The role of meiosis on the evolution of the sex chromosomes

理解从假定的祖先常染色体对形成性染色体的进化选择力量是生物学中的一个主要问题（见Khil和Camerini-Otero (2005) Crit）。学生物化学启。摩尔生物学，40,313)。几年前，有报道称，对秀丽隐杆线虫和黑腹线虫的全基因组分析表明，雄性特异性基因在x染色体上的代表性不足，而在大约24个小鼠精原细胞基因上的代表性不足。老鼠真的不同吗？我们对Spo11成年小鼠微阵列数据的分析提供了睾丸基因表达的时间线。我们发现，那些在减数分裂后期细胞中表达的基因，包括大多数睾丸富集基因，在x染色体上的表达不足。然而，早期基因被过度表达。由于先前发表的小鼠数据仅涉及早期精原基因，因此这些结果与所有数据一致。此外，我们提出，在大多数生物体中，X染色体的去雄化（雄性基因的去除）受到雄性减数分裂期间性染色体失活的影响，从而增加了这个谜题的缺失部分（见Reinke (2004) Nat. Genet. 6,548的评论）。实现X染色体重塑的一种方式是从X染色体到常染色体的反转录转位，其频率远高于常染色体之间的转位（Shiao等，in press）。

Understanding the evolutionary selective forces that fashioned the sex chromosomes from a putative ancestral autosome pair is a major problem in biology (reviewed in Khil and Camerini-Otero (2005) Crit. Rev. Biochem. Mol. Biol. 40, 313). A few years ago it was reported that genome-wide analyses of C. elegans and D. melanogaster revealed that male-specific genes are underrepresented on the X-chromosome whereas the opposite was reported for about 2 dozen mouse spermatogonial genes. Is the mouse really different? Our analysis of the Spo11 adult mice microarray data provided a time line for the expression of testis genes. We found that those genes expressed in cells late in meiosis, including the majority of testis-enriched genes, are underrepresented on the X. However, early genes are overrepresented. Since the previously published mouse data pertained to early spermatogonial genes only, these results reconcile all the data. Furthermore, we added a missing piece of the puzzle by proposing that the demasculinization (removal of male genes) of the X chromosome in most organisms is influenced by inactivation of the sex chromosomes during male meiosis (see commentary in Reinke (2004) Nat. Genet. 6, 548). One manner by which this reshaping of the X chromosome is achieved is by retrotransposition from the X chromosome to autosomes much more frequently than between autosomes (Shiao et al., in press).