The eyes have it: genetic, morphological and functional analysis of differences in compound eyes between Drosophila species

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Vision is one of the major sensory systems that helps animals navigate through the environment. There is great variation among the eyes of animals reflecting evolutionary adaptations in behaviour and habitat. Two major parameters that determine the quality of vision are: 1) How easily can objects be discriminated at different levels of light luminescence (contrast sensitivity) and 2) how far apart do two objects have to be to be detected discriminated (acuity). Contrast sensitivity depends on the amount of light absorbed by the eye's photoreceptors and therefore on the size of the eye and photoreceptor sensitivity. Acuity on the other hand depends on the density of those light-sensitive photoreceptors, for example, the density of rods and cones in the retina of the human eye. The same rules generally apply to the compound eyes of insects, which are made up of smaller eyes, called ommatidia. Larger ommatidia with larger lens diameters to collect photons gives higher contrast sensitivity. However, acuity depends on the density of ommatidia - more ommatidia per eye area gives higher acuity, and therefore there can be a trade off between contrast sensitivity and acuity determined by the size and number of ommatidia. The size of ommatidia and overall eye size varies greatly among insects, but we still know very little about the genes underlying these evolutionary differences, how this changes the development of eyes, and how these changes in eye morphology alter the vision of these animals. We have found that the gene orthodenticle (otd) causes differences in ommatidia diameter and overall eye size between the closely related species Drosophila mauritiana and Drosophila simulans. We now propose to use a range of approaches to understand how otd regulates ommatidia size and how this gene makes larger eyes in D. mauritiana, and the consequences for their vision. We will first further study the role of otd in specification of ommatidia size in the model organism D. melanogaster to learn more about the mechanisms involved. We will then put the D. mauritiana version of otd into a D. simulans genetic background and vice versa, resulting in a D. simulans fly with a D. mauritiana-like eye and the other way around. These flies can then be compared to pure D. simulans and D. mauritiana flies in their regulation of eye development, eye morphology, neuronal activity and their actual vision. The results of these experiments will reveal how natural genetic variation alters eye development to produce eyes with different optics and how this affects the vision of these flies. This will help us to better understand insect vision generally and provide new insights into how the great diversity of insect eyes and the vision of these animals have evolved. In addition, since there are major similarities between invertebrate and vertebrate eye development, including important roles of otd and its homologues, this project could provide new information about eye development more broadly in animals including humans.