Here's a refinement in the story of early pattern formation in the Drosophila embryo. Just to recap a little bit, I've told you that in flies there is a molecule, bicoid, that is expressed in a gradient and that is a transcription factor that regulates the expression of other genes. In particular, there is a set of gap genes, Hunchback and Krüppel and Knirps and Giant and Tailless that read the gradient of the bicoid morphogen and are turned on in specific bands along the length of the embryo.
All of this is more or less accurate, but I have to tell you now that it is also a great simplification. Development is much, much more complicated than that. From that description, you'd think that the bulk of the work in specifying identities along the longitudinal axis is done once the bicoid gradient is set up—everything from that point could require nothing but passive obedience from the downstream genes to what bicoid tells them to do. There are also a great many interactions between different downstream genes that are important in shaping the distribution of gene expression, however, and one thing developmental biologists can't do is get trapped into simple linear thinking.
Continue reading "Dynamic control of gap genes" (on Pharyngula)