Megha Chatterjee, M.Sc

Senior Research Fellow, Department of Zoology, University of Delhi

Drosophila melanogaster, the fruit fly, has proven to be a powerful model organism that can answer complex questions regarding the human disease pathology, development and genetics (Shulman et al., 2003). Adult Drosophila eye is a simple structure with a precise arrangement of repeating units called ommatidia. Each compound eye of wild-type Drosophila has ~800 ommatidia, with their number in females usually exceeding that of males.

Figure 1: Eye imaginal disc of a wandering Drosophila third instar larva captured under Nikon Ti-E confocal microscope showing the rows of photoreceptor neurons (Magnification: 60X).

Each ommatidium includes 8 photoreceptor neurons or rhabdomeres, 4 non-neuronal cone cells and 2 primary pigment cells. These ommatidial structures are separated by secondary and tertiary pigment cells making hexagonal boundaries around each ommatidium. Alongside, there are bristles at the ommatidial boundaries. Each adult eye emerges from an eye imaginal disc present in the larval stage (Figure 1 and 2). The eye and the antennal disc are present as a single epithelial tissue in the larva and later the eye disc metamorphoses into a red pigmented adult eye at pupal stage (Detailed description in Cagan, 2009).

Figure 2: Adult Drosophila compound eye visualized under high magnification Nikon SMZ745T light  microscope.

Can fly eyes be used to study human diseases?

Interestingly, a wide range of human diseases including neurodegenerative diseases like Huntington’s disease, Parkinson’s disease, Alzheimer’s disease, several developmental and genetic disorders as well as cancer have been well modeled and studied in Drosophila eye (Iyer et al., 2016, Lehmann et al., 2019). Using the UAS-Gal4 system, tissue specific expression of an exogenous gene and studying its in-vivo impact has become possible in the Drosophila system (Phelps and Brand, 1998). Eye specific Gal4 driver, GMR-Gal4, can drive the expression of any gene of interest with UAS promoter in the fly eye. Additionally, elav-Gal4 can drive gene expression in the photoreceptor neurons along with all the other neurons of fly body (Marsh et al., 2000).

How fly eyes can help us?

The analysis of adult Drosophila eye can begin with a basic manual inspection of the eye morphology such as depigmentation, the appearance of black patches signifying necrotic cell death and loss or de-patterning of bristles. These manual analyses can be executed by observing the eye of an anesthetized fly or as a whole mount under a high magnification light microscope (Mishra and Knust, 2012).  Histological sectioning and staining can also provide information regarding the inner morphological changes like cell loss and reduction in retinal thickness (Diez-Hermano et al., 2015; Wolff, 2010). Additionally, the pseudo-pupil analysis can quantitatively provide information on photoreceptor neuronal loss. It encompasses visualizing and counting complete rhabdomeres in each ommatidium visible in the field of view under the microscope. Immunohistochemistry of the larval eye disc can aid in the detection of proteins of interest. Also, staining the nucleic acid of cells with dyes like- acridine orange, Hoechst, propidium iodide and DAPI which detects the nucleic acid damage can give an indication of cell-death when visualized under a fluorescent microscope (Sahu and Mishra, 2020). The visual system of Drosophila thus provides an elegant and simple platform to understand and find solutions to a wide range of complicated human diseases.

Drosophila eye, simple answers to difficult questions!

References

1. Cagan, R. (2009). Principles of Drosophila Eye Differentiation. Curr Top Dev Biol, 89, 115–135.
2. Diez-Hermano, S., Valero, J., Rueda, C., Ganfornina, M.D., & Sanchez, D. (2015). An automated image analysis method to measure regularity in biological patterns: a case study in a Drosophila neurodegenerative model. Molecular Neurodegeneration, 10, 9.
3. Iyer, J., Wang, Q., Le, T., Pizzo, L., Grönke, S., Ambegaokar, S.S., Imai, Y., Srivastava, A., Troisí, B.L., Mardon, G., Artero, R., Jackson, G.R., Isaacs, A.M., Partridge, L., Lu, B., Kumar, J.P., & Girirajan, S. (2016). Quantitative assessment of eye phenotypes for functional genetic studies using Drosophila melanogaster. G3 (Bethesda), 6(5), 1427–1437.
4. Lehmann, M., Knust, E., & Hebbar, S. (2019). Drosophila melanogaster: A Valuable Genetic Model Organism to Elucidate the Biology of Retinitis Pigmentosa. In B.H.F. Weber & T. Langmann (Eds.), Retinal Degeneration, Methods in Molecular Biology, (2^nd, vol. 1834, pp. 221-249). Humana, New York, NY.
5. Marsh, J.L.,Walker, H., Theisen, H., Zhu, Y.Z., Fielder, T., Purcell, J., & ThompsonM. (2000). Expanded polyglutamine peptides alone are intrinsically cytotoxic and cause neurodegeneration in Drosophila. Human Molecular Genetics, 9(1), 13-25.
6. Mishra, M., & Knust, E. (2012). Analysis of the DrosophilaCompound Eye with Light and Electron Microscopy. In H.F. Weber & T. Langmann (Eds.), Retinal Degeneration, Methods in Molecular Biology (Methods and Protocols), (vol. 935, pp. 161-182). Humana Press, Totowa, NJ.
7. Phelps, C.B. & Brand A.H. (1998). Ectopic gene expression in Drosophila using GAL4 system. Methods: A Companion to Methods in Enzymology, 14(4), 367–379.
8. Sahu, S., & Mishra, M. (2020). Simple Histochemical Methods to Detect Cell Death in the Eye-Antennae Imaginal Disc of Drosophila. In M. Mishra (Ed.), Fundamental Approaches to Screen Abnormalities in Drosophila, Springer Protocols Handbooks, (pp. 77-86). Springer, New York, NY.
9. Shulman, J.M., Shulman, L.M., Weiner, W.J., & Feany, M.B. (2003). From fruit fly to bedside: translating lessons from Drosophila models of neurodegenerative disease. Current Opinion in Neurology, 16(4), 443–449.
10. Wolff, T. (2010). Cryosectioning and Immunocytochemistry of Adult Drosophila Eye Sections.  Cold Spring Harb Protoc, 2010(1), pdb.prot5370

Pictures in this content was taken by Megha Chatterjee, Senior Research Fellow, Department of Zoology, University of Delhi