A cytokine secreted from the suboesophageal body is essential for morphogenesis of the insect head

The suboesophageal body of insects was identified over a century ago in the silkworm embryo, but its biological function is still unknown. We discovered that this tissue is differentiated in the earliest embryonic stages of the cabbage armyworm and secretes the insect cytokine, growth-blocking peptide (GBP), transiently from 24 to 60 h after oviposition when gastrulation is in progress. Over-expression of GBP, achieved by microinjection of the GBP gene driven by a cytomegalovirus (CMV) constitutive promoter, resulted in complex deformities of the procephalon (embryonic head). Severe abnormal phenotypes of the head structure were produced by silencing the GBP expression in the embryo by treating with GBP double-stranded RNA: the procephalon-containing optic lobes diminished and completely separated into bilateral halves. This indicates that GBP secreted from the suboesophageal body plays an essential role in the formation of the procephalic domain during early embryogenesis. The cytokine-induced fusion of bilateral procephalic lobes is thought to be evolutionarily conserved at least in insects, because of the widespread occurrence of the suboesophageal body in insect embryos.