Distinct cellular and reproductive consequences of meiotic chromosome synapsis defects in syce2 and sycp1 mutant zebrafish.

The synaptonemal complex (SC) is a meiosis-specific structure that aligns homologous chromosomes and promotes the repair of meiotic DNA double-strand breaks (DSBs). To investigate how defects in SC formation affect gametogenesis in zebrafish, we analyzed mutations in two genes encoding core SC components: syce2 and sycp1. In syce2 mutants, chromosomes exhibit partial synapsis, primarily at sub-telomeric regions, whereas sycp1 mutant chromosomes display early prophase co-alignment but fail to synapse. Both mutants exhibit reduced efficiency in repairing meiotic DSBs compared to wild type. Despite these defects, syce2 and sycp1 mutant females are fertile. However, sycp1 mutant females produce a higher proportion of malformed progeny, correlating with increased univalent formation. While syce2 mutant males are fertile and produce normal offspring, sycp1 mutant males are sterile, with spermatocytes that transit prophase I but arrest at metaphase I or II. Additionally, sycp1 mutants display a male-biased sex ratio that can be suppressed by extending the developmental window for sex determination, suggesting that the absence of synapsis delays-but does not completely block-meiotic progression. Notably, embryos from syce2 and sycp1 mutant females exhibit widespread somatic mosaic aneuploidy, indicating that impaired meiotic chromosome dynamics can compromise genome stability during early development. In contrast to mouse SC mutants, the zebrafish syce2 and sycp1 mutants examined in this study progress through meiotic prophase I with minimal disruption, suggesting a less stringent surveillance mechanism for synapsis errors in zebrafish.