Micklem D R, Dasgupta R, Elliott H, Gergely F, Davidson C, Brand A, González-Reyes A, St Johnston D
Wellcome/CRC Institute, Department of Genetics, University of Cambridge, Cambridge, UK.
Curr Biol. 1997 Jul 1;7(7):468-78. doi: 10.1016/s0960-9822(06)00218-1.
Drosophila axis formation requires a series of inductive interactions between the oocyte and the somatic follicle cells. Early in oogenesis, Gurken protein, a member of the transforming growth factor alpha family, is produced by the oocyte to induce the adiacent follicle cells to adopt a posterior cell fate. These cells subsequently send an unidentified signal back to the oocyte to induce the formation of a polarised microtubule array that defines the anterior-posterior axis. The polarised microtubules also direct the movement of the nucleus and gurken mRNA from the posterior to the anterior of the oocyte, where Gurken signals a second time to induce the dorsal follicle cells, thereby polarising the dorsal-ventral axis.
In addition to its previously described role in the localisation of oskar mRNA, the mago nashi gene is required in the germ line for the transduction of the polarising signal from the posterior follicle cells. Using a new in vivo marker for microtubules, we show that mago nashi mutant oocytes develop a symmetric microtubule cytoskeleton that leads to the transient localisation of bicoid mRNA to both poles. Furthermore, the oocyte nucleus often fails to migrate to the anterior, causing the second Gurken signal to be sent in the same direction as the first. This results in a novel phenotype in which the anterior of the egg is ventralised and the posterior dorsalised, demonstrating that the migration of the oocyte nucleus determines the relative orientation of the two principal axes of Drosophila. The mago nashi gene is highly conserved from plants to animals, and encodes a protein that is predominantly localised to nuclei.
The mago nashi gene plays two essential roles in Drosophila axis formation: it is required downstream of the signal from the posterior follicle cells for the polarisation of the oocyte microtubule cytoskeleton, and has a second, independent role in the localisation of oskar mRNA to the posterior of the oocyte.
果蝇轴的形成需要卵母细胞与体细胞滤泡细胞之间进行一系列诱导相互作用。在卵子发生早期，转化生长因子α家族成员Gurken蛋白由卵母细胞产生，以诱导相邻的滤泡细胞采用后细胞命运。这些细胞随后向卵母细胞发送一个未知信号，以诱导形成定义前后轴的极化微管阵列。极化微管还引导细胞核和Gurken mRNA从卵母细胞的后部移动到前部，在那里Gurken再次发出信号以诱导背侧滤泡细胞，从而使背腹轴极化。
除了其先前描述的在oskar mRNA定位中的作用外，mago nashi基因在生殖系中是后滤泡细胞极化信号转导所必需的。使用一种新的微管体内标记，我们表明mago nashi突变型卵母细胞发育出对称的微管细胞骨架，导致bicoid mRNA短暂定位于两极。此外，卵母细胞核常常无法迁移到前部，导致第二个Gurken信号与第一个信号沿相同方向发送。这导致了一种新的表型，即卵的前部腹侧化而后部背侧化，表明卵母细胞核的迁移决定了果蝇两个主轴的相对方向。mago nashi基因从植物到动物高度保守，编码一种主要定位于细胞核的蛋白质。
mago nashi基因在果蝇轴形成中起两个重要作用：它在来自后滤泡细胞的信号下游是卵母细胞微管细胞骨架极化所必需的，并且在oskar mRNA定位于卵母细胞后部方面具有第二个独立作用。
