Switching from stochastic to deterministic patterning in fly retinas: mechanisms and behavioral significance

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The specification of cell fate occurs through extremely ordered and deterministic processes. This insures highly reproducible outcomes that reduce genetic and environmental noise during development. In many sensory systems, however, patterning processes produce random, “stochastic” distributions of sensory receptors that differ even in identical genetic backgrounds. The stochastic specification of photoreceptor (PR) subtypes in the Drosophila retina is one of the best-studied models of such patterning processes. The compound eyes of flies comprise ca. 750 ommatidia (unit-eyes), each containing eight PR cells. Color vision is mediated by two of these eight PRs, R7 and R8, which express different pairs of Rhodopsins in two distinct ommatidial subtypes. These two subtypes are distributed stochastically in most insect retinas, as in Drosophila. However, retinas found in some Dolichopodidae (long-legged flies, “Doli”) stand out: here, the two ommatidial subtypes are arranged in highly ordered columns producing vertical stripes across the retina. I will investigate the mechanisms that lead to the deterministic instead of a stochastic pattern in the dolichopodid retina and study its biological significance. The drastically different patterning in two otherwise equivalent systems, the retinas of Doli and Drosophila, provides a unique system to understand the regulation of two fundamental strategies of cell fate decisions. The change from a stochastic to a striped retina might have occurred due to evolutionary pressure and in favor of exploring new ecological niches. The project is divided into three main parts.(1) First, I will characterize the anatomical basis of vision in the Doli retina and combine detailed characterization of PR morphology and Rhodopsin expression patterns in a 3D model. I will determine if the columns align with the progress of early developmental programs, i.e. the wave of differentiation that crosses the eye imaginal disc from posterior to anterior. This ‘morphogenetic furrow’ produces columns of ommatidia that are specified in a temporal manner. (2) Second, to understand the origin of the deterministic pattern, I will investigate how the regulatory factors that control the retinal mosaic in Drosophila are expressed in Doli. I aim to discover the specific changes in the gene network that cause the switching between the patterns and are responsible for the development of stripes. I will be able to use the sophisticated tools of Drosophila genetics in an attempt to produce a deterministic patterning in Drosophila similar to that found in the Doli retina. (3) Finally, I will investigate the visual stimuli to which the Doli eye has adapted and compare the behavioral responses of Doli species with deterministic or stochastic patterning, and transgenic Drosophila. This will help to unravel why retinal stripes have evolved from a default stochastic patterning.