Ecdysone signaling controls early embryogenesis in the short-germ hemimetabolous insect Blattella germanica.

Steroid hormone signaling is a central regulator of insect development especially during the post-embryonic period. However, its role during embryonic development is less understood, particularly in short-germ band hemimetabolous insects. Here, we used the short-germ band hemimetabolous insect Blattella germanica to analyse the functions of 20-hydroxyecdysone (20E), the most active insect steroid, signaling during the embryonic stage. We show that the heterodimeric 20E receptor, BgEcR-A and BgRXR, is present from the beginning of embryogenesis and that a stereotypical 20E-dependent cascade of nuclear receptors is first detected at the early blastoderm stage. Using parental RNAi, we show that both receptors are required for the proper formation of the germ band by controlling cell proliferation during the blastoderm stage. In addition, they are also required for the coordinated induction of the 20E-induced cascade of nuclear receptors. Finally, we show that two of these nuclear receptors, BgHR3 and BgFTZ-F1, have relevant roles in germ band formation. Whereas BgHR3 controls the formation and development of the cephalic region of the germ band, BgFTZ-F1 stripped expression suggested a role in the segmentation of the germ band. In summary, our results show that 20E-signaling is required much earlier in short-germ band hemimetabolous than in long-germ band holometabolous embryogenesis, which raises the possibility that the loss of 20E-signaling during the initial stages of embryogenesis could be a key feature in the evolution of the different types of insect development.