The Physics of Blastoderm Flow during Early Gastrulation of Tribolium castaneum

The early embryo of the red flour beetle, Tribolium castaneum, initially consists of a single-layered blastoderm covering the yolk uniformly that differentiates into an embryonic rudiment as well as extraembryonic amnion and serosa. The germband forms inside the egg when the embryonic rudiment condenses and folds along the ventral midline; this process is accompanied by large-scale flow and expansion of the serosa which ultimately covers the entire surface of the egg. The mechanical properties of these tissues and the forces governing gastrulation in Tribolium are poorly understood. Here, we present our findings on the dynamics of myosin in the early blastoderm of Tribolium using multiview lightsheet live imaging. We quantify the global distribution of myosin throughout the flow phase and present a physical description that couples the contractile forces generated by myosin to the mechanical properties of the blastoderm. In particular, we describe the overall tissue as a thin, actively contractile, viscous bulk medium that exhibits friction with the vitelline membrane. This description accurately captures the large-scale deformation the tissue undergoes during the initial stages of gastrulation, thus laying a foundation for the physical description of gastrulation in Tribolium.