Genetic Control of Development

We can see how such a morphogen acts by considering the development of the anterior-posterior axis in the fruit fly embryo. In the fly egg case, the oocyte, or fertilized egg, is accompanied by "nurse cells" at what will become the head end of the fly. This is called the anterior end; the tail end is posterior. Nurse cells create messenger RNA for a protein called bicoid, which they transport to the oocyte. Because these mRNAs originate in the anterior end, their concentration is highest there, and is lower towards the posterior end. Once the oocyte begins to divide, the mRNA is translated, and the bicoid protein is synthesized. Anterior cells have more of it than posterior cells, and the difference in concentration sets each cell group down its own developmental pathway, with anterior cells developing head structures, and posterior cells tail structures. Note that, in keeping with the "American plan," the fate of each cell is determined not by its ancestry, but by the environment it is in.

A concentration gradient of bicoid mRNA (and later bicoid protein) helps establish the anterior-posterior axis within the fruit-fly oocyte.

The effect of bicoid can be seen in transgenic flies, which have too many or too few copies of the gene. With extra bicoid, a higher-than-normal concentration exists further back in the oocyte, and anterior structures develop further back on the fly. With no bicoid, the anterior structures don't develop at all.

As we might expect, the bicoid protein is a transcription factor, which helps regulate expression of other genes. Other gradients of other transcription factors also exist at this stage, and together, these overlapping gradients establish the dorsal-ventral (back-belly) axis and map out the body segments that characterize all insects. While the details are complex, the fundamental idea is that of combinatorial control: At each position, it is the combination of transcription factors and their concentrations that determines which genes will be expressed, and therefore what the identity of the cells will be.

As segmentation becomes more firmly established and segments begin to take on their unique identities, gradients become less important. Instead, local gene control and cell to cell interactions create the increasingly fine level of spatial patterning.