Dosage compensation rox!

Recent advances in our understanding of dosage compensation in flies have centered on characterizing its sex-specificity, identifying the structural RNAs involved in the process, and determining how dosage compensation is targeted to particular sites on the X chromosome.     

X染色体的剂量补偿机制

剂量补偿机制(Dosage compensation mechanism)是雌性和雄性X染色体表达平衡的关键,保证两性间由X染色体编码的蛋白质或其他酶类物质在数量上达到平衡。不同生物的剂量补偿机制各不相同,迄今研究表明剂量补偿机制主要有以下3种模式:通过雄性的单个X染色体表达加倍;通过雌性的一条X染色体失活;通过雌性的两个X染色体的表达减半来达到平衡。对剂量补偿的研究有助于揭示X连锁基因的调控机理、性染色体的进化和分化过程,以及解释性染色体畸变的机理,因此,文章将对这种重要的调控机制研究现状及进展进行简要论述。     

Dosage compensation of a retina-specific gene in Drosophila miranda

The X1R chromosome of Drosophila miranda and the 3L autosome of Drosophila melanogaster are thought to have originated from the ancestral D chromosomal element and therefore may contain the same set of genes. It is expected that these genes will be dosage compensated in D. miranda because of their X linkage. To test these possibilities and to study evolution of the dosage compensation mechanism, we used the 3L-linked autosomal head-specific gene 507ml of D. melanogaster to isolate the homologous gene (507 mr) from a D. miranda genomic library. In situ hybridization showed that gene 507 is located at the 12A region of the X1R chromosome of D. miranda, indicating that the chromosomal homology deduced by cytogenetic means is correct. Restriction analysis and cross-specific DNA and RNA blot hybridization revealed the presence of extensive restriction pattern polymorphism and lack of sequence similarity in some areas of the 507 mr and 507 ml DNA, including the 3 portion of the transcribed region. However, the 5 portion of the transcribed region and the DNA sequences, located approximately 0.8 kb upstream and 3 kb downstream from the 507 ml gene showed a high degreee of similarity with the DNA sequences of comparable regions of the 507 mr gene. In both species gene 507 codes for a highly abundant 1.8 kb RNA which is expressed in the retina of the compound eye. Although in D. miranda the males have one and the females have two copies of the 507 gene, the steady-state levels of the 507 mRNA in both sexes were found to be similar, indicating that gene 507 is dosage compensated in D. miranda. Thus, along with the disparate rates of evolution in different areas of the DNA associated with gene 507, in D. miranda this gene has come under the regulation of the X chromosomal dosage compensation mechanism.by M.L. Pardue     

Dosage compensation of X-linked heat-shock puffs in Drophila pseudoobscura

Four major puffs are inducible by heat shock in the larval salivary gland chromosomes of D. pseudoobscura. Two of these puffs are present at 23 and 39–40 on the right arm of the X chromosome and two are present at 53 and 58 on chromosome 2. By means of in situ hybridization, residual homologies were demonstrated between the puffs at 23 in D. pseudoobscura and at 63C in D. melanogaster, and between the two chromosome 2 puffs of D. pseudoobscura and 87A and 87C of D. melanogaster. RNA synthesis was monitored as a function of ³H-uridine incorporation in the major heat-induced puffs of D. pseudoobscura and was found to be equivalent in males and females indicating dosage compensation of the two X-linked loci. The evolution of the regulatory controls of these genes is discussed.     

Dosage compensation: making 1X equal 2X

Animals that have XX females and XY or XO males have differing doses of X-linked genes in each sex. Overcoming this is the most immediate and vital aspect of sexual differentiation. A number of systems that accurately compensate for sex-chromosome dosage have evolved independently: silencing a single X chromosome in female mammals, downregulating both X chromosomes in hermaphrodite Caenorhabditis elegans and upregulating the X chromosome in male Drosophila all equalize X-linked gene expression. Each organism uses a largely non-overlapping set of molecules to achieve the same outcome: 1X = 2X.