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The double stranded RNA binding protein Adad1 (adenosine deaminase domain containing 1) is a member of theadenosinedeaminaseacting onRNAs (Adar) protein family with germ cell-specific expression. In mice, Adad1 is necessary for sperm differentiation, however its function outside of mammals has not been investigated. Here, through an N-ethyl-N-nitrosourea (ENU) based forward genetic screen, we identified anadad1mutant zebrafish line that develop as sterile males. Further histological examination revealed complete lack of germ cells in adult mutant fish, however germ cells populated the gonad, proliferated, and entered meiosis in larval and juvenile fish. Although meiosis was initiated inadad1mutant testes, the spermatocytes failed to progress beyond the zygotene stage. Thus, Adad1 is essential for meiosis and germline maintenance in zebrafish. We tested if spermatogonial stem cells were affected using a label retaining cell (LRC) assay and found that the mutant testes had fewer LRCs compared to wild-type siblings, suggesting that failure to maintain the spermatogonial stem cells resulted in germ cell loss by adulthood. To identify potential molecular processes regulated by Adad1, we sequenced bulk mRNA from mutants and wild-type testes and found mis-regulation of genes involved in RNA stability and modification, pointing to a potential broader role in post-transcriptional regulation. Our findings suggest that Adad1 is an RNA regulatory protein required for fertility through regulation of spermatogonial stem cell maintenance in zebrafish.Author SummaryInfertility is a serious problem for millions of couples who wish to have children. Globally more than 10% of couples suffer from infertility due to genetic, epigenetic, and environmental factors. Among these about 50% of cases occur due to genetic factors such as aneuploidy and genetic mutations affecting development of the gametes (i.e. sperm and eggs). Although many genes are known to be involved in germ cell development, genetic causes of infertility are still largely unexplained. Therefore, it is imperative to investigate genes involved in reproductive processes. In this study, we report that theadad1gene is essential for germ cell maintenance and fertility in zebrafish. Our analysis of zebrafishadad1mutants demonstrates that it is required for maintenance of the germline stem cells and for completion of meiosis. This is in contrast to mouseAdad1, which functions later in gamete development to regulate differentiation of haploid sperm. Our work on zebrafishadad1has uncovered previously unknown roles ofadad1function in germline stem cell maintenance.