Neuronal development in the Drosophila compound eye: rap gene function is required in photoreceptor cell R8 for ommatidial pattern formation.

In the compound eye of Drosophila, cell-cell interactions are thought to play an important role in the determination of neuronal cell fate and pattern morphogenesis. Recent work on the bride of sevenless (boss) gene has demonstrated an inductive role for photoreceptor R8 in the differentiation of photoreceptor R7. These studies have shown that while R8 differentiates early in the scheme of ommatidial assembly, it continues to play an active role in subsequent patterning events. We describe studies on a new genetic locus rap (retina aberrant in pattern), whose functions are critical for normal pattern formation in the developing eye. Mutations in the rap gene perturb the early stages of pattern formation and lead to a variable number of photoreceptor cells (R cells) in each ommatidium. Experiments with a temperature-sensitive allele have shown that rap gene function is required during the period of development when pattern formation occurs. In addition, a somatic mosaic analysis of rap has shown that its function is required only in photoreceptor cell R8 for normal ommatidial patterning. These studies suggest an important role for rap in the initial events leading to pattern formation and are consistent with R8 playing a central role in directing ommatidial pattern formation.