Stochastic mechanisms diversify cell fates during development. How cells randomly choose between two or more fates remains poorly understood. In the Drosophila eye, the random mosaic of two R7 photoreceptor subtypes is determined by expression of the transcription factor Spineless (Ss). We investigated how cis-regulatory elements and trans factors regulate nascent transcriptional activity and chromatin compaction at the ss gene locus during R7 development. The ss locus is in a compact state in undifferentiated cells. An early enhancer drives transcription in all R7 precursors, and the locus opens. In differentiating cells, transcription ceases and the ss locus stochastically remains open or compacts. In Ss^ON R7s, ss is open and competent for activation by a late enhancer, whereas in Ss^OFF R7s, ss is compact, and repression prevents expression. Our results suggest that a temporally dynamic antagonism, in which transcription drives large-scale decompaction and then compaction represses transcription, controls stochastic fate specification.

随机机制使发育过程中的细胞命运多样化。细胞如何在两种或多种命运之间随机选择仍然知之甚少。在果蝇眼睛中，两种 R7 光感受器亚型的随机镶嵌是由转录因子 Spineless (Ss) 的表达决定的。我们研究了顺式调节元件和反式调节元件在 R7 发育过程中如何调节ss基因座的新生转录活性和染色质压缩。ss位点在未分化细胞中处于致密状态。早期增强子驱动所有 R7 前体中的转录，并且基因座打开。在分化细胞中，转录停止，ss基因座随机保持开放或紧凑。在Ss ^ON R7s中，ss是开放的并且能够被晚期增强子激活，而在Ss ^OFF R7s中，ss是紧凑的，并且抑制会阻止表达。我们的结果表明，时间动态拮抗作用控制着随机命运规范，其中转录驱动大规模解压缩，然后压缩抑制转录。