Background: During development, embryos decode maternal morphogen inputs into highly precise zygotic geneudexpression. The discovery of the morphogen Bicoid and its profound effect on developmental programming in theudDrosophila embryo has been a cornerstone in understanding the decoding of maternal inputs. Bicoid has been described asuda classical morphogen that forms a concentration gradient along the antero-posterior axis of the embryo by diffusion andudinitiates expression of target genes in a concentration-dependent manner in the syncytial blastoderm. Recent work hasudemphasized the stability of the Bicoid gradient as a function of egg length and the role of nuclear dynamics in maintainingudthe Bicoid gradient. Bicoid and nuclear dynamics were observed but not modulated under the ideal conditions usedudpreviously. Therefore, it has not been tested explicitly whether a temporally stable Bicoid gradient prior to cellularization isudrequired for precise patterning.udPrincipal Findings: Here, we modulate both nuclear dynamics and the Bicoid gradient using laminar flows of differentudtemperature in a microfluidic device to determine if stability of the Bicoid gradient prior to cellularization is essential forudprecise patterning. Dramatic motion of both cytoplasm and nuclei was observed prior to cellularization, and the Bicoidudgradient was disrupted by nuclear motion and was highly abnormal as a function of egg length. Despite an abnormal Bicoidudgradient during cycles 11–13, Even-skipped patterning in these embryos remained precise.udConclusions: These results indicate that the stability of the Bicoid gradient as a function of egg length is nonessentialudduring syncytial blastoderm stages. Further, presumably no gradient formed by simple diffusion on the scale of egg lengthudcould be responsible for the robust antero-posterior patterning observed, as severe cytoplasmic and nuclear motion woulduddisrupt such a gradient. Additional mechanisms for how the embryo could sense its dimensions and interpret the Bicoidudgradient are discussed.
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