An abundance of X-linked genes expressed in spermatogonia

Spermatogonia are the self-renewing, mitotic germ cells of the testis from which sperm arise by means of the differentiation pathway known as spermatogenesis. By contrast with hematopoietic and other mammalian stem-cell populations, which have been subjects of intense molecular genetic investigation, spermatogonia have remained largely unexplored at the molecular level. Here we describe a systematic search for genes expressed in mouse spermatogonia, but not in somatic tissues. We identified 25 genes (19 of which are novel) that are expressed in only male germ cells. Of the 25 genes, 3 are Y-linked and 10 are X-linked. If these genes had been distributed randomly in the genome, one would have expected zero to two of the genes to be X-linked. Our findings indicate that the X chromosome has a predominant role in pre-meiotic stages of mammalian spermatogenesis. We hypothesize that the X chromosome acquired this prominent role in male germ-cell development as it evolved from an ordinary, unspecialized autosome.     

Expression of Xist in mouse germ cells correlates with X-chromosome inactivation.

Mammals compensate for different doses of X-chromosome-linked genes in male (XY) and female (XX) somatic cells by terminally inactivating all but one X chromosome in each cell. A transiently inactive X chromosome is also found in germ cells, specifically in premeiotic oogenic cells and in meiotic and postmeiotic spermatogenic cells. Here we show that the Xist gene, which is a expressed predominantly from the inactive X-chromosome in female somatic cells, is also expressed in germ cells of both sexes, but only at those stages when an inactive X chromosome is present. This suggests support for the putative role of Xist as a regulator of X-chromosome inactivation and suggest a common mechanism for the initiation and/or maintenance of X-chromosome inactivation in all cell types.     

Rsx is a metatherian RNA with Xist-like properties in X-chromosome inactivation

In female (XX) mammals, one of the two X chromosomes is inactivated to ensure an equal dose of X-linked genes with males (XY). X-chromosome inactivation in eutherian mammals is mediated by the non-coding RNA Xist. Xist is not found in metatherians (marsupials), and how X-chromosome inactivation is initiated in these mammals has been the subject of speculation for decades. Using the marsupial Monodelphis domestica, here we identify Rsx (RNA-on-the-silent X), an RNA that has properties consistent with a role in X-chromosome inactivation. Rsx is a large, repeat-rich RNA that is expressed only in females and is transcribed from, and coats, the inactive X chromosome. In female germ cells, in which both X chromosomes are active, Rsx is silenced, linking Rsx expression to X-chromosome inactivation and reactivation. Integration of an Rsx transgene on an autosome in mouse embryonic stem cells leads to gene silencing in cis. Our findings permit comparative studies of X-chromosome inactivation in mammals and pose questions about the mechanisms by which X-chromosome inactivation is achieved in eutherians.     

Asynchronous replication of imprinted genes is established in the gametes and maintained during development.

Genomic imprinting is characterized by allele-specific expression of multiple genes within large chromosomal domains that undergo DNA replication asynchronously during S phase. Here we show, using both fluorescence in situ hybridization analysis and S-phase fractionation techniques, that differential replication timing is associated with imprinted genes in a variety of cell types, and is already present in the pre-implantation embryo soon after fertilization. This pattern is erased before meiosis in the germ line, and parent-specific replication timing is then reset in late gametogenesis in both the male and female. Thus, asynchronous replication timing is established in the gametes and maintained throughout development, indicating that it may function as a primary epigenetic marker for distinguishing between the parental alleles.