Abstract
The vertebrate eye is a vital sensory organ that has long fascinated scientists, but the details of how this organ evolved are still unclear. The vertebrate eye is distinct from the simple photoreceptive organs of other non-vertebrate chordates and there are no clear transitional forms of the eye in the fossil record. To investigate the evolution of the eye we can examine the eyes of the most ancient extant vertebrates, the hagfish and lamprey. These jawless vertebrates are in an ideal phylogenetic position to study the origin of the vertebrate eye but data on eye/retina development in these organisms is limited. New genomic and gene expression data from hagfish and lamprey suggest they have many of the same genes for eye development and retinal neurogenesis as jawed vertebrates, but functional work to determine if these genes operate in retinogenesis similarly to other vertebrates is missing. In addition, hagfish express a marker of proliferative retinal cells (Pax6) near the margin of the retina, and adult retinal growth is apparent in some species. This finding of eye growth late into hagfish ontogeny is unexpected given the degenerate eye phenotype. Further studies dissecting retinal neurogenesis in jawless vertebrates would allow for comparison of the mechanisms of retinal development between cyclostome and gnathostome eyes and provide insight into the evolutionary origins of the vertebrate eye.
Highlights
To survive in adverse environments organisms must be able to perceive and respond to their surroundings
Even more centrally several genes related to the differentiation of retinal cells are expressed. These results suggest that the cells from the ciliary marginal zone (CMZ) pass through several competence stages across the CMZ before taking on their final differentiated state
Under this model the extant hagfish eye represents a tissue that is akin to a transitional state between the relatively simple photoreceptive cells of other chordates and the more complex eyes of other vertebrate groups (Figure 4A)
Summary
To survive in adverse environments organisms must be able to perceive and respond to their surroundings. Through specialized cells (photoreceptors) animals can make use of light information to sense their environment (Arendt, 2003). Representatives of Cnidaria, Mollusca, Annelida, Onychophora, Arthropoda and Chordata are examples of organisms that have developed complex eyes, often associated with image formation (Zuker, 1994). Despite these groups being phylogenetically distant, their eye structures appear to have independently evolved to perform similar functions (often through similar mechanisms) reinforcing the value of visual information across organisms (Fernald, 2000; Nilsson, 2013). The retina of the vertebrate eye is specialized to form detailed images, detect motion, enhance contrast and/or
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