Dynamic Analysis of Non-Muscle Myosin II Recruitment During Germband Extension

Abstract

How the combined action of genetic patterning and self-organized mechanical feedback leads to the robust construction of biological form is a fundamental question. Here we present a dynamic, global analysis of anisotropic Non-Muscle Myosin II (MyoII) during Drosophila germband extension (GBE), with particular interest in elucidating the identity of the source responsible for MyoII recruitment. Previously we demonstrated that over 90 percent of tissue flow during GBE can be predicted from dynamic MyoII expression. However, the identity of the source responsible for anisotropic MyoII recruitment remains unclear. Two distinct possibilities have been proposed. Neighboring cells could sense and respond to juxtaposition of pair rule gene (PRG) expression across their shared interface, with the local gradient of PRG expression defining an instructive cue for orienting MyoII recruitment. Alternatively, MyoII recruitment may be mechanosensitive. To understand the relationship between PRGs and MyoII recruitment, we generated a global, dynamic atlas of PRG expression. We find that a cumulative description of PRG expression fails to predict the profile of anisotropic MyoII intensity. Additionally, the relative angle between the orientation of MyoII and Runt stripes becomes progressively misaligned during the course of GBE. Dynamic Runt expression is predictably Lagrangian, deforming with the tissue in response to flow. In contrast, anisotropic MyoII orientation remains aligned with the DV axis except in close proximity to the region of the germband where the magnitude of flow field vorticity is highest. FRAP analysis demonstrates that junctional MyoII has a finite lifetime and is fully mobile. Using a simple linear regression model, nearly the entire regional misalignment that occurs between MyoII orientation and the DV axis where vorticity is strongest can be accounted for using a single, constant parameter—the ‘effective lifetime’—of MyoII. This parameter reflects the average lifetime of MyoII association with the junctional F-actin cytoskeleton and is temporally consistent with the FRAP-derived experimental estimate of finite lifetime. Together these results suggest that junctional MyoII recruitment is instructed by a static Eulerian source aligned with the DV axis that is independent of PRG expression. We predict that the identity of this static source may be mechanical feedback.