C. elegans Germ Cells Switch between Distinct Modes of Double-Strand Break Repair During Meiotic Prophase Progression

Chromosome inheritance during sexual reproduction relies on deliberate induction of double-strand DNA breaks (DSBs) and repair of a subset of these breaks as interhomolog crossovers (COs). Here we provide a direct demonstration, based on our analysis of rad-50 mutants, that the meiotic program in Caenorhabditis elegans involves both acquisition and loss of a specialized mode of double-strand break repair (DSBR). In premeiotic germ cells, RAD-50 is not required to load strand-exchange protein RAD-51 at sites of spontaneous or ionizing radiation (IR)-induced DSBs. A specialized meiotic DSBR mode is engaged at the onset of meiotic prophase, coincident with assembly of meiotic chromosome axis structures. This meiotic DSBR mode is characterized both by dependence on RAD-50 for rapid accumulation of RAD-51 at DSB sites and by competence for converting DSBs into interhomolog COs. At the mid-pachytene to late pachytene transition, germ cells undergo an abrupt release from the meiotic DSBR mode, characterized by reversion to RAD-50-independent loading of RAD-51 and loss of competence to convert DSBs into interhomolog COs. This transition in DSBR mode is dependent on MAP kinase-triggered prophase progression and coincides temporally with a major remodeling of chromosome architecture. We propose that at least two developmentally programmed switches in DSBR mode, likely conferred by changes in chromosome architecture, operate in the C. elegans germ line to allow formation of meiotic crossovers without jeopardizing genomic integrity. Our data further suggest that meiotic cohesin component REC-8 may play a role in limiting the activity of SPO-11 in generating meiotic DSBs and that RAD-50 may function in counteracting this inhibition.

有性生殖过程中的染色体遗传依赖于双链DNA断裂（double-strand DNA breaks, DSBs）的精准诱导，以及将其中部分断裂修复为同源交叉（interhomolog crossovers, COs）的过程。本研究通过对rad-50突变体的分析，直接证实秀丽隐杆线虫（Caenorhabditis elegans）的减数分裂程序包含一种特殊双链断裂修复（double-strand break repair, DSBR）模式的获得与丧失。在有丝分裂前生殖细胞中，RAD-50并不参与在自发性或电离辐射（ionizing radiation, IR）诱导的DSBs位点加载链交换蛋白RAD-51的过程。减数分裂前期起始时，一种特殊的减数分裂DSBR模式被激活，该过程与减数分裂染色体轴结构的组装同步。这种减数分裂DSBR模式具有两个核心特征：一是RAD-51在DSBs位点的快速聚集依赖RAD-50，二是具备将DSBs转化为同源交叉COs的能力。在粗线期中期向粗线期晚期过渡阶段，生殖细胞会突然脱离该减数分裂DSBR模式，表现为恢复RAD-50非依赖型的RAD-51加载，以及丧失将DSBs转化为同源交叉COs的能力。这种DSBR模式的转变依赖于丝裂原活化蛋白激酶（MAP kinase）介导的减数分裂前期进程，且在时间上与染色体结构的大规模重塑相吻合。我们提出，秀丽隐杆线虫生殖细胞中至少存在两种由发育程序调控的DSBR模式切换事件，该过程大概率由染色体结构变化所介导，从而在不损害基因组完整性的前提下实现减数分裂交叉的形成。本研究数据进一步表明，减数分裂黏连蛋白组分REC-8可能参与限制SPO-11生成减数分裂DSBs的活性，而RAD-50则可能通过拮抗这一抑制作用发挥功能。