Evolution of photoreceptors as illuminated by fossils and development

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The long-term Aim of my Program is to make foundational insights into photoreceptor evolution and visual ecology. We accomplish this principally by comparing the development (evo-devo) of photoreceptors between disparate vertebrates and between ontogenetic stages. We often use zebrafish as a platform to experimentally test the novel hypotheses generated from exploring other more interesting animals (jawless fishes, migrating fishes, crustaceans). In the coming years we will investigate provocative new ideas emerging from our recent work:*** The majority of Mammalian rod photoreceptors develop and evolved from cone photoreceptors, especially from the short-wavelength-sensitive (SWS- or UV-) cones;*** To refine cell type homology concepts, and to test if rod photoreceptors evolved from SWS cones, we will compare the propensity of the 4 cone types to transdifferentiate into rods;*** The identity and evolution of rods vs. cones can be revealed in hagfish (thus revealing early vertebrate evolution) by assessing expression of key developmental genes alongside incisively testing hagfish gene function in our zebrafish platform. ***My HQP will be well-positioned to make foundational insights into the biology and evolution of the duplex retina, i.e. the biology behind possessing both rods and cones that has empowered the immense success of the vertebrate radiation.******Further, we remain puzzled and fascinated by the elaborately arrayed patterns of photoreceptors observed in the retinas of most animals (though lost in mammals). Our genetic approaches in zebrafish continue to reveal mechanisms of this photoreceptor mosaic formation and mosaic functions. To complement this, we will expand our Program now to compare to the development of photoreceptor arrays in crustaceans. Ontogenetic changes in ommatidia and photoreceptor patterning therein has striking (presumably convergent?) similarities to those of fish photoreceptors. Decapod crustaceans (crabs, crayfish, etc.) of special interest have square (“reflecting mirror box”) ommatidia, however they BEGIN larval life with the familiar hexagonal ommatidia. Amazingly, and unlike in vertebrates, these photoreceptor morphologies are well-preserved in the fossil record. We will compare fossils to the ommatidia and photoreceptor complement in extant crabs and shrimps, with special focus on arrays that change during ontogeny and/or have regional specialization such that multiple morphologies occur simultaneously in each individual eye. ******Understanding the evolution of vertebrate photoreceptors is hampered by lack of fossil record. My HQP overcome this challenge via two strategies: First: by comparing the developmental program that produces photoreceptors, we will uniquely reveal ancient homologies. Second: we will identify the pitfalls and opportunities of this evo-devo approach, as applied to sensory systems, by interrogating a developmental system that can be calibrated against fossil photoreceptors.**