Evidence for the priming role of the central retinula cell in ommatidium differentiation of Ephestia kuehniella

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Abstract
In the developing compound eye of Ephestia kuehniella, within the advancing front of differentiation, regular cell clusters arise which consist of a central cell and two flanking cells. The central cell is destined to become the basal retinula cell later in development. Its crucial role in ommatidium formation is confirmed by 3H-thymidine labelling. Eye anlagen labelled early in the pupal stage incorporate thymidine within two distinct zones along the front of differentiation. After the ommatidia are completely differentiated, both zones contain labelled nuclei of all cell types which participate in ommatidia formation. Within the posterior zone, however, the basal retinula cells are always unlabelled, whereas in the anterior they show labelled nuclei. From this observation it must be concluded that the basal retinula cell first terminates proliferation (either alone or together with a few other cells) to become differentiated as the central retinula cell. These results agree with those found in Drosophila and indicate that the ordered stepwise addition of cells to a central founder cell is a widespread principle of ommatidia formation in insects.