THE DROSOPHILA BIFOCAL GENE ENCODES A NOVEL PROTEIN WHICH COLOCALIZESWITH ACTIN AND IS NECESSARY FOR PHOTORECEPTOR MORPHOGENESIS

Photoreceptor cells of the Drosophila compound eye begin to develop sp ecialized membrane foldings at the apical surface in midpupation. The microvillar structure ultimately forms the rhabdomere, an actin-rich l ight-gathering organelle with a characteristic shape and morphology. I n a P-element transposition screen, we isolated mutations in a gene, b ifocal (bif), which is required for the development of normal rhabdome res. The morphological defects seen in bif mutant animals, in which th e distinct contact domains established by the newly formed rhabdomeres are abnormal, first become apparent during midpupal development, The later defects seen in the mutant adult R cells are more dramatic, with the rhabdomeres enlarged, elongated, and frequently split, bif encode s a novel putative protein of 1063 amino acids which is expressed in t he embryo and the larval eye imaginal disc in a pattern identical to t hat of F actin, During pupal development, Bif localizes to the base of the filamentous actin associated with the forming rhabdomeres along o ne side of the differentiating R cells. On the basis of its subcellula r localization and loss-of-function phenotype, we discuss possible rol es of Bif in photoreceptor morphogenesis.