Dissecting the determinants of light sensitivity in amphioxus microvillar photoreceptors: Possible evolutionary implications for melanopsin signaling

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Summary

This study uses electrophysiology in amphioxus photoreceptors to trace the evolutionary pathway by which melanopsin-based photoreceptors transitioned from high-sensitivity visual receptors to low-sensitivity circadian light sensors in vertebrates. The findings help explain why ipRGCs in vertebrates require relatively high light levels for circadian entrainment, with implications for understanding the biological basis of melanopic sensitivity thresholds used in lighting standards.

Key Findings

The gain of the melanopsin phototransduction cascade in amphioxus is lower than in canonical rhabdomeric (microvillar) photoreceptors of invertebrates, as determined by comparing single-photon current fluctuations to single-channel activation sizes.
Melanopsin expression levels in amphioxus photoreceptors are comparable to those in canonical visual receptors, suggesting that reduced photosensitivity in vertebrate circadian receptors arose from decreased pigment expression and loss of microvillar anatomy rather than low pigment abundance.
The study concludes that a modest amplification in melanopsin-using receptors is apparent in early chordates, with vertebrates subsequently evolving lower photosensitivity optimized for reporting mean ambient illumination rather than spatial vision.