Role of short-wave-sensitive 1 (sws1) in cone development and first feeding in larval zebrafish

Dietary carotenoids enhance SWS1 expression in female western mosquitofish (Gambusia affinis) but do not impair their likelihood of pregnancy in the presence of male guppy

The various cone opsin genes are responsible for distinct ecological tasks, with the altered expression profiles in teleost fishes representing an excellent paradigm for studying how fishes can quickly adapt to diverse habitats within their lifecycles. The molecular mechanisms underlying transcriptional switching among cone opsin genes are still being investigated, but factors such as light conditions, developmental stages, sex hormones, and diet are known to play a role in changing cone opsin expression profiles. Based on previous research on guppies, we hypothesized that a diet rich in carotenoids could enhance expression of the opsin gene LWS in western mosquitofish (Gambusia affinis) and potentially influence female mate choice. We raised female western mosquitofish under low-level or high-level carotenoid diets and then conducted female mating preference experiments, with or without the presence of male guppy (Poecilia reticulata). qPCR revealed that high carotenoid intake upregulates SWS1 rather than LWS transcription. This positive feedback loop may promote foraging efficiency and also protect the visual system from UV damage. The carotenoid diets had no effect on pregnancy likelihood, possibly because UV light is not a critical cue in western mosquitofish female mate choice and/or the light source we used did not encompass the UV spectrum. Presence of male guppies had no effect on pregnancy likelihood, though a previous study reported that it significantly reduced brood size. Therefore, interactions between male guppies and western mosquitofish likely reduces the number of copulations and/or disrupts parenting to reduce the number of offspring.

Single cones give rise to multi-cone types in the retinas of fishes

Retinal cone photoreceptors are specialized neurons that capture light to begin the process of daylight vision. They occur as individual cells (i.e., single cones), or as combinations of structurally linked cells, such as the double and triple cones found in the retinas of non-eutherian vertebrates. These different morphological cone types form mosaics of varying regularity, with single and double cones patterned as nearly perfect lattices in the retinas of many bony fishes (teleosts) and some geckos. Although double cones were first reported over 150 years ago, how they form (i.e., whether from coalescing single cones, or from structurally linked cone progenitors) remains uncertain. In turn, whether there is a general vertebrate sequence in appearance of morphological cone types and mosaics is unknown. Here, the developing retinas of seven species of teleosts were examined revealing that only single cones, arranged in hexagonal-like mosaics, were present at the earliest stages of photoreceptor differentiation. Double cones arose from coalescing single cones and the formation of multi-cone type mosaics (such as the square mosaic, where each single cone is surrounded by four double cones) followed different dynamics depending on whether the species was altricial or precocial. Single cones were therefore the primordial cells from which all multi-cone types arose and hexagonal-like mosaics preceded other mosaic patterns. Based on observations from transitional retinas, we propose a model for mosaic transformation from hexagonal to square. The double cones of fishes and those of land vertebrates constitute an example of convergent evolution to achieve the elliptical waveguide structure, likely for improved spatio-temporal resolution.

Knockout of lws1 in zebrafish (Danio rerio) reveals its role in regulating feeding and vision-guided behavior

Long-wave sensitive (LWS) is a G protein-coupled receptor expressed in the retina, and zebrafish is a better model organism for studying vision, but the role of LWS1 in vision-guided behavior of larvae fish has rarely been reported. In this study, we found that zebrafish lws1 and lws2 are tandemly replicated genes, both with six exons, with lws1 being more evolutionarily conserved. The presence of Y277F in the amino acid sequence of lws2 may have contributed to the shift of λmax to green light. We established a lws1 knockout zebrafish model using CRISPR/Cas9 technology. Lws1-/- larvae showed significantly higher levels of feeding and appetite gene (agrp) expression than WT, and significantly lower levels of anorexia gene (pomc, cart) expression. In addition, green light gene compensation was observed in lws1-/- larvae with significantly increased expression levels of rh2-1. The light-dark movement test showed that lws1-/- larvae were more active under light-dark transitions or vibrational stimuli, and the expression of phototransduction-related genes was significantly up-regulated. This study reveals the important role of lws1 gene in the regulation of vision-guided behavior in larvae.