Foxq2 determines blue cone identity in zebrafish.

Abstract

Most vertebrate lineages retain a tetrachromatic visual system, which is supported by a functional combination of spectrally distinct multiple cone photoreceptors, ultraviolet (UV), blue, green, and red cones. The blue cone identity is ensured by selective expression of blue (<i>sws2</i>) opsin, and the mechanism is poorly understood because <i>sws2</i> 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 blue cone–specific factor driving <i>sws2</i> gene expression. Foxq2 has dual functions acting as an activator of <i>sws2</i> transcription and as a suppressor of UV (<i>sws1</i>) opsin transcription in blue cones. A wide range of vertebrate species retain both <i>foxq2</i> and <i>sws2</i> genes. We propose that Foxq2-dependent <i>sws2</i> expression is a prevalent regulatory mechanism that was acquired at the early stage of vertebrate evolution.