Hypothetical interference of aUbx and Ubx/Antp transcription with Ubx and Antp expression. (A) Inversion model explaining complementary expression patterns of Ubx and aUbx based on the hypothetical conservation of bxd/aUbx promoters. In a hypothetical ancestor of myriapods, bxd/aUbx and Ubx must be inverted (inversion_1) (indicated by question mark). A second inversion must have transferred Ubx back onto the leading strand. (B) De novo evolution of bxd-like aUbx promoter on the complementary strand upstream of Ubx. (C) Inferred negative interference of bicistronic transcription on Antp expression. If Ubx/Antp does not code for UBX protein, its expression may also repress monocistronic (translated) Ubx in Glomeris. Promoters and direction of transcription are indicated by arrows; genes are represented by shaded bars; crossbars indicate for the split Hox cluster in Drosophila; relevant transcripts are highlighted by coloured bars. Blue T-bars indicate suggested repressor function. Question marks indicate hypothetical functions/facts. Light shading indicates areas involved in inversion events.