Derepression of a Subset of Meiotic Proteins in Primordial Germ Cells of max Mutant Zebrafish

Germ cells in zebrafish are specified by a preformation mechanism, unlike in mice. Max (Myc associated factor X) and its associated PRC1.6 (polycomb repressive complex 1.6) act as negative regulators of germ cell- and meiosis-related genes in mice; however, their function in zebrafish remains unknown. Here, we generated max mutants in zebrafish using genome-editing techniques to investigate the role of Max in germ cell development and meiosis. No obvious morphological abnormalities were observed in the mutants up to 4 days post fertilization (dpf), likely due to maternally supplied max mRNAs. However, the mutants failed to form an inflated swim bladder and gradually died after 10 dpf. The number of primordial germ cells (PGCs) did not differ between the max mutant and wild-type larvae up to 12 dpf. In contrast, analysis of protein expression levels of germ cell-related and meiosis-related genes in PGCs at 9 dpf revealed that the levels of Sycp2, Sycp3, and Dmc1 proteins were significantly increased in max mutants, whereas the expression of Ddx4 and Piwil1 remained unchanged. Together, these results suggest that zebrafish Max selectively represses the expression of a subset of meiotic genes in PGCs.