de Saint Phalle B, Sullivan W
Department of Biology, University of California at Santa Cruz, 95064, USA.
Development. 1996 Dec;122(12):3775-84. doi: 10.1242/dev.122.12.3775.
Sex in Sciara coprophila is determined by maternally supplied factors that control the number of paternal X chromosomes eliminated during the syncytial embryonic divisions. Confocal microscopy and FISH demonstrate that the centromeres of the X chromosomes separate at anaphase and remain functional during the cycle in which the X chromosomes are eliminated. However, a region of the sister chromatids fails to separate and the X chromosomes remain at the metaphase plate. This indicates that failure of sister chromatid separation is the mechanism of chromosome elimination. Elimination of the X chromosomes requires the presence of a previously discovered Controlling Element that acts in cis during male meiosis. Using an X-autosome translocation, we demonstrate that the Controlling Element acts at-a-distance to prevent sister chromatid separation in the arm of an autosome. This indicates that the region in which sister chromatid separation fails is chromosome-independent. Although chromosome elimination occurs in all somatic nuclei and is independent of location of the nuclei within the embryo, the decision to eliminate is made at the level of the individual nucleus. Programmed X chromosome elimination occurs at different cycles in male and female embryos. These observations support a model in which elements on the X chromosome are titrating maternally supplied factors controlling the separation of sister X chromatids.
粪蝇（Sciara coprophila）的性别由母体提供的因子决定，这些因子控制着在合胞体胚胎分裂过程中被消除的父本X染色体的数量。共聚焦显微镜和荧光原位杂交显示，X染色体的着丝粒在后期分离，并在X染色体被消除的周期中保持功能。然而，姐妹染色单体的一个区域未能分离，X染色体仍留在中期板上。这表明姐妹染色单体分离失败是染色体消除的机制。X染色体的消除需要一个先前发现的控制元件的存在，该元件在雄性减数分裂过程中顺式作用。利用X-常染色体易位，我们证明控制元件远距离作用以防止常染色体臂上的姐妹染色单体分离。这表明姐妹染色单体分离失败的区域与染色体无关。虽然染色体消除发生在所有体细胞核中，并且与细胞核在胚胎内的位置无关，但消除的决定是在单个细胞核水平上做出的。程序性X染色体消除在雄性和雌性胚胎的不同周期发生。这些观察结果支持一个模型，其中X染色体上的元件正在滴定母体提供的控制姐妹X染色单体分离的因子。
