Myosin II activity is not required forDrosophilatracheal branching morphogenesis

The Drosophila tracheal system consists of an interconnected network of monolayered epithelial tubes that ensures oxygen transport in the larval and adult body. During tracheal dorsal branch (DB) development, individual DBs elongate as a cluster of cells, led by tip cells at the front and trailing cells in the rear. Branch elongation is accompanied by extensive cell intercalation and cell lengthening of the trailing stalk cells. While cell intercalation is governed by Myosin II (MyoII)-dependent forces during tissue elongation in the Drosophila embryo leading to germ-band extension, it remained unclear whether MyoII plays a similar active role during tracheal branch elongation and intercalation. Here, we use a nanobody-based approach to selectively knock-down MyoII in tracheal cells. Our data shows that despite the depletion of MyoII function, tip cells migration and stalk cell intercalation (SCI) proceeds at a normal rate. Therefore, our data confirms a model in which DB elongation and SCI in the trachea occurs as a consequence of tip cell migration, which produces the necessary forces for the branching process. Summary statement Branch elongation during Drosophila tracheal development mechanistically resembles MyoII-independent collective cell migration; tensile forces resulting from tip cell migration are reduced by cell elongation and passive stalk cell intercalation. Abbreviations DB Dorsal branch DC Dorsal closure E-Cad E-Cadherin GBE Germ-band extension MRLC Myosin regulatory light chain MyoII Myosin II SCI stalk cell intercalation Sqh Spaghetti squash Sxll Sex lethal TC Tip cell Tr Tracheomere