Proper neural function depends on the correct specification of individual neural fates, controlled by combinations of neuronal transcription factors. Different neural types are sequentially generated by neural progenitors in a defined order, and this temporal patterning process can be controlled by Temporal Transcription Factors (TTFs) that form temporal cascades in neural progenitors. The Drosophila medulla, part of the visual processing center of the brain, contains more than 70 neural types generated by medulla neuroblasts which sequentially express several TTFs, including Homothorax (Hth), eyeless (Ey), Sloppy paired 1 and 2 (Slp), Dichaete (D) and Tailless (Tll). However, it is not clear how such a small number of TTFs could give rise to diverse combinations of neuronal transcription factors that specify a large number of medulla neuron types. Here we report how temporal patterning specifies one neural type, the T1 neuron. We show that the T1 neuron is the only medulla neuron type that expresses the combination of three transcription factors Ocelliless (Oc or Otd), Sox102F and Ets65A. Using CRISPR-Cas9 system, we show that each transcription factor is required for the correct morphogenesis of T1 neurons. Interestingly, Oc, Sox102F and Ets65A initiate expression in neurons beginning at different temporal stages and last in a few subsequent temporal stages. Oc expressing neurons are generated in the Ey, Slp and D stages; Sox102F expressing neurons are produced in the Slp and D stages; while Ets65A is expressed in subsets of medulla neurons born in the D and later stages. The TTF Ey, Slp or D is required to initiate the expression of Oc, Sox102F or Ets65A in neurons, respectively. Thus, the neurons expressing all three transcription factors are born in the D stage and become T1 neurons. In neurons where the three transcription factors do not overlap, each of the three transcription factors can act in combination with other neuronal transcription factors to specify different neural fates. We show that this way of expression regulation of neuronal transcription factors by temporal patterning can generate more possible combinations of transcription factors in neural progeny to diversify neural fates.

正确的神经功能取决于个体神经命运的正确规范，并由神经元转录因子的组合控制。不同的神经类型由神经祖细胞按照定义的顺序依次生成，并且这种时间模式过程可以由在神经祖细胞中形成时间级联的时间转录因子（TTF）控制。果蝇髓质是大脑视觉处理中心的一部分，包含由髓质神经母细胞产生的 70 多种神经类型，这些神经类型依次表达多种 TTF，包括 Homothorax (Hth)、eyeless (Ey)、Sloppypaired 1 and 2 (Slp)、 Dichaete (D) 和 Tailless (Tll)。然而，尚不清楚如此少量的 TTF 如何产生指定大量髓质神经元类型的神经元转录因子的不同组合。在这里，我们报告时间模式如何指定一种神经类型，即 T1 神经元。我们发现 T1 神经元是唯一表达三种​​转录因子 Ocelliless（Oc 或 Otd）、Sox102F 和 Ets65A 组合的髓质神经元类型。使用 CRISPR-Cas9 系统，我们证明每个转录因子都是 T1 神经元正确形态发生所必需的。有趣的是，Oc、Sox102F 和 Ets65A 在神经元中从不同的时间阶段开始表达，并在随后的几个时间阶段持续。表达 Oc 的神经元在 Ey、Slp 和 D 阶段产生；表达 Sox102F 的神经元在 Slp 和 D 阶段产生；而 Ets65A 在 D 期及后期出生的髓质神经元亚群中表达。 TTF Ey、Slp 或 D 分别需要启动神经元中 Oc、Sox102F 或 Ets65A 的表达。 因此，表达所有三种转录因子的神经元在 D 阶段诞生并成为 T1 神经元。在三个转录因子不重叠的神经元中，三个转录因子中的每一个都可以与其他神经元转录因子结合起作用，以指定不同的神经命运。我们表明，这种通过时间模式对神经元转录因子进行表达调节的方式可以在神经后代中产生更多可能的转录因子组合，从而使神经命运多样化。