Key words and terms
Embryonic development, pattern formation, morphogen, transcription, morphogenesis, gastrulation, diffusion, cooperativity, Drosophila, GFP, degradation, confocal microscopy, 2-photon microscopy.
Following fertilization the Drosophila embryo undergoes 13 synchronous rounds of nuclear division by the end of which zygotic genes are expressed in distinct patterns in the embryo. These genes regulate subsequent events in the embryo such as gastrulation. The pattern of zygotic gene expression is regulated by a set of maternally provided cues.
One such signal is the morphogen Bicoid, which forms an anterior-posterior nuclear concentration gradient in the embryo. This gradient arises from maternal bicoid mRNA deposited at the anterior pole of the egg. Synthesis of Bicoid protein at the anterior pole, and diffusion from this site of production, results in the observed graded distribution of Bicoid. Nuclei can distinguish their position in the embryo based on the concentration of Bicoid. Some genes respond only to high concentrations of Bicoid in nuclei, as is found close to the anterior pole. Further from the source, the concentration of Bicoid declines and different genes are transcribed in response to lower concentrations of Bicoid. It is believed that the affinity of Bicoid for DNA binding sites determines this concentration dependent gene expression.
The genes that are regulated by Bicoid are called gap genes and encode transcription factors themselves. Cells expressing a certain combination/subset of these genes adopt a given fate (fate determination by combinatorial gene expression). For example, nuclei/cells at the anterior that were exposed to high concentrations of Bicoid, express the gap gene orthodenticle which confers a head fate onto these cells.
The activation of Bicoid target genes and the expression of most zygotic genes takes place at the mid blastula transition (MBT). In addition to the activation of the zygotic genome, maternally deposited mRNAs, such as bicoid mRNA, are degraded; in other words, the embryo switches from maternal to zygotic control of development. One consequence of the patterned expression of these genes is the correct spatio-temporal order of the morphogenetic movements such as ventral furrow formation.
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