On the development of ectopic eyes in Drosophila melanogaster produced by the mutation extra eye (ee).

The mutation ee often produces an ectopic eye on the vertex that is a mirror image partial duplication of the normal eye on the ipsilateral side of the head. The pattern of the duplication and a clonal analysis by mitotic recombination indicate that the duplications are of dorsal eye and orbital structures. Large ectopic eyes (more than 100 ommatidia) and their surrounding bristles may be produced without cuticular deficiencies. The penetrance of ee is temperature dependent with penetrance higher (72%) at 25 degrees and 29 degrees than at 19 degrees (43%). Temperature shift experiments show two temperature-sensitive periods: one at midembryogenesis, the other at mid-first larval instar. Microscopic examination of ee late-second and third instar imaginal cephalic discs show no indication of growth of the extra tissue needed to produce the duplication until after mid-third instar. This was confirmed by cell counts of ee and wild-type discs. There is no evidence of differential cell death in the two types of discs at this stage, although much earlier cell death is postulated. Tests for cell autonomy of the mutation by the production of morphogenetic clones suggest nonautonomy. Formation of pattern duplications by mutant genes is discussed in terms of cell death that eliminates whole developmental compartments, restricted cell death that occurs within a compartment, extensive cell death within a compartment and proliferative growth unassociated with cell lethality.