Role of Repressors in Fine Regulation of Development: Sxl and Its New Repressors Hey and Myc

In Drosophila, XX embryos express Sxl from the early promoter, SxlPe, and become females. At the same time, XY embryos with only one X chromosome become males. I investigated the role of repression in the establishment of the strict regulation of SxlPe. I found that the co-repressor Groucho, is responsible for amplification of the two-fold difference in X-encoded activator genes into an all-or-nothing difference in Sxl expression. Three new basic helix-loop-helix repressors of Sxl were identified: Hey, Cwo and the prooncogene Myc, all of which are maternally supplied. I have shown that Myc specific repression is important as early as cycle 10, which is 2 cycles earlier than the onset of normal Sxl expression.