Diversification of neuronal subtypes often requires stochastic gene regulatory mechanisms. How stochastically expressed transcription factors interact with other regulators in gene networks to specify cell fates is poorly understood. The random mosaic of color-detecting R7 photoreceptor subtypes in Drosophila is controlled by the stochastic on/off expression of the transcription factor Spineless (Ss). In Ss^ON R7s, Ss induces expression of Rhodopsin 4 (Rh4), whereas in Ss^OFF R7s, the absence of Ss allows expression of Rhodopsin 3 (Rh3). Here, we find that the transcription factor Runt, which is initially expressed in all R7s, is sufficient to promote stochastic Ss expression. Later, as R7s develop, Ss negatively feeds back onto Runt to prevent repression of Rh4 and ensure proper fate specification. Together, stereotyped and stochastic regulatory inputs are integrated into feedforward and feedback mechanisms to control cell fate.

神经元亚型的多样化通常需要随机基因调控机制。随机表达的转录因子如何与基因网络中的其他调节因子相互作用以指定细胞命运尚不清楚。果蝇中颜色检测 R7 光感受器亚型的随机镶嵌受转录因子 Spineless (Ss) 的随机开/关表达控制。在 Ss ^ON R7s 中，Ss 诱导视紫红质 4 (Rh4) 的表达，而在 Ss ^OFF中R7s，Ss 的缺失允许视紫红质 3 (Rh3) 的表达。在这里，我们发现最初在所有 R7 中表达的转录因子 Runt 足以促进随机 Ss 表达。后来，随着 R7s 的发展，Ss 负反馈到 Runt 以防止抑制 Rh4 并确保正确的命运规范。总之，刻板和随机的调节输入被整合到前馈和反馈机制中，以控制细胞命运。