Interplay between Synaptonemal Complex, Homologous Recombination, and Centromeres during Mammalian Meiosis

The intimate synapsis of homologous chromosome pairs (homologs) by synaptonemal complexes (SCs) is an essential feature of meiosis. In many organisms, synapsis and homologous recombination are interdependent: recombination promotes SC formation and SCs are required for crossing-over. Moreover, several studies indicate that initiation of SC assembly occurs at sites where crossovers will subsequently form. However, recent analyses in budding yeast and fruit fly imply a special role for centromeres in the initiation of SC formation. In addition, in budding yeast, persistent SC–dependent centromere-association facilitates the disjunction of chromosomes that have failed to become connected by crossovers. Here, we examine the interplay between SCs, recombination, and centromeres in a mammal. In mouse spermatocytes, centromeres do not serve as SC initiation sites and are invariably the last regions to synapse. However, centromeres are refractory to de-synapsis during diplonema and remain associated by short SC fragments. Since SC–dependent centromere association is lost before diakinesis, a direct role in homolog segregation seems unlikely. However, post–SC disassembly, we find evidence of inter-centromeric connections that could play a more direct role in promoting homolog biorientation and disjunction. A second class of persistent SC fragments is shown to be crossover-dependent. Super-resolution structured-illumination microscopy (SIM) reveals that these structures initially connect separate homolog axes and progressively diminish as chiasmata form. Thus, DNA crossing-over (which occurs during pachynema) and axis remodeling appear to be temporally distinct aspects of chiasma formation. SIM analysis of the synapsis and crossover-defective mutant Sycp1−/− implies that SCs prevent unregulated fusion of homolog axes. We propose that SC fragments retained during diplonema stabilize nascent bivalents and help orchestrate local chromosome reorganization that promotes centromere and chiasma function.

联会复合体（synaptonemal complexes, SCs）介导同源染色体对（homologous chromosome pairs）与同源染色体（homologs）的紧密联会，是减数分裂（meiosis）的核心特征之一。在多数生物类群中，联会过程与同源重组（homologous recombination）相互依存：同源重组可促进联会复合体的组装，而联会复合体亦是染色体交换（crossing-over）得以发生的必要条件。此外，多项研究显示，联会复合体的组装起始位点往往就是后续将发生染色体交换的区域。不过，近期针对酿酒酵母（budding yeast）与果蝇（fruit fly）的研究分析表明，着丝粒（centromeres）在联会复合体组装起始过程中可能承担特殊功能。此外，在酿酒酵母中，持续存在的、依赖联会复合体的着丝粒关联，可助力那些未能通过染色体交换建立连接的同源染色体完成分离。
本研究以哺乳动物为研究对象，探究联会复合体、同源重组与着丝粒之间的相互调控关系。在小鼠精母细胞（mouse spermatocytes）中，着丝粒并非联会复合体的组装起始位点，且始终是最后完成联会的染色体区域。但在双线期（diplonema）阶段，着丝粒对去联会（de-synapsis）过程具有抗性，仍会通过短小的联会复合体片段保持相互关联。由于依赖联会复合体的着丝粒关联在终变期（diakinesis）前便已消失，其在同源染色体分离（homolog segregation）中发挥直接作用的可能性较低。不过，在联会复合体解离后，我们观测到着丝粒间连接（inter-centromeric connections）的相关证据，这类结构或许在促进同源染色体双向定向（homolog biorientation）与分离的过程中发挥更为直接的作用。
另一类持久存在的联会复合体片段则被证实依赖于染色体交换过程。超分辨率结构照明显微镜（super-resolution structured-illumination microscopy, SIM）观测结果显示，这类结构最初连接分离的同源染色体轴，并随着交叉结（chiasmata）的形成逐渐消失。由此可见，发生在粗线期（pachynema）的DNA交换与染色体轴重塑，在交叉结形成过程中呈现出时间上的先后差异。对联会与交换缺陷型突变体Sycp1−/−的超分辨率结构照明显微镜分析表明，联会复合体可阻止同源染色体轴发生不受调控的融合。据此我们提出假说：双线期留存的联会复合体片段可稳定新生二价体（bivalents），并协助调控局部染色体的重组过程，进而促进着丝粒与交叉结发挥正常功能。