Schoenmakers Sam, Wassenaar Evelyne, Laven Joop S E, Grootegoed J Anton, Baarends Willy M
Department of Obstetrics and Gynaecology, Erasmus MC-University Medical Center, PO BOX 2040, 3000 CA, Rotterdam, The Netherlands.
Chromosoma. 2010 Jun;119(3):311-24. doi: 10.1007/s00412-010-0258-9. Epub 2010 Feb 17.
During male meiotic prophase in mammals, X and Y are in a largely unsynapsed configuration, which is thought to trigger meiotic sex chromosome inactivation (MSCI). In avian species, females are ZW, and males ZZ. Although Z and W in chicken oocytes show complete, largely heterologous synapsis, they too undergo MSCI, albeit only transiently. The W chromosome is already inactive in early meiotic prophase, and inactive chromatin marks may spread on to the Z upon synapsis. Mammalian MSCI is considered as a specialised form of the general meiotic silencing mechanism, named meiotic silencing of unsynapsed chromatin (MSUC). Herein, we studied the avian form of MSUC, by analysing the behaviour of the peculiar germline restricted chromosome (GRC) that is present as a single copy in zebra finch spermatocytes. In the female germline, this chromosome is present in two copies, which normally synapse and recombine. In contrast, during male meiosis, the single GRC is always eliminated. We found that the GRC in the male germline is silenced from early leptotene onwards, similar to the W chromosome in avian oocytes. The GRC remains largely unsynapsed throughout meiotic prophase I, although patches of SYCP1 staining indicate that part of the GRC may self-synapse. In addition, the GRC is largely devoid of meiotic double strand breaks. We observed a lack of the inner centromere protein INCENP on the GRC and elimination of the GRC following metaphase I. Subsequently, the GRC forms a micronucleus in which the DNA is fragmented. We conclude that in contrast to MSUC in mammals, meiotic silencing of this single chromosome in the avian germline occurs prior to, and independent of DNA double strand breaks and chromosome pairing, hence we have named this phenomenon meiotic silencing prior to synapsis (MSPS).
在哺乳动物雄性减数分裂前期，X和Y染色体处于基本未联会的状态，这种状态被认为会引发减数分裂性染色体失活（MSCI）。在鸟类中，雌性为ZW型，雄性为ZZ型。尽管鸡卵母细胞中的Z和W染色体显示出完全的、基本为异源的联会，但它们也会经历MSCI，不过只是短暂的。W染色体在减数分裂前期早期就已经失活，并且失活的染色质标记可能在联会时扩散到Z染色体上。哺乳动物的MSCI被认为是一般减数分裂沉默机制的一种特殊形式，称为未联会染色质的减数分裂沉默（MSUC）。在此，我们通过分析斑胸草雀精母细胞中以单拷贝形式存在的特殊种系限制性染色体（GRC）的行为，研究了鸟类的MSUC形式。在雌性生殖系中，这条染色体以两个拷贝存在，它们通常会联会并重组。相反，在雄性减数分裂过程中，单拷贝的GRC总是被消除。我们发现雄性生殖系中的GRC从细线期早期就开始沉默，这与鸟类卵母细胞中的W染色体类似。在整个减数分裂前期I中，GRC基本保持未联会状态，尽管SYCP1染色斑块表明GRC的一部分可能会自我联会。此外，GRC基本没有减数分裂双链断裂。我们观察到GRC上缺乏着丝粒内部蛋白INCENP，并且在中期I后GRC被消除。随后，GRC形成一个微核，其中的DNA发生片段化。我们得出结论，与哺乳动物的MSUC不同，鸟类生殖系中这条单染色体的减数分裂沉默发生在DNA双链断裂和染色体配对之前且与之无关，因此我们将这种现象命名为联会前减数分裂沉默（MSPS）。
