Sycp1 Is Not Required for Subtelomeric DNA Double-Strand Breaks but Is Required for Homologous Alignment in Zebrafish Spermatocytes

Abstract

In meiotic prophase I, homologous chromosomes are bound together by the synaptonemal complex, in which two axial elements are connected by transverse filaments and central element proteins. In human and zebrafish spermatocytes, homologous recombination and assembly of the synaptonemal complex initiate predominantly near telomeres. In mice, synapsis is not required for meiotic double-strand breaks (DSBs) and homolog alignment but is required for DSB repair; however, the interplay of these meiotic events in the context of peritelomeric bias remains unclear. In this study, we identified a premature stop mutation in the zebrafish gene encoding the transverse filament protein Sycp1. In sycp1 mutant zebrafish spermatocytes, axial elements were formed and paired at chromosome ends between homologs during early to mid-zygonema. However, they did not synapse, and their associations were mostly lost in late zygotene- or pachytene-like stages. In sycp1 mutant spermatocytes, γH2AX signals were observed, and Dmc1/Rad51 and RPA signals appeared predominantly near telomeres, resembling wild-type phenotypes. We observed persistent localization of Hormad1 along the axis in sycp1 mutant spermatocytes, while the majority of Iho1 signals appeared and disappeared with kinetics similar to those in wild-type spermatocytes. Notably, persistent Iho1 foci were observed in spo11 mutant spermatocytes, suggesting that Iho1 dissociation from axes occurs in a DSB-dependent manner. Our results demonstrated that Sycp1 is not required for peritelomeric DSB formation but is necessary for complete pairing of homologs in zebrafish meiosis.

The zebrafish meiotic cohesion complex protein Smc1b is required for key events in meiotic prophase I

“…In zebrafish spermatocytes, Smc1b is required for Sycp3 and Sycp1 to associate along chromosomes suggesting that interactions between these proteins is necessary for SC formation. This notion is further supported by recent analysis of zebrafish SC mutants, which also show defects in homolog pairing and synapsis (Takemoto et al, 2020;Imai et al, 2021). For instance, spermatocytes mutant for sycp2, encoding an SC axial element protein, failed to form an SC, which disrupted homologue pairing (Takemoto et al, 2020).…”

“…Mutations disrupting the SC transverse element protein Sycp1 displayed somewhat less severe defects than those observed for sycp2 and smc1b mutants. Homologous chromosomes initially paired in Sycp1 mutant spermatocytes, however synapsis did not occur which caused loss of pairing and germ cell arrest at late zygotene/early pachytene stage (Imai et al, 2021). Analysis of these mutants demonstrates that the zebrafish cohesion complex is intimately associated with SC formation and function during meiosis, as has been observed in other organisms.…”

“…The axial element of the SC assembles along chromosomes beginning near the chromosome ends, as has been visualized by Sycp2 and Sycp3 localization (Saito et al, 2011;Blokhina et al, 2018;Takemoto et al, 2020). Synapsis ensues, as seen by visualization of the transverse element protein, Sycp1, which follows Sycp3 localization (Blokhina et al, 2018;Imai et al, 2021). Thus, homologue pairing and synapsis begins at the chromosomes ends and zippers closed towards the chromosome center.…”

“…Meiosis-I resumes at the maturation stage, which occurs just prior to ovulation in zebrafish (stage IV), then arrests again in meiosis-II until fertilization takes place (Selman et al, 1993). Multiple meiotic mutants have all male phenotypes in zebrafish, including mutants disrupting sypc1 and sycp2 genes (Saito et al, 2011;Takemoto et al, 2020;Imai et al, 2021). In mutants affecting synaptonemal complex components, oogenesis was initiated, as juvenile fish had ovarian follicles at 40 dpf (sycp1 -/-) and 28 dpf (sycp2 -/-) (Takemoto et al, 2020;Imai et al, 2021).…”

“…Multiple meiotic mutants have all male phenotypes in zebrafish, including mutants disrupting sypc1 and sycp2 genes (Saito et al, 2011;Takemoto et al, 2020;Imai et al, 2021). In mutants affecting synaptonemal complex components, oogenesis was initiated, as juvenile fish had ovarian follicles at 40 dpf (sycp1 -/-) and 28 dpf (sycp2 -/-) (Takemoto et al, 2020;Imai et al, 2021). Therefore, in the sycp1 and sycp2 mutants, oogenesis did not proceed to stages that could support ovary development into adulthood.…”