Jacobson David J, Raju Namboori B, Freitag Michael
Department of Biological Sciences, Stanford University, 371 Serra Mall, Stanford, CA 94305-5020, USA.
Fungal Genet Biol. 2008 Mar;45(3):351-62. doi: 10.1016/j.fgb.2007.09.014. Epub 2007 Oct 11.
Meiotic silencing by unpaired DNA is a posttranscriptional gene silencing process in Neurospora crassa. Any gene without a homolog in the same chromosomal position during meiotic prophase generates a sequence-specific signal that prevents expression of all copies of that gene, but only during meiosis. Meiotic silencing is epigenetic and involves components of a meiosis-specific RNA silencing machinery. Although N. tetrasperma is closely related to N. crassa, its sexual biology is significantly different. N. tetrasperma was used here to evaluate both the generality of meiotic silencing within the genus and its possible evolutionary significance. A reporter gene for meiotic silencing, a histone H1-GFP fusion construct, was introgressed from N. crassa into various chromosome locations in N. tetrasperma. Whereas we did not observe meiotic silencing in four out of five introgression series, we obtained inconclusive results in the fifth series. Thus, we propose that meiotic silencing in N. tetrasperma is either absent or is substantially reduced when compared to N. crassa, possibly because the sad-1 gene (RNA-directed RNA polymerase, RdRP) is naturally unsynapsed (although "paired") and self-silenced during meiosis by structural differences between N. tetrasperma mating-type chromosomes. In N. crassa, wild-type sad-1 function is essential for meiotic silencing. Many point mutations in or deletion of sad-1 result in self-silencing of RdRP, and consequently suppression of meiotic silencing in heterozygous asci. The apparent absence or reduced meiotic silencing in N. tetrasperma raises the possibility that this form of silencing is not necessarily a major genome defense mechanism or responsible for reproductive isolation among the species of the genus Neurospora.
未配对DNA介导的减数分裂沉默是粗糙脉孢菌中的一种转录后基因沉默过程。在减数分裂前期，任何在同一染色体位置上没有同源物的基因都会产生一个序列特异性信号，该信号会阻止该基因所有拷贝的表达，但仅在减数分裂期间。减数分裂沉默是表观遗传的，涉及减数分裂特异性RNA沉默机制的组成部分。尽管四极性脉孢菌与粗糙脉孢菌密切相关，但其有性生物学却有显著差异。本文利用四极性脉孢菌来评估该属内减数分裂沉默的普遍性及其可能的进化意义。一个用于减数分裂沉默的报告基因，即组蛋白H1-GFP融合构建体，从粗糙脉孢菌导入到四极性脉孢菌的各个染色体位置。虽然在五个导入系列中的四个中未观察到减数分裂沉默，但在第五个系列中得到了不确定的结果。因此，我们提出，与粗糙脉孢菌相比，四极性脉孢菌中的减数分裂沉默要么不存在，要么大幅减少，这可能是因为sad-1基因（RNA指导的RNA聚合酶，RdRP）在减数分裂期间由于四极性脉孢菌交配型染色体之间的结构差异而自然未配对（尽管“配对”）并自我沉默。在粗糙脉孢菌中，野生型sad-1功能对于减数分裂沉默至关重要。sad-1中的许多点突变或缺失会导致RdRP的自我沉默，从而抑制杂合子子囊中的减数分裂沉默。四极性脉孢菌中明显不存在或减数分裂沉默减少，这增加了这种沉默形式不一定是主要的基因组防御机制或导致脉孢菌属物种间生殖隔离的可能性。
