Light exposure during embryonic and yolk‐sac alevin development of Chinook salmon Oncorhynchus tshawytscha does not alter the spectral phenotype of photoreceptors

Abstract

Colour vision is mediated by the expression of different visual pigments in photoreceptors of the vertebrate retina. Each visual pigment is a complex of a protein (opsin) and a vitamin A chromophore; alterations to either component affects visual pigment absorbance and, potentially, the visual capabilities of an animal. Many species of fish undergo changes in opsin expression during retinal development. In the case of salmonid fishes the single cone photoreceptors undergo a switch in opsin expression from SWS1 (ultraviolet sensitive) to SWS2 (blue‐light sensitive) starting at the yolk‐sac alevin stage, around the time when they first experience light. Whether light may initiate this event or produce a plastic response in the various photoreceptors is unknown. In this study, Chinook salmon Oncorhynchus tshawytscha were exposed to light from the embryonic (5 days prior to hatching) into the yolk sac alevin (25 days post hatching) stage and the spectral phenotype of photoreceptors assessed with respect to that of unexposed controls by in situ hybridization with opsin riboprobes. Light exposure did not change the spectral phenotype of photoreceptors, their overall morphology or spatial arrangement. These results concur with those from a variety of fish species and suggest that plasticity in photoreceptor spectral phenotype via changes in opsin expression may not be a widespread occurrence among teleosts.

Growth hormone regulates opsin expression in the retina of a salmonid fish

“…Salmonid fishes in the genus Oncorhynchus hatch with a retina in which all the single cones express SWS1 opsin . Around the time when the alevin absorbs its yolk sac and rises from the gravel to begin a life of active foraging, single cones in the ventro‐temporal retina start switching opsin expression from SWS1 to SWS2 . This transformation proceeds from the ventral to the dorsal retina such that, by the time the fish becomes a young smolt, only the dorso‐temporal retina has some single cones that still express predominantly SWS1 opsin .…”

“…Following the start of the SWS1 to SWS2 opsin switch, single corner cones begin to disappear from the square mosaic in the ventral retina of alevin salmonids . It has been hypothesised, although never proven conclusively, that these cones become apoptotic .…”

“…24,65 Around the time when the alevin absorbs its yolk sac and rises from the gravel to begin a life of active foraging, single cones in the ventro-temporal retina start switching opsin expression from SWS1 to SWS2. 15,16,24,65,68 This transformation proceeds from the ventral to the dorsal retina such that, by the time the fish becomes a young smolt, only the dorso-temporal retina has some single cones that still express predominantly SWS1 opsin. 15,16 In returning adults, all the single cones in the main retina express SWS2 opsin.…”

“…74,75 Following the start of the SWS1 to SWS2 opsin switch, single corner cones begin to disappear from the square mosaic in the ventral retina of alevin salmonids. 15,16,23,24,68 It has been hypothesised, although never proven conclusively, that these cones become apoptotic. 16 Growth hormone exerts a neuroprotective role in the retina of chick and human by promoting ganglion cell differentiation and survival, and this anti-apoptotic effect is thought to be mediated via the insulin-like growth factor 1.…”

The pink salmon genome: uncovering the genomic consequences of a strict two-year life-cycle

“…In the eye, this transition requires the development of new functionality such as night vision to chase prey. One example of such a transition is the change of UV opsins observed in hatched Pacific salmon to blue later in life by opsin changeover (144,145). Variation in vision related genes have previously been observed between sockeye salmon populations (77).…”