Cell fate in the Drosophila ommatidium.

Lesions in the Drosophila gene sevenless cause the cell normally destined to differentiate as photoreceptor 7 of the ommatidium to become an accessory lens-secreting cell, the equatorial cone cell. In both P-element- and EMS-induced alleles, the developmental transformation occurs identically. The mutation is cell autonomous, showing that the developmental failure is intrinsic to the transformed cell. Histological and immunological analyses indicate that the cell fails to operate any photoreceptor differentiation machinery prior to adopting the cone cell pathway.