<i>Soul</i> is a master control gene governing the development of the <i>Drosophila</i> prothoracic gland

The prothoracic gland (PG) is a major insect endocrine organ. It is the principal source of insect steroid hormones, and critical for key developmental events such as the molts, the establishment of critical weight (CW), pupation, and sexual maturation. However, little is known about the developmental processes that regulate PG morphology. In this study, we identified <i>soul</i>, which encodes a PG-specific basic helix-loop-helix (bHLH) transcription factor. We demonstrate that Tap, also a bHLH protein, dimerizes with Soul. Both are expressed in the developing PG. Interfering with either <i>soul</i> or <i>tap</i> function caused strikingly similar phenotypes, resulting in small and fragmented PGs, the abolishment of steroid hormone-producing gene expression, larval arrest, and a failure to undergo metamorphosis. Furthermore, both <i>soul</i> and <i>tap</i> showed expression peaks just prior to the CW checkpoint. Disrupting <i>soul-</i> or <i>tap-</i>function before, but not after, the CW checkpoint caused larval arrest, and perturbed highly similar gene cohorts, which were enriched for regulators and components of the steroid hormone biosynthesis pathway. Intriguingly, a chitin-based cuticle gene, <i>Cpr49Ah</i>, and a POU domain transcription factor gene, <i>pdm3</i>, are direct target genes of the Soul/Tap complex, and disruption of either phenocopied key aspects of <i>soul/tap</i> loss-of-function phenotypes. Taken together, our findings demonstrate that the Soul/Tap heterodimer resides at the top of a complex gene hierarchy that drives PG development, CW establishment, and steroid hormone production.