Fruit Fly 4, Embryo

The fertilized egg of a fruit fly has a set of cells from the mother. Those cells produce molecules into the egg itself and work to make the beginning of the new larva. There is a wonderful picture which of what goes on from a book on the NCBI website called "Developmental Biology" by Scott Gilbert, published by Sinaur (6th ed.). The link for the fruit fly embryo picture is HERE .

It is a remarkable process to say the least. The female cells which come along with the embryo are called "nurse cells." They therefore have their own production of RNA available for the process of protein production in the embryo. The embryo has to start differentiating so it will eventually produce the anterior (head), middle (thorax and abdomen) and posterior (tail end) of the organism.

One of the startling aspects of this process is that the diffusion of these molecules is critical in how the embryo responds, so that concentrations of mRNA and proteins at particular places within the egg are important in how the animal develops. The genes are sensitive to the concentration and type of proteins within the embryo. A higher concentration produces the head and a lower one produces the thorax and so on. There are also other proteins acting together to form different concentrations of each in different areas. These are all proteins which must be co-ordinated in the embryo and is true for all animals which have front and back, which means it would have all had to be coordinated from the first worm or other such organism.

The number of molecules made from those "nurse" mother cells is important. Research by Gregor, Tank, Wieschaus and Bialek, "Probing the Limits to Positional Information," Cell (Elsevier, 2007) has shown that it is not just a matter of the cells activating from a bunch of molecules being thrown at them. They can tell the difference between large and small quantities in the range of about 70 molecules.