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