Segment polarity genes act here too, especially gooseberry and naked . See Chris Doe's Hyper-Neuroblast map site for information on the origin and lineage of specific neuroblasts in the CNS and for information about the expression of specific genes regulating neuroblast origin. The pair rule genes act through this region, situated in the intervening chromosomal segment between achaete and scute . Thus this region is under the initial control of products of the pair rule genes and is later maintained by selected segment polarity genes. Consider the amazing patterning present in insect epidermis.
Flies have hairs that appear as reproducable patterns from one individual to another.
These patterns are species specific and therefore distinguish one species from another. Ultimately it is the capacity of site specific enhancers to kindle AS-C transcription at very localized regions in the proneural clusters of imaginal discs that is responsible for the patterning of sensory elements seen in adults. Each of these enhancers, regulated by a specific set of transcription factors defines a prepattern for achaete and scute expression. The enhancers are named after the sensilla in which they direct achaete and scute expression. It is rather remarkable that each sensory organ or group is specificied by a special enhancer selected through evolution for just that purpose.