A Cambrian origin for vertebrate rods

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Abstract

Vertebrates acquired dim light vision when an ancestral cone evolved into the rod photoreceptor at an unknown stage preceding the last common ancestor of extant jawed vertebrates (~420 million years ago Ma). The jawless lampreys provide a unique opportunity to constrain the timing of this advance, as their line diverged ~505 Ma and later displayed high morphological stability. We recorded with patch electrodes the inner segment photovoltages and with suction electrodes the outer segment photocurrents of Lampetra fluviatilis retinal photoreceptors. Several key functional features of jawed vertebrate rods are present in their phylogenetically homologous photoreceptors in lamprey: crucially, the efficient amplification of the effect of single photons, measured by multiple parameters, and the flow of rod signals into cones. These results make convergent evolution in the jawless and jawed vertebrate lines unlikely and indicate an early origin of rods, implying strong selective pressure toward dim light vision in Cambrian ecosystems.

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Sabrina Asteriti

Department of Translational Research, University of Pisa, Pisa, Italy

Competing interests
The authors declare that no competing interests exist.
Sten Grillner

Department of Neuroscience, Karolinska Institute, Stockholm, Sweden

Competing interests
The authors declare that no competing interests exist.
Lorenzo Cangiano

Department of Translational Research, University of Pisa, Pisa, Italy

For correspondence
lorenzo.cangiano@unipi.it

Competing interests
The authors declare that no competing interests exist.

Ethics

Animal experimentation: Animal experimentation: All procedures involving the handling of experimental animals were approved by the Ethical Committee of the University of Pisa (prot. n. 2891/12) and were conducted in accordance with Italian (D.lgs.vo 116/92) and EU regulations (Council Directive 86/609/EEC).

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This article is distributed under the terms of the Creative Commons Attribution License permitting unrestricted use and redistribution provided that the original author and source are credited.