We study the patterning of photoreceptor neurons. Color vision is achieved through photoreceptors containing opsin photopigments with different wavelength specificity. These photopigments are expressed in mutually exclusive patterns in the compound eye of Drosophila. This implies there is a process for choosing a given opsin gene and transcriptionally repressing all others. The restricted expression of a transcription factor called Prospero in R7 neurons is critical for their development as photoreceptors with unique wavelength sensitivity. We use molecular and genetic approaches to identify the signaling pathways responsible for the elaboration of this system. This involves studying the prospero gene and others like it at the transcriptional level. We have found that ligands secreted from neighboring cells activate prospero transcription in R7 cells through a transcription enhancer within the prospero gene. Our current model suggests that extracellular signals provide a general timing mechanism to activate transcription of specific genes such as prospero, but activation is limited to specific genes by signal convergence within cells.

Understanding how cells fit together in space is an area of science that has started to gain serious momentum in the last decade. Cells are different shapes and pack together in different ways depending on where they are located in a living thing and what their function is. We have been looking at this phenomenon by focusing on the cone and pigment cells in the eye. Differential expression of N-cadherin within cone cells causes them to form an overall shape that minimizes their surface contact with surrounding pigment cells. The cells within this group, in both normal and experimentally manipulated conditions, pack together as if they are soap bubbles. The shaping of the cone-cell group and packing of its components precisely imitate the physical tendency for surfaces to be minimized. Thus, simple patterned expression of N-cadherin results in a complex spatial pattern of cells due to cellular surface mechanics. This system illustrates the importance of mathematics and physics in biology and points to a general principle of patterning found in a wide range of living things.