Toll ligand Spätzle3 controls melanization in the stripe pattern formation in caterpillars.

Abstract

A stripe pattern is an aposematic or camouflage coloration often observed among various caterpillars. However, how this ecologically important pattern is formed is largely unknown. The silkworm dominant mutant <i>Zebra</i> (<i>Ze</i>) has a black stripe in the anterior margin of each dorsal segment. Here, fine linkage mapping of 3,135 larvae revealed a 63-kbp region responsible for the <i>Ze</i> locus, which contained three candidate genes, including the Toll ligand gene <i>spätzle3</i> (<i>spz-3</i>). Both electroporation-mediated ectopic expression and RNAi analyses showed that, among candidate genes, only processed <i>spz-3</i> induced melanin pigmentation and that <i>Toll-8</i> was the candidate receptor gene of <i>spz-3</i> This Toll ligand/receptor set is also involved in melanization of other mutant <i>Striped</i> (<i>p</i>^<i>S</i> ), which has broader stripes. Additional knockdown of 5 other <i>spz</i> family and 10 <i>Toll-</i>related genes caused no drastic change in the pigmentation of either mutant, suggesting that only <i>spz-3/Toll-8</i> is mainly involved in the melanization process rather than pattern formation. The downstream pigmentation gene <i>yellow</i> was specifically up-regulated in the striped region of the <i>Ze</i> mutant, but <i>spz-3</i> showed no such region-specific expression. Toll signaling pathways are known to be involved in innate immunity, dorsoventral axis formation, and neurotrophic functions. This study provides direct evidence that a Toll signaling pathway is co-opted to control the melanization process and adaptive striped pattern formation in caterpillars.