The structure of the USP/RXR of Xenos pecki indicates that Strepsiptera are not closely related to Diptera.

The receptor for the insect molting hormone, ecdysone, is a heterodimer consisting of the Ecdysone Receptor and Ultraspiracle (USP) proteins. The ligand binding domain sequences of arthropod USPs divide into two distinct groups. One group consists of sequences from members of the holometabolous Lepidoptera and Diptera, while the other arthropod sequences group with vertebrate retinoid-X-receptors (RXRs). We therefore wondered whether USP/RXR structure could be used to clarify the contentious phylogenetic position of the order Strepsiptera, which has proposed affinities with either Diptera or Coleoptera. We have cloned and sequenced the USP/RXR from the strepsipteran Xenos pecki. Phylogenetic analyses are not consistent with a close affinity between Strepsiptera and Diptera.