foxr1is a novel maternal-effect gene in fish that regulates embryonic cell growth viap21andrictor

The family of forkhead box (Fox) transcription factors regulate gonadogenesis and embryogenesis, but the role offoxr1/foxn5in reproduction is unknown. Evolution offoxr1in vertebrates was examined and the gene found to exist in most vertebrates, including mammals, ray-finned fish, amphibians, and sauropsids. By quantitative PCR and RNA-seq, we found thatfoxr1had an ovarian-specific expression in zebrafish, a common feature of maternal-effect genes. In addition, it was demonstrated usingin situhybridization thatfoxr1was a maternally-inherited transcript that was highly expressed even in early-stage oocytes and accumulated in the developing eggs during oogenesis. We also analyzed the function offoxr1in female reproduction using a zebrafish CRISPR/Cas9 knockout model. It was observed that embryos from thefoxr1-deficient females had a significantly lower survival rate whereby they either failed to undergo cell division or underwent abnormal division that culminated in growth arrest at around the mid-blastula transition and early death. These mutant-derived eggs contained a dramatically increased level ofp21, a cell cycle inhibitor, and reducedrictor, a component of mTOR and regulator of cell survival, which were in line with the observed growth arrest phenotype. Our study shows for the first time thatfoxr1is an essential maternal-effect gene and is required for proper cell division and survival via the p21 and mTOR pathways. These novel findings will broaden our knowledge on the functions of specific maternal factors stored in the developing egg and the underlying mechanisms that contribute to reproductive fitness.Summary sentenceThefoxr1gene in zebrafish is a novel maternal-effect gene that is required for proper cell division in the earliest stage of embryonic development possibly as a transcriptional factor for cell cycle progression regulators,p21andrictor.