Spatiotemporal expression of the zebrafish pax9 gene that is essential for median fin patterning

PAX9 is an evolutionarily conserved paired-box transcription factor that is critical for embryonic development and human diseases. The mouse model has been predominantly used to investigate Pax9 functions. Zebrafish has emerged as a complementary vertebrate model for various human diseases, including cancers. Until recently, the functions of the zebrafish pax9 gene in jaw and hematopoiesis have started to be uncovered. However, detailed pax9 spatiotemporal expression, molecular mechanisms, and potential functions in other zebrafish organs remain largely unknown. With the technical advances in CRISPR-Cas9, non-homologous end joining (NHEJ) has made knockin and knockout a convenient way to examine endogenous gene expression in vivo and to generate a loss-of-function allele simultaneously. Here, we first generated pax9 knockin fish lines by inserting fluorescent proteins at the start of the endogenous pax9 coding region. Then, we examined pax9 expression in real time from early embryonic stages through adulthood. Except for previously reported expression domains, we were able to identify pax9 expression in high resolution in the paired and median fins, where pax9 marks anterior fin rays. Moreover, our knockin and knockout mutants showed increased fin ray number in median fins, but no evident effect on paired fins in pax9 null mutants. Thus, PAX9 is critical for median fin patterning in zebrafish.