Sycp2 is essential for synaptonemal complex assembly, early meiotic recombination and homologous pairing in zebrafish spermatocytes

Abstract

Meiotic recombination is essential for faithful segregation of homologous chromosomes during gametogenesis. The progression of recombination is associated with dynamic changes in meiotic chromatin structures. However, whether Sycp2, a key structural component of meiotic chromatin, is required for the initiation of meiotic recombination is still unclear in vertebrates. Here, we describe that Sycp2 is required for assembly of the synaptonemal complex and early meiotic events in zebrafish spermatocytes. Our genetic screening by N-ethyl-N-nitrosourea mutagenesis revealed that ietsugu (its), a mutant zebrafish line with an aberrant splice site in the sycp2 gene, showed a defect during meiotic prophase I. The its mutation appeared to be a hypomorphic mutation compared to sycp2 knockout mutations generated by TALEN mutagenesis. Taking advantage of these sycp2 hypomorphic and knockout mutant lines, we demonstrated that Sycp2 is required for the assembly of the synaptonemal complex that is initiated in the vicinity of telomeres in wild-type zebrafish spermatocytes. Accordingly, homologous pairing, the foci of the meiotic recombinases Dmc1/ Rad51 and RPA, and γH2AX signals were largely diminished in sycp2 knockout spermatocytes. Taken together, our data indicate that Sycp2 plays a critical role in not only the assembly of the synaptonemal complex, but also early meiotic recombination and homologous pairing, in vertebrates.

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

“…Similar staining pattern is seen in females ( Fig 1B). This localization is similar to that described in a recent study using a different antibody to zebrafish Rad21l1 [41]. Localization to axes and the transverse filament of the SC is also similar to the localization of RAD21L protein in mouse as viewed by super-resolution microscopy [46].…”

“…Shortly thereafter, the axes elongate toward the middle of the chromosome during zygotene until they reach full length at early pachytene [38][39][40][41]. Synapsis, as detected by IF staining of the transverse filament protein Sycp1, initiates near the chromosome ends and extends inward, slightly trailing the elongation of the AE until pachytene [39,40].…”

“…and Msh5 are suppressed by the elimination of DSBs by deleting Spo11 [62]. In zebrafish, spo11 mutants have normal sex ratios and females are fertile, yet give rise to malformed embryos [40,41,63]. We reasoned that if the rad21l1 -/mutation was inducing meiotic arrest by preventing the repair of Spo11-induced DSBs, eliminating DSBs by deleting Spo11 would rescue the rad21l1 -/sex reversal phenotype.…”

“…Although homologs of RAD21L have been identified in other vertebrate genomes, there is a dearth of studies of this cohesin in vertebrates other than mouse. Zebrafish has emerged as an excellent model to use genetic approaches to study the chromosome events in meiosis [38][39][40][41]. Both sexes produce gametes throughout their lives, thus providing a window to study sexually dimorphic features of female and male meiosis.…”

Meiotic cohesins mediate initial loading of HORMAD1 to the chromosomes and coordinate SC formation during meiotic prophase

“…Following pre-meiotic DNA replication at preleptotene, chromosomes are organized into proteinaceous structures, termed axial element (AE) or chromosome axis, which is assembled by the main components SYCP2 [5,6] and SYCP3 [7]. AE provides a scaffold for recruiting meiotic recombination machineries to promote double-strand break (DSB) introduction and repair [8,9] and for assembly of the synaptonemal complex (SC) [3,10,11].…”