Expression profiling of zebrafish sox9 mutants reveals that Sox9 is required for retinal differentiation

The transcription factor gene Sox9 plays various roles in development, including differentiation of the skeleton, gonads, glia, and heart. Other functions of Sox9 remain enigmatic. Because Sox9 protein regulates expression of target genes, the identification of Sox9 targets should facilitate an understanding of the mechanisms of Sox9 action. To help identify Sox9 targets, we used microarray expression profiling to compare wild-type embryos to mutant embryos lacking activity for both sox9a and sox9b, the zebrafish co-orthologs of Sox9. Candidate genes were further evaluated by whole-mount in situ hybridization in wild-type and sox9 single and double mutant embryos. Results identified genes expressed in cartilage (col2a1a and col11a2), retina (calb2a, calb2b, crx, neurod, rs1, sox4a and vsx1) and pectoral fin bud (klf2b and EST AI722369) as candidate targets for Sox9. Cartilage is a well-characterized Sox9 target, which validates this strategy, whereas retina represents a novel Sox9 function. Analysis of mutant phenotypes confirmed that Sox9 helps regulate the number of Müller glia and photoreceptor cells and helps organize the neural retina. These roles in eye development were previously unrecognized and reinforce the multiple functions that Sox9 plays in vertebrate development.