Foxq2 determines blue cone identity in zebrafish

Most vertebrate lineages retain a tetrachromatic visual system, which is supported by a functional combination of spectrally distinct multiple cone photoreceptors, UV, blue, green, and red cones. The blue cone identity is ensured by selective expression of blue ( sws2 ) opsin, and the mechanism is poorly understood because SWS2 gene has been lost in mammalian species such as mouse, whose visual system has been extensively studied. Here we pursued loss-of-function studies on transcription factors expressed predominantly in zebrafish cone photoreceptors and identified Foxq2 as a core factor driving sws2 gene expression. foxq2 is expressed only in the blue cone, and loss of foxq2 selectively abolishes sws2 expression. Comparative genomic analysis revealed that a wide range of vertebrate species retain both FOXQ2 and SWS2 genes. We propose that FOXQ2-dependent SWS2 expression is a prevalent regulatory mechanism that was acquired at the early stage of vertebrate evolution