Spatial Bistability Generates hunchback Expression Sharpness in the Drosophila Embryo

Uwe Ohler,Francisco J P Lopes,Fernando M C Vieira,Paulo M Bisch,Alexander V Spirov,David M Holloway

doi:10.1371/journal.pcbi.1000184

Abstract

During embryonic development, the positional information provided by concentration gradients of maternal factors directs pattern formation by providing spatially dependent cues for gene expression. In the fruit fly, Drosophila melanogaster, a classic example of this is the sharp on–off activation of the hunchback (hb) gene at midembryo, in response to local concentrations of the smooth anterior–posterior Bicoid (Bcd) gradient. The regulatory region for hb contains multiple binding sites for the Bcd protein as well as multiple binding sites for the Hb protein. Some previous studies have suggested that Bcd is sufficient for properly sharpened Hb expression, yet other evidence suggests a need for additional regulation. We experimentally quantified the dynamics of hb gene expression in flies that were wild-type, were mutant for hb self-regulation or Bcd binding, or contained an artificial promoter construct consisting of six Bcd and two Hb sites. In addition to these experiments, we developed a reaction–diffusion model of hb transcription, with Bcd cooperative binding and hb self-regulation, and used Zero Eigenvalue Analysis to look for multiple stationary states in the reaction network. Our model reproduces the hb developmental dynamics and correctly predicts the mutant patterns. Analysis of our model indicates that the Hb sharpness can be produced by spatial bistability, in which hb self-regulation produces two stable levels of expression. In the absence of self-regulation, the bistable behavior vanishes and Hb sharpness is disrupted. Bcd cooperative binding affects the position where bistability occurs but is not itself sufficient for a sharp Hb pattern. Our results show that the control of Hb sharpness and positioning, by hb self-regulation and Bcd cooperativity, respectively, are separate processes that can be altered independently. Our model, which matches the changes in Hb position and sharpness observed in different experiments, provides a theoretical framework for understanding the data and in particular indicates that spatial bistability can play a central role in threshold-dependent reading mechanisms of positional information.

Highlights

How an embryo achieves pattern and form from an initially undifferentiated state has fascinated people at least since the time of Aristotle
By applying dynamical systems theory to the model, we show that Hb sharpness is due to spatial bistability stemming from hb self-regulation
We have shown, using a predictive reaction–diffusion model as well as a series of experimental data, that hb reads out the positional information of the morphogenetic Bcd gradient with bistable kinetics, resulting from hb self-regulation

Summary

Introduction

Scientific advances on this began over a century ago, with, for example, the experiments of Hans Driesch on sea urchin embryos [1], from which he proposed that the embryo has a coordinate system specifying cellular position; and from the experiments of Ethel Browne [2], who showed that a piece of hydra mount induced a secondary axis when grafted into the body of another hydra. In the late 1980’s, the Bicoid (Bcd) protein gradient was characterized and its concentration-dependent effect on downstream target genes in Drosophila was demonstrated [7,8,9] This has since become one of the most studied examples of morphogen gradient signaling in developmental biology [10,11]. These techniques can be applied to data, as well as HSR model results

Methods

Results

Conclusion