The M.R. Bauer
Claude Desplan, D.Sc., Ph.D.
Professor of Biology
New York University
Week of March 21, 2011
Color Vision in Drosophila
Our capacity for vision is one we often take for granted (at least until something goes wrong). In order for us to view the world, a remarkably large number of circuits and mechanisms must work in concert to construct the perception of a visual environment. Dr. Claude Desplan studies vision in the fruit fly, Drosophila melanogaster, whose compound eyes contain hundreds of retina-like structures known as ommatidia. These ommatidia contain several types of neurons, which act in a manner similar to the rods and cones that are integral for human vision. Dr. Desplan is interested in how these different types of neurons arise in the ommatidia of the fruit fly and how the signals from these cells are interpreted and integrated by the fly brain.
The fruit fly Drosophila uses a "compound eye" for visual functions. It is made up of 800 unit eyes called ommatidia that form an image with 800 pixels. Each ommatidium contains eight photoreceptors. Six of the photoreceptors (called R1-R6) are like the human rods; they are involved in motion detection and are identical in all ommatidia. The remaining two photoreceptor types (R7 and R8) play a major role in color vision and differ in different parts of the retina: p ommatidia contain an R7 photoreceptor that is UV-sensitive, while p R8 photoreceptors are blue-sensitive. The y ommatidia have an R7 that is sensitive to a different UV light wavelength and an R8 that is green-sensitive. The p and y subsets are distributed stochastically throughout the retina in a 30:70 ratio. Comparison between R7 and R8, and between p and y ommatidia, allows flies to discriminate between colors, with p ommatidia involved in the detection of short wavelengths and y ommatidia for longer wavelengths. In the dorsal rim area of the retina (DRA), ommatidia serve to measure the vector of light polarization for navigation on cloudy days. A fourth subset located in the dorsal third of the eye serves to detect the orientation of the sun for navigation on sunny days.
Both of Dr. Desplan's talks focused on the cascade of genes that specify the different subsets of photoreceptors through a series of fate restrictions and how this cascade is modified to define the various regions of the retina in Drosophila. The gene homothorax, for example, is required for the formation of DRA ommatidia, while another gene called spineless is expressed in a stochastic manner in a subset of R7 cells (y R7). These molecular mechanisms allow for the specification of the whole retina by specifying the y choice in R7 and allowing R7 to instruct R8 of its choice. Finally, IroC genes determine the region where ommatidia detect the orientation of the sun and co-express Rh3 and Rh4 in R7 of the y subtype.
Processing of color information occurs in the medulla part of the optic lobes that receives input from R7 and R8. The medulla is formed by about 40,000 neurons surrounding a neuropil where photoreceptors and medulla neurons interconnect. Associated with each set of R7/R8 projections, there are about 800 "columns," which are the functional units in the medulla. Dr. Desplan described his work addressing how more than 70 cell types are specified in the medulla and connect to photoreceptors in a retinotopic manner to process color information. This information is then sent to higher brain centers in the lobula complex and central brain to mediate color behavior. The function of each of these neuronal subtypes was addressed by silencing small populations of medulla neurons and testing the consequence for color discrimination. For this purpose, the Desplan lab utilizes a flight simulation paradigm, where the fly is trained to associate color with a reward or punishment before being tested in the absence of the reward. Further insight into the mechanisms underlying color discrimination will hopefully shed light on the fundamental processes likely to be shared by all visual systems, from flies to mammals.