Parkhurst S M, Lipshitz H D, Ish-Horowicz D
Division of Basic Sciences, Fred Hutchinson Cancer Research Center, Seattle, Washington 98104.
Development. 1993 Feb;117(2):737-49. doi: 10.1242/dev.117.2.737.
Sex determination in Drosophila depends on X-linked 'numerator' genes activating early Sex-lethal (Sxl) transcription in females. One numerator gene, sisterless-b (sis-b), corresponds to the achaete-scute (AS-C) T4 basic-helix-loop-helix (bHLH) gene. Two other closely related AS-C bHLH genes, T3 and T5, appear not to function as numerator elements. We analyzed endogenous AS-C expression and show that T4 is the major AS-C numerator gene because it is expressed earlier and more strongly than are T3 and T5. Only T4 expression is detectable during the early syncytial stages when Sxl state is being determined. Nevertheless, the effects of ectopic AS-C gene expression show that T3 and T5 proteins display weak but significant feminizing activities, enhancing male-lethality, and rescuing the female-lethality of sis mutations. Detailed examination of Sxl expression in rescued embryos suggests that female cells may be viable in the absence of detectable Sxl protein expression.
果蝇的性别决定取决于X连锁的“分子”基因在雌性中激活早期性致死基因（Sex-lethal，Sxl）的转录。其中一个分子基因，无姐妹基因b（sisterless-b，sis-b），对应于achaete-scute（AS-C）T4碱性螺旋-环-螺旋（bHLH）基因。另外两个密切相关的AS-C bHLH基因，T3和T5，似乎不发挥分子元件的作用。我们分析了内源性AS-C的表达，结果表明T4是主要的AS-C分子基因，因为它比T3和T5表达得更早且更强。在决定Sxl状态的早期合胞体阶段，只有T4的表达可以检测到。然而，异位AS-C基因表达的结果表明，T3和T5蛋白表现出微弱但显著的雌性化活性，增强了雄性致死率，并挽救了sis突变体的雌性致死性。对挽救胚胎中Sxl表达的详细检查表明，在没有可检测到的Sxl蛋白表达的情况下，雌性细胞可能是存活的。
