Broad complex and wing development in cockroaches

In hemimetabolan insects, the transcription factor Broad complex (Br-C) promotes wing growth and development during the nymphal period. We wondered whether Br-C could trigger the initiation of wing development, using the cockroach Blattella germanica as a model. We show that first instar nymphs have their unique identity of these three thoracic segments specified. During embryogenesis, the expression of Br-C and some wing-related genes show two matching waves. The first takes place before the formation of the germ band, which might be involved in the establishment of various developmental fields including a potential "wing field", and the second wave around organogenesis, possibly involved in the initiation of wing development. However, the expression of Br-C in early embryogenesis concentrates in the developing central nervous system, thus not co-localizing with the expression of the typical wing-related gene vestigial, which is expressed at the edge of the thoracic and abdominal segments. This suggests that Br-C is not specifically involved in the establishment of a potential "wing field" in early embryogenesis. Moreover, maternal RNAi for Br-C depletes the first wave of Br-C expression but does not affect the early expression of wing-related genes. As maternal Br-C RNAi did not deplete the second expression wave of Br-C, we could not evaluate if Br-C is involved in the initiation of wing development. Alternatively, using nymphal RNAi of Br-C and Sex combs reduced (Scr), we show that Br-C contributes to the formation of ectopic wing structures that develop in the prothorax when Scr is depleted. The gene most clearly influenced by Br-C RNAi is nubbin (nub), which, in nymphs is crucial for wing growth. Together, these results suggest that Br-C does not specifically contribute to the establishment of the "wing field", but it does seem important later, in the initiation of wing development, enhancing the expression of wing-related genes, especially nub. This supports the hypothesis previously proposed by the authors, whereby Br-C might have facilitated the evolution of holometaboly. However, there is no doubt that other factors have also contributed to this evolution.