Rat PRDM9 shapes recombination landscapes, duration of meiosis, gametogenesis, and age of fertility

Vertebrate recombination concentrates in meiotic chromatin regions (hotspots) that are opened in some species by the DNA-sequence-specific-binding histone H3 trimethyltransferase PRDM9, while other species recombine in regions with already opened chromatin and other function. Inactivation of the mouse Prdm9 gene induces the shift of hotspots to functional regions, gross fertility reduction in males, and sterility in females. In contrast, the other vertebrate species lacking PRDM9 remain fertile. To resolve this discrepancy, we generated Prdm9 deletions in the Rattus norvegicus genome and generated the first rat genome-wide maps of recombination-initiating double-strand break hotspots. Rat strains carrying the same wild-type Prdm9 allele shared 88% hotspots but strains with different Prdm9 alleles only 3%. After Prdm9 deletion, rat hotspots relocated to functional regions, 40% to positions corresponding to Prdm9-independent mouse hotspots. Despite of hotspot relocation and of decreased fertility, Prdm9-deficient rats of the SHR/OlaIpcv strain produced apparently normal offspring. Rat PRDM9 thus makes recombination landscape unique, but it is unnecessary for recombination. This peculiarity is likely similar for human PRDM9 and may resolve the paradox between the apparently species-specific functions. PRDM9 is known to play a role in speciation, as it causes mouse hybrid sterility via meiotic asynapsis. Besides the expected mild meiotic arrest, we also detected apoptosis of postmeiotic spermatids, suggesting that PRDM9 has an additional role during spermatogenesis and perhaps also in speciation.脊椎动物的重组事件富集于减数分裂染色质区域（重组热点，recombination hotspots），部分物种中这类热点由结合DNA序列特异性的组蛋白H3三甲基转移酶PRDM9介导开放；而其余物种的重组则发生在已开放的染色质及兼具其他功能的区域。敲除小鼠的Prdm9基因会导致重组热点向功能区域转移，同时造成雄性个体生育能力大幅下降，雌性个体完全不育。与之相对，缺乏PRDM9的其他脊椎动物物种仍可维持正常可育性。为解决这一矛盾，我们在褐家鼠（Rattus norvegicus）基因组中构建了Prdm9基因敲除模型，并绘制了首张全基因组范围内的重组起始双链断裂（double-strand break, DSB）热点图谱。携带相同野生型Prdm9等位基因的大鼠品系，其共享的重组热点比例达88%；而携带不同Prdm9等位基因的品系，该比例仅为3%。在Prdm9基因敲除后，大鼠的重组热点转移至功能区域，其中40%的热点位置与不依赖PRDM9的小鼠重组热点相吻合。尽管出现了重组热点转移且生育能力下降，SHR/OlaIpcv品系的Prdm9缺陷大鼠仍可产下外观正常的后代。由此可见，大鼠的PRDM9塑造了独特的重组调控图谱，但它并非重组过程所必需的因子。人类PRDM9大概率也存在类似特性，这或可解开此前关于其功能存在明显物种特异性的悖论。已知PRDM9在物种形成过程中发挥关键作用，它可通过引发减数分裂联会异常导致小鼠杂交不育。除了预期出现的轻度减数分裂阻滞外，我们还检测到了减数分裂后精子细胞的凋亡现象，这表明PRDM9在精子发生过程中具备额外功能，且可能同样参与物种形成进程。