Figure 5.

A proposal for the evolution of mirrregulation in D. melanogaster. (A) Two separate input modules regulate mirr expression: the green module uses only the input of the EGFr pathway via Cic, whereas the orange module requires both EGFr and Dpp input. (B) The two regulatory modules drive different expression levels of mirr. pip repression requires only low levels of mirr, which are provided through the green module, whereas the br(L) enhancer is activated only when mirr levels are sufficiently high, which is achieved through the orange module. (C) A proposal for the evolution of two regulatory modules, using two enhancers. A single enhancer (mirLo) is responsive to EGFr signaling only, and is sufficient to provide the low level of mirr expression required to repress pip as part of an ancestral signaling cassette. A second enhancer (mirHi) has evolved in Drosophila, which now drives mirr expression in response to both EGFr and Dpp signaling, in high levels that are sufficient for the activation of the br(L) enhancer.