The evolution of rod photoreceptors

Feb 15, 2017Philosophical transactions of the Royal Society of London. Series B, Biological sciences

How rod photoreceptors have evolved

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Abstract

Vertebrate ciliary photoreceptors may use less ATP than rhabdomeric photoreceptors.

Ciliary photoreceptors are found in many animal phyla, but vertebrates uniquely possess two types that cover a wide vision range.
The evolution of rods from less sensitive cone-like photoreceptors is linked to the development of a duplex retina.
Present evidence suggests that rods evolved very early in vertebrate history, prior to the divergence of jawed and jawless vertebrates.
Outer-segment discs in rods do not contribute to sensitivity but may enhance the efficiency of protein renewal.
Rod evolution was incremental and involved various changes in protein isoforms and expression, without any single change significantly affecting transduction.

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Photoreceptors in animals are generally of two kinds: the ciliary or c-type and the rhabdomeric or r-type. Although ciliary photoreceptors are found in many phyla, vertebrates seem to be unique in having two distinct kinds which together span the entire range of vision, from single photons to bright light. We ask why the principal photoreceptors of vertebrates are ciliary and not rhabdomeric, and how rods evolved from less sensitive cone-like photoreceptors to produce our duplex retina. We suggest that the principal advantage of vertebrate ciliary receptors is that they use less ATP than rhabdomeric photoreceptors. This difference may have provided sufficient selection pressure for the development of a completely ciliary eye. Although many of the details of rod evolution are still uncertain, present evidence indicates that (i) rods evolved very early before the split between the jawed and jawless vertebrates, (ii) outer-segment discs make no contribution to rod sensitivity but may have evolved to increase the efficiency of protein renewal, and (iii) evolution of the rod was incremental and multifaceted, produced by the formation of several novel protein isoforms and by changes in protein expression, with no one alteration having more than a few-fold effect on transduction activation or inactivation.This article is part of the themed issue 'Vision in dim light'.

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The evolution of rod photoreceptors

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