Department of Zoology, University of Cologne, Cologne, Germany.
PLoS Biol. 2018 Jul 3;16(7):e2005093. doi: 10.1371/journal.pbio.2005093. eCollection 2018 Jul.
In Drosophila melanogaster, the germband forms directly on the egg surface and solely consists of embryonic tissue. In contrast, most insect embryos undergo a complicated set of tissue rearrangements to generate a condensed, multilayered germband. The ventral side of the germband is embryonic, while the dorsal side is thought to be an extraembryonic tissue called the amnion. While this tissue organisation has been accepted for decades and has been widely reported in insects, its accuracy has not been directly tested in any species. Using live cell tracking and differential cell labelling in the short germ beetle Tribolium castaneum, I show that most of the cells previously thought to be amnion actually give rise to large parts of the embryo. This process occurs via the dorsal-to-ventral flow of cells and contributes to germband extension (GBE). In addition, I show that true 'amnion' cells in Tribolium originate from a small region of the blastoderm. Together, my findings show that development in the short germ embryos of Tribolium and the long germ embryos of Drosophila is more similar than previously proposed. Dorsal-to-ventral cell flow also occurs in Drosophila during GBE, and I argue that the flow is driven by a conserved set of underlying morphogenetic events in both species. Furthermore, the revised Tribolium fate map that I present is far more similar to that of Drosophila than the classic Tribolium fate map. Lastly, my findings show that there is no qualitative difference between the tissue structure of the cellularised blastoderm and the short/intermediate germ germband. As such, the same tissue patterning mechanisms could function continuously throughout the cellularised blastoderm and germband stages, and easily shift between them over evolutionary time.
在黑腹果蝇中，原肠胚直接在卵表面形成，仅由胚胎组织组成。相比之下，大多数昆虫胚胎经历了一系列复杂的组织重排，以产生一个浓缩的、多层的原肠胚。原肠胚的腹侧是胚胎组织，而背侧被认为是一种叫做羊膜的胚胎外组织。虽然这种组织学结构已经被接受了几十年，并在昆虫中得到了广泛的报道，但它在任何物种中的准确性都没有被直接测试过。使用活体细胞跟踪和短体甲虫 Tribolium castaneum 中的差异细胞标记，我表明，以前被认为是羊膜的大多数细胞实际上产生了胚胎的大部分。这个过程是通过细胞从背侧向腹侧的流动来实现的，并有助于原肠胚延伸（GBE）。此外，我还表明，Tribolium 中的真正的“羊膜”细胞来源于胚盘的一小部分区域。总的来说，我的研究结果表明，Tribolium 的短体胚胎和 Drosophila 的长体胚胎的发育比以前提出的更为相似。在 GBE 期间，Drosophila 中也发生了从背侧向腹侧的细胞流动，我认为这种流动是由两种物种中保守的一组基础形态发生事件驱动的。此外，我提出的经修订的 Tribolium 命运图谱与 Drosophila 的命运图谱更为相似，而不是经典的 Tribolium 命运图谱。最后，我的研究结果表明，细胞化胚盘和短体/中间原肠胚的组织结构之间没有质的区别。因此，相同的组织模式形成机制可以在细胞化胚盘和原肠胚阶段连续发挥作用，并在进化过程中轻松在它们之间转换。
