Functional Equivalence of the SOX2 and SOX3 Transcription Factors in the Developing Mouse Brain and Testes.

Gene duplication provides spare genetic material that evolution can craft into new functions. and are evolutionarily related genes with overlapping and unique sites of expression during embryogenesis. It is currently unclear whether SOX2 and SOX3 have identical or different functions. Here, we use CRISPR/Cas9-assisted mutagenesis to perform a gene-swap, replacing the ORF with the ORF to investigate their functional equivalence in the brain and testes. We show that increased expression of SOX2 can functionally replace SOX3 in the development of the infundibular recess/ventral diencephalon, and largely rescues pituitary gland defects that occur in null mice. We also show that ectopic expression of SOX2 in the testes functionally rescues the spermatogenic defect of null mice, and restores gene expression to near normal levels. Together, these data provide strong evidence that SOX2 and SOX3 proteins are functionally equivalent.