I'm learning about development via the example of Drosophila embryogenesis. I understand that bicoid regulates hunchback, among other genes. My question whether the regulation is direct or indirect? In other words, does the level of bicoid directly govern the expression of hunchback, or are there steps in between?
These two papers  argue that bcd is a direct activator of hb, although be aware that this does not rule out downstream events feeding back to further activate hb. (In fact, given the complexity of the embryonic gene network, it is likely that both mechanisms have a role.)
- Struhl G, Struhl K, Macdonald PM. 1989. The gradient morphogen bicoid is a concentration-dependent transcriptional activator. Cell, 57(7): 1259-1273, doi:10.1016/0092-8674(89)90062-7.
- Driever W, Nüsslein-Volhard C. 1989. The bicoid protin is a positive regulator of hunchback transcription in the early Drosophila embryo. Nature, 337: 138-143.