In our presentation on Friday, we didn’t really have time to talk about the specifics of how dorsal ventral patterning occurs in Drosophila. The key protein in this pathway is a morphogen called Dorsal, which is responsible for activating the genes associated with ventral identity. Dorsal is found throughout a developing embryo, but it only enters the nucleus of ventral cells, indicating that something else is responsible for signaling Dorsal to enter the nucleus. The Spz-Toll complex comes into play because cleavage of Spz only occurs on the ventral side of the embryo (and thus Toll is only activated on the ventral side). Activation of Toll turns on a kinase which frees Dorsal from its chaperone protein and allows Dorsal to enter the nucleus.