A proposed path by which genes common to mammalian X and Y chromosomes evolve to become X inactivated
A proposed path by which genes common to mammalian X and Y chromosomes evolve to become X inactivated
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作者：
Jegalian, K;Page, DC
哺乳动物的X和Y染色体由一对常染色体进化而来；X染色体保留，Y染色体逐渐失去大多数祖先基因(1，2)，在雌性动物中，一条X染色体因X染色体失活而沉默，这一过程通常被认为是在广泛的区域或染色体基础上进化的(3)。在这里，我们认为X和Y染色体共有的基因或簇(X-Y基因)沿着多步路径独立进化，最终在X染色体上获得剂量补偿。这里研究的三个基因，以及其他现存的基因，似乎是中间体。通过检测CpG岛甲基化来监测ZFX、RPS4X和SMCX在不同物种中的X失活情况，CpG岛甲基化反映了许多真兽类的X失活。ZFX在原真兽类中明显逃脱了X的失活，后者也具有非常相似的Y连锁基因；这两个特征在大多数现存的目中都被保留了下来，但在肌型啮齿动物中却没有。对于RPS4X来说，从X失活中逃脱似乎是灵长类动物独有的。SMCX在灵长类动物和肌型动物中可以逃脱失活，但在其他几个谱系中则不能。因此，X失活可以针对这些基因中的每一个独立进化。我们认为它是对同源Y连锁基因衰变的一种适应。
Mammalian X and Y chromosomes evolved from an autosomal pair; the X retained and the Y gradually lost most ancestral genes(1,2), In females, one X chromosome is silenced by X inactivation, a process that is often assumed to have evolved on a broadly regional or chromosomal basis(3). Here we propose that genes or clusters common to both the X and Y chromosomes (X-Y genes) evolved independently along a multistep path, eventually acquiring dosage compensation on the X chromosome. Three genes studied here, and other extant genes, appear to be intermediates. ZFX, RPS4X and SMCX were monitored for X inactivation in diverse species by assaying CpG-island methylation, which mirrors X inactivation in many eutherians. ZFX evidently escaped X inactivation in proto-eutherians, which also possessed a very similar Y-linked gene; both characteristics were retained in most extant orders, but not in myomorph rodents. For RPS4X, escape from X inactivation seems unique to primates. SMCX escapes inactivation in primates and myomorphs but not in several other lineages. Thus, X inactivation can evolve independently for each of these genes. We propose that it is an adaptation to the decay of a homologous, Y-linked gene.