Dosage compensation

About: Dosage compensation is a research topic. Over the lifetime, 1920 publications have been published within this topic receiving 124589 citations.

Noncoding RNAs and Epigenetic Mechanisms During X-Chromosome Inactivation

Abstract: In mammals, the process of X-chromosome inactivation ensures equivalent levels of X-linked gene expression between males and females through the silencing of one of the two X chromosomes in female cells. The process is established early in development and is initiated by a unique locus, which produces a long noncoding RNA, Xist. The Xist transcript triggers gene silencing in cis by coating the future inactive X chromosome. It also induces a cascade of chromatin changes, including posttranslational histone modifications and DNA methylation, and leads to the stable repression of all X-linked genes throughout development and adult life. We review here recent progress in our understanding of the molecular mechanisms involved in the initiation of Xist expression, the propagation of the Xist RNA along the chromosome, and the cis-elements and trans-acting factors involved in the maintenance of the repressed state. We also describe the diverse strategies used by nonplacental mammals for X-chromosome dosage compensation and highlight the common features and differences between eutherians and metatherians, in particular regarding the involvement of long noncoding RNAs.

Mammalian X chromosome inactivation.

Abstract: The initial step in mammalian sexual differentiation is based on the XX: XY chromosomal system. In order to function properly, this chromosomal mechanism must be regulated to eliminate the aneuploidy effects in somatic tissues and still insure normal sexual differentiation and development. In mammalian forms, an X-chromosome regulatory mechanism has evolved to carry out these developmental functions. The two X chromosomes in the female germ line remain active through most of their ontogeny to bring about normal ovarian function; a single X chromosome is active in the female soma so as to eliminate gross aneuploidy effects between males and females; and in the male germ line the single X chromosome is inactivated or eliminated at an apparently critical stage in spermiogenesis. This is the broad outline of mammalian X-chromosome regulation. The specifics vary in different forms: random X-chromosome inactivation in most eutherian mammals, a possible nonrandom mechanism in marsupials, and a chromosomal elimination system in the creeping vole, Micron’s oregoni.

Sex-Specific Assembly of a Dosage Compensation Complex on the Nematode X Chromosome

Abstract: In nematodes, flies, and mammals, dosage compensation equalizes X-chromosome gene expression between the sexes through chromosome-wide regulatory mechanisms that function in one sex to adjust the levels of X-linked transcripts. Here, a dosage compensation complex was identified in the nematode Caenorhabditis elegans that reduces transcript levels from the two X chromosomes in hermaphrodites. This complex contains at least four proteins, including products of the dosage compensation genes dpy-26 and dpy-27. Specific localization of the complex to the hermaphrodite X chromosomes is conferred by XX-specific regulatory genes that coordinately control both sex determination and dosage compensation.

Reversal of an ancient sex chromosome to an autosome in Drosophila

Abstract: Although transitions of sex-determination mechanisms are frequent in species with homomorphic sex chromosomes, heteromorphic sex chromosomes are thought to represent a terminal evolutionary stage owing to chromosome-specific adaptations such as dosage compensation or an accumulation of sex-specific mutations. Here we show that an autosome of Drosophila, the dot chromosome, was ancestrally a differentiated X chromosome. We analyse the whole genome of true fruitflies (Tephritidae), flesh flies (Sarcophagidae) and soldier flies (Stratiomyidae) to show that genes located on the dot chromosome of Drosophila are X-linked in outgroup species, whereas Drosophila X-linked genes are autosomal. We date this chromosomal transition to early drosophilid evolution by sequencing the genome of other Drosophilidae. Our results reveal several puzzling aspects of Drosophila dot chromosome biology to be possible remnants of its former life as a sex chromosome, such as its minor feminizing role in sex determination or its targeting by a chromosome-specific regulatory mechanism. We also show that patterns of biased gene expression of the dot chromosome during early embryogenesis, oogenesis and spermatogenesis resemble that of the current X chromosome. Thus, although sex chromosomes are not necessarily evolutionary end points and can revert back to an autosomal inheritance, the highly specialized genome architecture of this former X chromosome suggests that severe fitness costs must be overcome for such a turnover to occur.