Before the Black Box

Abstract

Ecdysteroid hormones are responsible for regulating arthropod moulting, but unlike other taxa which used sterol-derived hormones (e.g. vertebrate sex-determining hormones), arthropods cannot synthesis sterols de novo. Ecdysteroid synthesis is thus dependent on funnelling diverse dietary sterols into the conserved reactions of ecdysteroidogenesis. While the later steps of ecdysteroid synthesis are dominated by cytochrome P450s, earlier stages of this pathway (i.e. those acting in or before the Black Box in which 7-dehydrocholesterol is converted to 5,22,25-trideoxyecdysone) involve more diverse enzymes. While numerous comparative analyses of arthropod cytochrome P450s have been conducted, little comparable work has been done on the pre-Black Box FAD-dependent oxidoreductase DHCR24 and the Rieske domain oxygenase Neverland and the Black Box short-chain dehydrogenase/reductase Shroud. However, there is some evidence for evolutionary shifts among these enzymes across arthropods, e.g. the absence of DHCR24 from Drosophila melanogaster. We have sought to systematically identify such changes in copy number in DHCR24, Neverland and Shroud across arthropods, as well as to identify correlations between these changes and dietary factors with reference to the literature. Among other results, we find that Neverland is absent from all beetles (providing support for mycophagy as an important element of the beetle ancestral diet) and that losses of DHCR24 correlate with production of alkylated ecdysteroids.