A revised understanding of Tribolium morphogenesis further reconciles short and long germ development

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, multi-layered 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. 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 germband extension, and I argue that the flow is driven by a conserved set of underlying morphogenetic events in both species. Furthermore, the revised Tribolium fatemap that I present is far more similar to that of Drosophila than the classic Tribolium fatemap. 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.Author summaryIn many animals, certain groups of cells in the embryo do not directly contribute to adult structures. Instead, these cells generate so-called ‘extra-embryonic tissues’ that support and facilitate development, but degenerate prior to birth/hatching. In most insect species, embryos are described as having two major extra-embryonic tissues; the serosa, which encapsulates the entire embryo and yolk, and the amnion, which covers one side of the embryo. This tissue structure has been widely reported for over a century, but detailed studies on the amnion are lacking. Working in the beetle Tribolium castaneum, I used long-term fluorescent live imaging, cell tracking and differential cell labelling to investigate amnion development. In contrast to our current understanding, I show that most cells previously thought to be amnion actually form large parts of the embryo. In addition, I show how these cells ‘flow’ as a whole tissue and contribute to elongation of the embryo, and how only a relatively small number of cells form the actual amnion. Lastly, I describe how my findings show that despite exhibiting substantial differences in overall structure, embryos of Tribolium and the fruit fly, Drosophila melanogaster, utilise a conserved set of morphogenetic processes.