Multiscale diffusion in the mitotic Drosophila melanogaster syncytial blastoderm

Abstract

Despite the fundamental importance of diffusion for embryonic morphogen gradient formation in the early Drosophila melanogaster embryo, there remains controversy regarding both the extent and the rate of diffusion of well-characterized morphogens. Furthermore, the recent observation of diffusional “compartmentalization” has suggested that diffusion may in fact be nonideal and mediated by an as-yet-unidentified mechanism. Here, we characterize the effects of the geometry of the early syncytial Drosophila embryo on the effective diffusivity of cytoplasmic proteins. Our results demonstrate that the presence of transient mitotic membrane furrows results in a multiscale diffusion effect that has a significant impact on effective diffusion rates across the embryo. Using a combination of live-cell experiments and computational modeling, we characterize these effects and relate effective bulk diffusion rates to instantaneous diffusion coefficients throughout the syncytial blastoderm nuclear cycle phase of the early embryo. This multiscale effect may be related to the effect of interphase nuclei on effective diffusion, and thus we propose that an as-yet-unidentified role of syncytial membrane furrows is to temporally regulate bulk embryonic diffusion rates to balance the multiscale effect of interphase nuclei, which ultimately stabilizes the shapes of various morphogen gradients. Positional information is a general mechanism for pattern formation in developing organisms: Cell fate specification is determined in a dosage-dependent manner by embryonic morphogen gradients. In the Drosophila syncytial blastoderm, maternal factors establish the axes and set up a system of positional information on which further patterning is built. There is a cascade of gene activity that leads both to the development of periodic structures, the segments, and to their acquiring a unique identity. This cascade involves the binding of transcription factors to regulatory regions of genes to produce sharp thresholds. There are striking similarities in the mechanisms for specifying and recording positional identity in Drosophila and vertebrates. It has been shown that nuclei and cytoplasm within the Drosophila syncytial blastoderm become organized into independent nuclear/cytoplasmic “protoplasmic islands” known as “energids” (1). It has also been observed that the cortical plasma membrane and various cytoplasmic components—including secretory machinery, Golgi, endoplasmic reticulum, and even cytoplasmic proteins—become “compartmentalized” such that they effectively belong to a single unique nucleus before cellularization (2⇓–4). Despite the concept that the syncytium is essentially a shared sheet of cortical cytoplasm, photobleaching has revealed that diffusion between energids takes place much more slowly than diffusion within individual energids during mitosis (2). This diffusive “compartmentalization” has clear implications for the way in which diffusion-based gradients are formed within syncytial embryos and has begun to be incorporated into morphogen gradient modeling (5, 6). However, despite the fundamental importance of this phenomenon, it remains poorly characterized and so far unexplained. Here, using a combination of mathematical/computational modeling and live-cell imaging, we characterize the effects of dynamic syncytial geometry on the effective diffusivity of cytoplasmic proteins in the Drosophila syncytial blastoderm.