Wavefronts and Mechanical Signaling in Early Drosophila Embryos

Abstract

Many developmental processes use signaling for synchronization. One possible and well-known method is for each component to measure the local concentration of some diffusing biochemical species, and if that concentration exceeds a certain threshold, to release more of the same biochemical, giving rise to wavefront propagation in the chemically excitable medium. There are however more ways in which a medium can be excited and wavefronts can be formed. We suggest a new kind of signaling based not on diffusion of a chemical species, but on the propagation of mechanical stress. We construct a theoretical approach to describe mechanical signaling in an overdamped system as a nonlinear wavefront propagation problem. We apply the theory to mitosis in the early syncytial Drosophila embryo, which is highly correlated in space and time, as manifested in mitotic wavefronts that propagate across the embryo. We compare our results to data taken on Drosophila embryos in which histones in the nuclear chromosomes are labeled with GFP. By analyzing confocal microscopy videos of the mitotic wavefront, we find that the wavefront can be resolved into two distinct wavefronts in each cycle, corresponding to the onset of metaphase and of anaphase, respectively. The two wavefronts have the same speed and are separated by a time interval that is independent of cycle, indicating that they are two different markers for the same process. We find that the dependence of wavefront speed on cell cycle number is most naturally explained by mechanical signaling via stress diffusion, and that the entire process suggests a scenario in which biochemical and mechanical signaling are coupled.