32Many animal embryos pull and close an epithelial sheet around the spherical or ellipsoidal 33 egg surface during a gastrulation process known as epiboly. The ovoidal geometry 34 dictates that the epithelial sheet first expands and subsequently compacts. Moreover, the 35 epithelial sheet spreading over the sphere is mechanically stressed and this stress needs 36 to be released. Here we show that during extraembryonic tissue (serosa) epiboly in the red 37 flour beetle Tribolium castaneum, the non-proliferative serosa becomes regionalized into 38 two distinct territories: a dorsal region under higher tension away from the leading edge 39 with larger non-rearranging cells, and a more fluid ventral region under lower tension 40surrounding the leading edge with smaller cells undergoing cell intercalation. Our results 41suggest that fluidization of the leading edge is caused by a heterogeneous actomyosin 42 cable that drives sequential eviction and intercalation of individual cells away from the 43 serosa margin. Since this developmental solution utilized during epiboly resembles the 44 mechanism of wound healing in other systems, we propose actomyosin cable-driven local 45 tissue fluidization as a conserved morphogenetic module for closure of epithelial gaps. 46 47Epiboly is one of the evolutionarily conserved morphogenetic movements during animal 48 gastrulation 1 . It involves spreading of an epithelial sheet over the spherical or ellipsoidal egg. The 49sheet eventually forms a continuous layer that entirely surrounds the embryo and the yolk sac. 50During this morphogenetic event, fundamental geometrical and mechanical problems arise. First, 51in order to cover the entire egg, the epithelium has to expand in surface area. However, once the 52 egg equator is reached, the expanding tissue must also undergo a regional compaction at its 53 leading edge in order to seal seamlessly at the bottom of the sphere (Fig 1A). Studies in fish 54showed that the tissue spreading is mediated by changes in cell shape, cell number and cell 55 arrangement coupled to constriction of an actomyosin ring in the yolk at the leading edge of the 56 sheet 2-5 . However, it remains unclear whether pulling forces at the leading edge expand cells 57 uniformly throughout the tissue and how cells behave at the leading edge that needs to compact. 58Second, spreading over a sphere induces mechanical stress in the tissue. In zebrafish, 59 mechanical stress during epibolic expansion is released by oriented cell divisions in the tissue 4 . 60In other epiboly systems however, cell division does not occur, and thus other unknown 61 mechanisms have to alleviate built-up stress. 62 63For example, in many insect taxa, the developing embryo is completely surrounded by a 64 protective epithelial cell layer of extraembryonic fate called the serosa 6-10 . In the red flour beetle, 65Tribolium castaneum, extraembryonic serosal cells are initially specified as an anterior cap of the 66 cellular blastoderm, which subsequently spreads over the gastrulating embryonic...
