1During development, coordinated cell shape changes and cell divisions sculpt 2 tissues. While these individual cell behaviors have been extensively studied, how cell 3 shape changes and cell divisions that occur concurrently in epithelia influence tissue 4 shape is less understood. We addressed this question in two contexts of the early 5Drosophila embryo: premature cell division during mesoderm invagination, and native 6 ectodermal cell divisions with ectopic activation of apical contractility. Using quantitative 7 live-cell imaging, we demonstrated that mitotic entry reverses apical contractility by 8 interfering with medioapical RhoA signaling. While premature mitotic entry inhibits 9 mesoderm invagination, which relies on apical constriction, mitotic entry in an artificially 10 contractile ectoderm induced ectopic tissue invaginations. Ectopic invaginations 11 resulted from medioapical myosin loss in neighboring mitotic cells. This myosin loss 12 enabled non-mitotic cells to apically constrict through mitotic cell stretching. Thus, the 13 spatial pattern of mitotic entry can differentially regulate tissue shape through signal 14 interference between apical contractility and mitosis. 15To determine whether loss of medioapical myosin was a general feature of 131 dividing, contractile epithelial cells, we took advantage of the stereotyped cell divisions 132 in the early mitotic domains that occur on the dorsal side of the head (Foe, 1989), 133 particularly focusing on mitotic domains 1 and 5 ( Figure 2C). We artificially increased 134 ectoderm apical contractility by ectopically expressing folded gastrulation (fog), a ligand 135 for a G-protein-coupled receptor (GPCR) that is expressed in the mesoderm and 136 functions upstream of apical myosin activation (Costa et al., 1994; Dawes-Hoang et al., 137 ,-.

Cell divisions can either promote or inhibit tissue morphogenesis. In contractile epithelia, mitotic entry disrupts medioapical myosin activation and reverses apical constriction. We found that different spatiotemporal patterns of mitotic entry and the resultant changes in force generation at the tissue level dictate distinct tissue shape outcomes.

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