Asymmetrical actomyosin network formation during T1 transition and AJ elongation. (A–C) Representative images of wild-type NE cells expressing sf9-mNeonGreen and Scarlet-UtrCH. The white arrowheads indicate the accumulation of sf9-mNeon at tricellular vertices. (B) Representative snapshot images of NMII and F-actin during T1 transition and AP elongation of newly formed AJs from Video 5. The white circles represent T1 junctions. The arrowheads indicate crescent-shaped sf9 accumulation at the anterior and posterior sides of T1 junctions and retaining accumulation at the same vertices. The double arrows represent elongating AP junctions. The paired arrows represent shrinking ML junctions prior to the formation of T1 junctions. (C) Enlarged images of sf9-mNeonGreen and Scarlet-UtrCH at T1 junctions (left, top panel) or vertices on both ends of elongating AP junctions (left, middle panels). A line intensity profile plot shows two sf9 peaks and a single UtrCH peak at T1 junction (right panel). Schematic of NMII and actomyosin distribution during the T1 transition (bottom panel). (D) Representative snapshot images of sf9-mNeonGreen and Scarlet-UtrCH during T1 transition in Pk2-KD NEs from Video 5. The white circles represent T1 junctions. The arrowheads represent the accumulation of crescent-shaped sf9 at the anterior and posterior sides of T1 junction and the remaining accumulation at both ends of elongating AP junctions. The double arrows represent elongating AP junctions. (E) Enlarged images of representative T1 junctions (top panels) and newly formed AP junctions (middle panels). Line intensity profile plot of T1 junctions in the Pk2 KD NE (bottom panel). Scale bars: 10 μm in A, 5 μm in B and D, and 2 μm in C and E.