A Precise Bicoid Gradient Is Nonessential during Cycles 11–13 for Precise Patterning in the Drosophila Blastoderm

Elena M Lucchetta,Rustem F Ismagilov,Meghan E Vincent

doi:10.1371/journal.pone.0003651

Abstract

BackgroundDuring development, embryos decode maternal morphogen inputs into highly precise zygotic gene expression. The discovery of the morphogen Bicoid and its profound effect on developmental programming in the Drosophila embryo has been a cornerstone in understanding the decoding of maternal inputs. Bicoid has been described as a classical morphogen that forms a concentration gradient along the antero-posterior axis of the embryo by diffusion and initiates expression of target genes in a concentration-dependent manner in the syncytial blastoderm. Recent work has emphasized the stability of the Bicoid gradient as a function of egg length and the role of nuclear dynamics in maintaining the Bicoid gradient. Bicoid and nuclear dynamics were observed but not modulated under the ideal conditions used previously. Therefore, it has not been tested explicitly whether a temporally stable Bicoid gradient prior to cellularization is required for precise patterning.Principal FindingsHere, we modulate both nuclear dynamics and the Bicoid gradient using laminar flows of different temperature in a microfluidic device to determine if stability of the Bicoid gradient prior to cellularization is essential for precise patterning. Dramatic motion of both cytoplasm and nuclei was observed prior to cellularization, and the Bicoid gradient was disrupted by nuclear motion and was highly abnormal as a function of egg length. Despite an abnormal Bicoid gradient during cycles 11–13, Even-skipped patterning in these embryos remained precise.ConclusionsThese results indicate that the stability of the Bicoid gradient as a function of egg length is nonessential during syncytial blastoderm stages. Further, presumably no gradient formed by simple diffusion on the scale of egg length could be responsible for the robust antero-posterior patterning observed, as severe cytoplasmic and nuclear motion would disrupt such a gradient. Additional mechanisms for how the embryo could sense its dimensions and interpret the Bicoid gradient are discussed.

Highlights

Embryos produce precise patterns of protein expression in spite of perturbations such as differences in gene dosage and uniform changes in the temperature at which they develop
These results indicate that the stability of the Bicoid gradient as a function of egg length is nonessential during syncytial blastoderm stages
Presumably no gradient formed by simple diffusion on the scale of egg length could be responsible for the robust antero-posterior patterning observed, as severe cytoplasmic and nuclear motion would disrupt such a gradient

Summary

Introduction

Embryos produce precise patterns of protein expression in spite of perturbations such as differences in gene dosage and uniform changes in the temperature at which they develop. The Bicoid (Bcd) protein in Drosophila embryos has been used as a model to determine how embryos translate a maternal input into robust patterns of zygotic gene expression. When shifts in the Bcd gradient have been induced by altering the copy number of maternal bcd genes, embryos have had corresponding shifts in downstream Even-skipped (Eve) expression, these shifts were not as great as was predicted by a simple gradient model [3]. Embryos compensated for these shifts in Eve expression by apoptosis after gastrulation [3]. It has not been tested explicitly whether a temporally stable Bicoid gradient prior to cellularization is required for precise patterning

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