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.
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