodies, such as pairs of protruding structures in Byasa alcinous (Fig. 1A) and spot pigmentation in some Papilio species [1, 2], which may involve their defensive strategies
Insects have evolved various ways to avoid predation
We have demonstrated that Wnt1 designates the region where protrusions emerge on the flat cuticle of the larval body in the K mutant of B. mori
Summary
Some lepidopteran species exhibit aposematic colors and shapes on their larval bodies, such as pairs of protruding structures in Byasa alcinous (Fig. 1A) and spot pigmentation in some Papilio species [1, 2], which may involve their defensive strategies. Protruding structures on the larval body are among the conserved structures characterized in troidine swallowtails (Lepidoptera: Papilionidae) and are often observed in distantly related species such as the nymphalid butterfly, Hestina assimilis (Fig. 1B), and ailanthus silkworm Samia cynthia pryeri (Fig. 1C). Bombyx mori, hundreds of spontaneous mutants have been obtained, many of which involve larval characteristics. Knobbed (K), one of these larval mutants, shows characteristic knobs (protrusions) paired on specific dorsal regions in the 2nd, 3rd, 5th and 8th
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