Six6 and Six7 coordinately regulate expression of middle-wavelength opsins in zebrafish.

Abstract

Color discrimination in the vertebrate retina is mediated by a combination of spectrally distinct cone photoreceptors, each expressing one of multiple cone opsins. The opsin genes diverged early in vertebrate evolution into four classes maximally sensitive to varying wavelengths of light: UV (SWS1), blue (SWS2), green (RH2), and red (LWS) opsins. Although the tetrachromatic cone system is retained in most nonmammalian vertebrate lineages, the transcriptional mechanism underlying gene expression of the cone opsins remains elusive, particularly for SWS2 and RH2 opsins, both of which have been lost in the mammalian lineage. In zebrafish, which have all four cone subtypes, <i>rh2</i> opsin gene expression depends on a homeobox transcription factor, <i>sine oculis</i> homeobox 7 (Six7). However, the <i>six7</i> gene is found only in the ray-finned fish lineage, suggesting the existence of another evolutionarily conserved transcriptional factor(s) controlling <i>rh2</i> opsin expression in vertebrates. Here, we found that the reduced <i>rh2</i> expression caused by <i>six7</i> deficiency was rescued by forced expression of <i>six6b</i>, which is a <i>six7-</i>related transcription factor conserved widely among vertebrates. The compensatory role of <i>six6b</i> was reinforced by ChIP-sequencing analysis, which revealed a similar pattern of Six6b- and Six7-binding sites within and near the cone opsin genes. TAL effector nuclease-induced genetic ablation of <i>six6b</i> and <i>six7</i> revealed that they coordinately regulate SWS2 opsin gene expression. Mutant larvae deficient for these transcription factors showed severely impaired visually driven foraging behavior. These results demonstrate that in zebrafish, <i>six6b</i> and <i>six7</i> govern expression of the SWS2 and RH2 opsins responsible for middle-wavelength sensitivity, which would be physiologically important for daylight vision.