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Colour vision in flies

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Abstract: The Drosophila compound eye is made of 800 unit eyes (ommatidia) that each contains eight photoreceptors: Six are involved in motion detection while two (R7 and R8) play a major role in color vision. These ommatidia can be grouped into four categories: p ommatidia contain UV-sensitive Rh3 in photoreceptors R7 and Blue-Rh5 in R8 while y ommatidia express another UV-Rh4 in R7 and green-Rh6 in R8. The p and y subsets are distributed stochastically throughout the retina in a 30:70 ratio. Comparison between the inputs of 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. Dorsal Rim Area (DRA) ommatidia express UV-Rh3 in both R7 and R8. They function as polarizing filters that allow the fly to measure the vector of light polarization for navigation on cloudy days. A fourth subset located in the dorsal third of the eye co-expresses UV-Rh3 and -Rh4 in yR7 and serves to detect solar vs. anti-solar orientations, also for navigation on sunny days.

I will describe 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 and how this spatial organization is used in other insect species: homothorax is required for the formation of the DRA . spineless is expressed in a stochastic manner in R7 cells that express Rh4 (yR7). It allows 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 yR7 co-express Rh3 and Rh4. Processing of color information occurs in the medulla that receives input from R7 and R8. The medulla is formed by approximatly 40,000 neurons surrounding a neuropil where photoreceptors and medulla neurons interconnect. Associated with each set of R7/R8 projections, there are approximatly 800 ‘columns’, the functional units in the medulla. We are addressing how medulla cells process color information coming from R7 (sensitive to UV) and R8 (sensitive to blue or green) and send it to higher processing centers in the lobula complex and central brain to mediate color behavior.

We are silencing subsets of medulla neurons using specific Gal4 lines and testing the consequence for color discrimination. We have adapted to color vision the flight simulator originally designed by the Dickinson/Frye labs. In an operant paradigm, the fly is trained to associate color with a reward or punishment before being tested in the absence of the reward.

Biosketch: Dr Desplan was trained at the Ecole Normale Superieure in St Cloud, France where he obtained the ‘agregation’ in 1975. He did his PhD in Paris at the INSERM with Drs. Moukhtar and Thomasset on Calcium regulation before joining Pat O’Farrell’s lab at UCSF . This is where he initiated his studies of the homeodomain and demonstrated that this conserved signature of many developmental genes was a DNA binding motif. In 1987, he joined the Faculty of Rockefeller University and was a Howard Hughes investigator. He pursued structural studies of the homeodomain and initiated his work on the evolution of axis formation in insects. In 1997, he embarked into the investigation of color vision in Drosophila that occupies most of his current laboratory. He moved as a professor to New York University in 1999. His team has described the molecular mechanisms of patterning of the fly retina that underlies color vision. He is now studying processing of color vision with an investigation of the functional anatomy of the medulla part of the optic lobe. In parallel, his lab developed the wasp Nasonia as a model system to compare early developmental events in the embryo (Evo-Devo). He contributed extensively to the understanding of how insect embryos pattern their antero-posterior axis through the utilization of many of the same genes that are used in Drosophila with significant changes in the network, in particular through mRNA localization.


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