Rad21l1 cohesin subunit is dispensable for spermatogenesis but not oogenesis in zebrafish

During meiosis I, ring-shaped cohesin complexes play important roles in aiding the proper segregation of homologous chromosomes. RAD21L is a meiosis-specific vertebrate cohesin that is required for spermatogenesis in mice but is dispensable for oogenesis in young animals. The role of this cohesin in other vertebrate models has not been explored. Here, we tested if the zebrafish homolog Rad21l1 is required for meiotic chromosome dynamics during spermatogenesis and oogenesis. We found that Rad21l1 localizes to unsynapsed chromosome axes. It is also found between the axes of the mature tripartite synaptonemal complex (SC) in both sexes. We knocked out rad21l1 and found that nearly all rad21l1(-/-) mutants develop as fertile males, suggesting that the mutation causes a defect in juvenile oogenesis, since insufficient oocyte production triggers female to male sex reversal in zebrafish. Sex reversal was partially suppressed by mutation of the checkpoint gene tp53, suggesting that the rad21l1 mutation activates Tp53-mediated apoptosis or arrest in females. This response, however, is not linked to a defect in repairing Spo11-induced double-strand breaks since deletion of spo11 does not suppress the sex reversal phenotype. Compared to tp53 single mutant controls, rad21l1(-/-) tp53(-/-) double mutant females produce poor quality eggs that often die or develop into malformed embryos. Overall, these results indicate that the absence of rad21l1(-/-) females is due to a checkpoint-mediated response and highlight a role for a meiotic-specific cohesin subunit in oogenesis but not spermatogenesis. Author summary Cohesins are multi-protein, ring-shaped complexes that tether DNA duplexes to one another and are important for nuclear organization of DNA and proper chromosome segregation during cell division. Specialized cohesin subunits play important roles in meiosis, a cell division required to produce gametes. Segregation errors in meiosis can lead to the formation of gametes with the incorrect number of chromosomes (aneuploidy), a major cause of birth defects and pregnancy loss in humans. Here we assess the role of Rad21l1, a cohesin subunit specific to vertebrate animals, by knocking out the gene in zebrafish and examining how oogenesis and spermatogenesis are affected. While males produce normal sperm in the absence of Rad21l1, oocyte production is severely compromised. Oocytes develop partway but then undergo cell death. Deleting tp53, a gene involved in sensing cellular stress, prevents cell death and allows oogenesis to be completed, though most of the resulting eggs are severely poor quality. Thus, our work shows that zebrafish oogenesis requires a special cohesin subunit, without which oocytes die. In contrast, spermatogenesis does not require this subunit.