Lobe mediates Notch signaling to control domain-specific growth in the Drosophila eye disc.

Notch (N) activation at the dorsoventral (DV) boundary of the Drosophila eye is required for early eye primordium growth. Despite the apparent DV mirror symmetry, some mutations cause a preferential loss of the ventral domain, suggesting that the growth of individual domains is asymmetrically regulated. We show that the Lobe (L) gene is required non-autonomously for ventral growth but not dorsal growth, and that it mediates the proliferative effect of midline N signaling in a ventral-specific manner. L encodes a novel protein with a conserved domain. Loss of L suppresses the overproliferation phenotype of constitutive N activation in the ventral, but not in the dorsal eye, and gain of L rescues ventral tissue loss in N mutant background. Furthermore, L is necessary and sufficient for the ventral expression of a N ligand, Serrate (Ser), which affects ventral growth. Our data suggest that the control of ventral Ser expression by L represents a molecular mechanism that governs asymmetrical eye growth.