Pleiotropy in Drosophila organogenesis: Mechanistic insights from Combgap and the retinal determination gene network.

Master regulatory transcription factors cooperate in networks to shepherd cells through organogenesis. In the Drosophila eye, a collection of master control proteins known as the retinal determination gene network (RDGN) switches the direction and targets of its output to choreograph developmental transitions, but the molecular partners that enable such regulatory flexibility are not known. We recently showed that two RDGN members, Eyes absent (Eya) and Sine oculis (So), promote exit from the terminal cell cycle known as the second mitotic wave (SMW) to permit differentiation. A search for co-factors identified the ubiquitously expressed Combgap (Cg) as a novel transcriptional partner that impedes cell cycle exit and interferes with Eya-So activity specifically in this context. Here, we argue that Cg acts as a flexible transcriptional platform that contributes to numerous gene expression outcomes by a variety of mechanisms. For example, Cg provides repressive activities that dampen Eya-So output, but not by recruiting Polycomb chromatin-remodeling complexes as it does in other contexts. We propose that master regulators depend on both specifically expressed co-factors that assemble the combinatorial code and broadly expressed partners like Cg that recruit the diverse molecular activities needed to appropriately regulate their target enhancers.