FRIZZLED REGULATES MIRROR-SYMMETRICAL PATTERN-FORMATION IN THE DROSOPHILA EYE

Coordinated morphogenesis of ommatidia during Drosophila eye developme nt establishes a mirror-image symmetric pattern across the entire eye bisected by an anteroposterior equator. We have investigated the mecha nisms by which this pattern formation occurs and our results suggest t hat morphogenesis is coordinated by a graded signal transmitted bidire ctionally from the presumptive equator to the dorsal and ventral poles . This signal is mediated by frizzled, which encodes a cell surface tr ansmembrane protein, Mosaic analysis indicates that frizzled acts non- autonomously in an equatorial to polar direction. It also indicates th at relative levels of frizzled in photoreceptor cells R3 and R4 of eac h ommatidium affect their positional fate choices such that the cell w ith greater frizzled activity becomes an R3 cell and the cell with les s frizzled activity becomes an R4 cell, Moreover, this bias affects th e choice an ommatidium makes as to which direction to rotate. Equator- outwards progression of elav expression and expression of the nemo gen e in the morphogenetic furrow are regulated by frizzled, which itself is dynamically expressed about the morphogenetic furrow, We propose th at frizzled mediates a bidirectional signal emanating from the equator .