Figure 3. Ectopic expression of eye regulatory network genes in flies.
A: Ectopic expression of optix in antennal discs leads to ectopic red pigment in adult antennae (photo courtesy of Makiko Seimiya and Walter Gehring). B: Ectopic expression of eyeless in wing discs leads to ectopic eyes [11] (photo courtesy of Georg Halder). C: The pattern of gene connections between several top regulatory genes in the eye network.
One possibility involves the co-option of the master regulator into a novel context, but at a different expression level from the endogenous eye context, leading to the activation of only those sub-networks that require lower levels of master gene expression to pass an activation threshold.
Previous work characterized the red pigments in Heliconius wings as ommochromes, and proposed that the ommochrome biosynthetic pathway originally deployed in the eye, had been co-opted to the wing. It appears that this was indeed the case, but the co-option was not done in a stepwise fashion. Instead a single gene, that is known to regulate all the elements of this pathway as well as other eye components, optix, was co-opted to the wing.