Abstract

Recent findings shed light on the steps underlying the evolution of vertebrate photoreceptors and retina. Vertebrate ciliary photoreceptors are not as wholly distinct from invertebrate rhabdomeric photoreceptors as is sometimes thought. Recent information on the phylogenies of ciliary and rhabdomeric opsins has helped in constructing the likely routes followed during evolution. Clues to the factors that led the early vertebrate retina to become invaginated can be obtained by combining recent knowledge about the origin of the pathway for dark re-isomerization of retinoids with knowledge of the inability of ciliary opsins to undergo photoreversal, along with consideration of the constraints imposed under the very low light levels in the deep ocean. Investigation of the origin of cell classes in the vertebrate retina provides support for the notion that cones, rods and bipolar cells all originated from a primordial ciliary photoreceptor, whereas ganglion cells, amacrine cells and horizontal cells all originated from rhabdomeric photoreceptors. Knowledge of the molecular differences between cones and rods, together with knowledge of the scotopic signalling pathway, provides an understanding of the evolution of rods and of the rods' retinal circuitry. Accordingly, it has been possible to propose a plausible scenario for the sequence of evolutionary steps that led to the emergence of vertebrate photoreceptors and retina.

Highlights

  • The purpose of this review is to consider how it may have been that photoreception in our eyes evolved

  • The consideration of this evolutionary line will run from the primitive bilaterally symmetric organisms that predated the divergence of protostomes and deuterostomes, through the emergence of early chordates, through the Cambrian explosion when craniates and vertebrates appeared, and through to the near-perfection of vertebrate retinal photoreception that had been reached by the time that jawed vertebrates appeared

  • SUMMARY: A SCENARIO FOR THE EVOLUTION OF VERTEBRATE RETINAL PHOTORECEPTION Combining the information set out above, it is possible to present the following as a likely scenario for the sequence of events by which vertebrate retinal photoreception arose

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Summary

INTRODUCTION

The purpose of this review is to consider how it may have been that photoreception in our eyes evolved. The emphasis will be on events that occurred in our own ancestors; i.e. in the line leading to mammals, as illustrated in the evolutionary trees in several figures in this Theme Issue Larhammar et al 2009; figure 2) The consideration of this evolutionary line will run from the primitive bilaterally symmetric organisms that predated the divergence of protostomes and deuterostomes, through the emergence of early chordates, through the Cambrian explosion when craniates and vertebrates appeared, and through to the near-perfection of vertebrate retinal photoreception that had been reached by the time that jawed vertebrates appeared. A scenario will be described for the overall sequence and likely timing of the principal events that led to the evolution of our retina and its photoreceptors

ORIGIN OF VERTEBRATE RETINAL PHOTORECEPTORS AND OPSINS
ORIGIN OF THE VERTEBRATE EYECUP
ORIGIN OF VERTEBRATE RETINAL CELLS AND CIRCUITRY
ORIGIN OF VERTEBRATE SCOTOPIC VISION
Findings
SUMMARY: A SCENARIO FOR THE EVOLUTION OF VERTEBRATE RETINAL PHOTORECEPTION
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