Overall Model of Drosophila Early Development
From knowledge of developmental genes and their expression patterns, we can form a rough picture of Drosophila development. Initially two gradients are set up. The anterior gradient in the bicoid product is generated by activating bicoid mRNA for translation at the time of fertilization. A posterior morphogen is also utilized. This is the nanos product, which is located in the rear half of the egg. Next the develop-mental genes responsible for sizeable portions of the embryo are activated.
Hunchback, krüppel, and knirps are examples of such genes. Defects in these genes lead to sizeable gaps in the shapes of developing embryos. The concentrations of bicoid and nanos regulate the expres-sion of these proteins. Likely they are synthesized only in regions in which proteins lie in certain concentration ranges. Next, the pair rule genes function. These eight or so genes further subdivide the developing embryo into six to eight stripes. A gene like engrailed helps sharpen the boundaries between the segments. After this, the runt and hairy genes are expressed in alternate stripes. The homeotic genes then function to specify the actual identity of the different areas.