Topic
X chromosome
About: X chromosome is a research topic. Over the lifetime, 9862 publications have been published within this topic receiving 407354 citations. The topic is also known as: GO:0000805 & chrX.
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TL;DR: It is shown that Smchd1 is required for the silencing of around 10% of the genes on Xi, apparently independent of CGI hypomethylation, and, moreover, that these genes nonrandomly occur in clusters.
Abstract: The Smchd1 gene encodes a large protein with homology to the SMC family of proteins involved in chromosome condensation and cohesion. Previous studies have found that Smchd1 has an important role in CpG island (CGI) methylation on the inactive X chromosome (Xi) and in stable silencing of some Xi genes. In this study, using genome-wide expression analysis, we showed that Smchd1 is required for the silencing of around 10% of the genes on Xi, apparently independent of CGI hypomethylation, and, moreover, that these genes nonrandomly occur in clusters. Additionally, we found that Smchd1 is required for CpG island methylation and silencing at a cluster of four imprinted genes in the Prader-Willi syndrome (PWS) locus on chromosome 7 and genes from the protocadherin-alpha and -beta clusters. All of the affected autosomal loci display developmentally regulated brain-specific methylation patterns which are lost in Smchd1 homozygous mutants. We discuss the implications of these findings for understanding the function of Smchd1 in epigenetic regulation of gene expression.
99 citations
TL;DR: The phenotypes of embryos hemizygous for terminal and interstitial deletions of region 1B1-1B10, and of embryos carrying different mutations in certain genes of this region are described.
Abstract: Genes within subdivision IB of the X-chromosome of Drosophila melanogaster are known to affect the development of both the central (CNS) and the peripheral (PNS) embryonic nervous system. In this paper we describe the phenotypes of embryos hemizygous for terminal and interstitial deletions of region 1B1–1B10, and of embryos carrying different mutations in certain genes of this region. A minimum of 6 genetic functions that are involved in neural development can be defined in this region. Three of these genes, mapping to the I'sc (a gene of the achaete-scute complex, ASC), elav and md loci, affect major and apparently different aspects of CNS development. Two additional genes of the ASC, ac and sea, also play a role in CNS development, although their participation can only be demonstrated under certain conditions. Finally, in the rightmost part of the region uncovered by the deletion Df(1)260.1, two not yet well separated functions are found to be required for embryonic CNS and compound eye development, res...
99 citations
TL;DR: It is suggested that some loci on the distal short arm of the X chromosome (Xp) are not randomly inactivated in the female, because they are within the proximal part of the pairing segment between Xp and Yp, thereby giving rise to exceptions to X-or Y-linked inheritance.
Abstract: In this paper observations are summarized and speculations discussed, and it is suggested that some loci on the distal short arm of the X chromosome (Xp) are not randomly inactivated in the female, because they are within the proximal part of the pairing segment between Xp and Yp. This peculiarity of gene expression may be a remnant of the evolutionary history of the sex chromosomes, the pairing segment of which may involve at least 27% of Xp and 95% of Yp. Crossing over seems to occur mostly in the terminal third of the X/Y pairing segment. However, crossing-over inhibition control may lapse, or may be somewhat variable, within the pairing segment, so that some loci on the X and Y (e.g. Xg. H-Y, STS, and perhaps others) might cross over with a variable frequency which is proportional to their distances from the telomeres of the short arms. It is postulated that the DNA of the pairing segment is composed in a way which may also permit unequal crossing over to occur between the X and the Y, thereby giving rise to exceptions to X-or Y-linked inheritance. The peculiarities of behaviour and the position of other loci on the sex chromosomes are also discussed briefly.
99 citations
TL;DR: Cytological evidence does not support the hypothesis that the microchromosomes of Birds are non-chromosomal particles and the observation and identification of an unpaired Z chromosome in the female sex of Birds fully justifies the conclusions of Oguma, Yamashina c.
Abstract: 1.
In nine species of Reptiles (Crocodilians, Lacertilians, Ophidians, Chelonians) no difference between the chromosome sets of male and female could be observed.
2.
Morphologically differentiated heterochromosomes thus do not exist in this class.
3.
This result agrees with the conclusions of Matthey and is in contradiction with those of Oguma, Makino and other Japanese cytologists.
4.
In three species of Birds one of the macrochromosomes was found to be without a mate in the female sex; it is impossible to state whether a W chromosome is present.
5.
The observation and identification of an unpaired Z chromosome in the female sex of Birds fully justifies the conclusions of Oguma, Yamashina c. s.; however, Matthey's scepticism remains legitimate in so far as it is impossible to specify the type of female heterogamety (ZO or ZW).
6.
Cytological evidence does not support the hypothesis that the microchromosomes of Birds are non-chromosomal particles.
7.
In the Spiny Ant Eater (Monotremata) an unpaired X chromosome was found in the male; the material at hand did not allow for a decision between XO and XY type of male heterogamety.
8.
The type of sex chromosomes is the only cytological character that is exclusively typical of each of the three higher Vertebrate classes.
9.
The evidence hitherto obtained concerning the sex chromosomes in the different classes of Vertebrates can be summarized as follows:
Fishes: no cytologically recognizable sex chromosomes; Amphibians: no cytologically recognizable sex chromosomes; Reptiles: no cytologically recognizable sex chromosomes; Birds: female heterogamety (ZO, ZW ?); Monotremes: male heterogamety (XO, XY ?); Other Mammals: male heterogamety of the XY type; rarely, multiple sex chromosomes.
99 citations
01 Jan 2012
TL;DR: It is concluded that Ohno’s hypothesis is rejected for the vast majority of genes, reopening the search for the evolutionary force driving the origin of chromosome-wide X inactivation in female mammals.
Abstract: Susumu Ohno proposed in 1967 that, during the origin of mammalian sex chromosomes from a pair of autosomes, per-allele expression levels of X-linked genes were doubled to compensate for the degeneration of their Y homologs. This conjecture forms the foundation of the current evolutionary model of sex chromosome dosage compensation, but has been tested in mammals only indirectly via a comparison of expression levels between X-linked and autosomal genes in the same genome. The test results have been controversial, because examinations of different gene sets led to different conclusions that either support or refute Ohno’s hypothesis. Here we resolve this uncertainty by directly comparing mammalian X-linked genes with their one-to-one orthologs in species that diverged before the origin of the mammalian sex chromosomes. Analyses of RNA sequencing data and proteomic data provide unambiguous evidence for expression halving (i.e., no change in per-allele expression level) of X-linked genes during evolution, with the exception of only ∼5% of genes that encode members of large protein complexes. We conclude that Ohno’s hypothesis is rejected for the vast majority of genes, reopening the search for the evolutionary force driving the origin of chromosome-wide X inactivation in female mammals.
99 citations