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T. D. Lamb, S. P. Collin, E. N. Pugh, Jr.; Evolution of the Vertebrate Eye, Retina, and Photoreceptors: Hypotheses and Tests. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1265.
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To analyse the sequence of events recently hypothesized  to underlie the evolution of the vertebrate eye, retina, and photoreceptors. To refine the steps in the proposed sequence (and their paleontological timing), and to design further tests of the proposals.
We have reviewed previously-reported findings from studies covering paleontology, chordate evolution, photoreceptor comparative morphology, photoreceptor electrophysiology, opsin phylogeny, eyecup development, and retinal circuitry, in light of a recent model of eye evolution.
Hagfish are primitive craniates, with "eyes" that are much simpler than those of vertebrates, resembling the pineal organ of non-mammalian vertebrates in many respects. Detailed study of the genes, photoreceptors, retina, and eye of hagfish are likely to provide important clues in determining the origin of the vertebrate eye. However, the phylogenetic position of hagfish remains unclear, and clarification of their correct phylogenetic position will be crucial in interpreting data on the evolution of the eye. Until the entire genome of a hagfish species is available, it will be necessary to rely on a relatively small number of available gene sequences. We propose the following sets of experiments: (1) Extraction and sequencing of the genes for hagfish opsin(s) and phototransduction proteins. (2) Ultrastructural examination of hagfish photoreceptors, with emphasis on the outer segment and synaptic terminal, and on the identity of the post-synaptic neurons. (3) Retrograde transport of labels via axons of the optic nerve, to test whether photoreceptors make direct contact with retinal projection neurons. (4) A search for bipolar cells (using markers characterized in vertebrates), since the model predicts that bipolar cells should not be present in the hagfish retina. Results from the literature, from other groups, and from our own investigations will be presented.
In light of a recently-proposed scheme to account for the evolution of the vertebrate eye, it is possible to propose a variety of tests, that should help to either rule out, or support, revise, or refine the proposed sequence of eye designs, and thereby provide a comprehensive account of the evolutionary origin of the human eye. Lamb, Collin & Pugh (2007). Nature Reviews Neuroscience 8, 960-975.
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