Genetic Controls
A number of genetic pathways are responsible for the occurrence and positioning of the ocelli. The gene orthodenticle is allelic to ocelliless, a mutation that stops ocelli from being produced. In Drosophila, the rhodopsin Rh2 is only expressed in simple eyes.
While (in Drosophila at least) the genes eyeless and dachshund are both expressed in the compound eye but not the simple eye, no reported 'developmental' genes are uniquely expressed in the simple eye.
Epidermal growth factor receptor (Egfr) promotes the expression of orthodenticle and as such is essential for simple eye formation.
Read more about this topic: Invertebrate Photoreceptor
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