Roughest is also involved in terminal stages of the development of the regular pattern of ommatidia in the eye. The final step of pattern formation in the developing retina is the elimination of excess cells between the ommatidia and the differentiation of the remaining cells into secondary and tertiary pigment cells. Roughest protein accumulates strongly at the borders between primary pigment and interommatidial cells. Mutant and misexpression analyses show that this accumulation of the Roughest protein is necessary for aligning interommatidial cells in a single row. This reorganization is a prerequisite for the identification of death candidates.
Ommatidial formation is characterized by an accretive mode of growth.
Since the last cells to develop have common borders with several ommatidia, such an accretive process cannot be continued to complete retinal development. Corrective measures are necessary to integrate the individually formed ommatidial elements into a regular lattice. Such measures are provided by the initial overproduction of cells and the wave of cell death that eliminates surplus interommatidial cells. This is made possible by a reorganization of cell contacts, constraining interommatidial cells into chains lying end to end. In a following step, surplus cells are eliminated.