Molecular Biology

Less Expression from X

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Science  10 Dec 2010:
Vol. 330, Issue 6010, pp. 1456
DOI: 10.1126/science.330.6010.1456-a

Sex is genetically determined in many organisms and has often involved the evolution of specialized sex chromosomes—the X and Y chromosomes in mammals, for example. The sex chromosomes originally evolved from a pair of normal chromosomes, or autosomes. To balance gene dosage between mammalian female (XX) cells and male (XY) cells, one of the pair of female XX chromosomes is inactivated, a process known as X inactivation. Because dosage compensation can involve inactivating or reducing gene expression on the X chromosome, it has been hypothesized by Susumu Ohno that, during evolution, gene expression on an individual X must have been boosted chromosome-wide by a factor of 2 to balance expression with the (paired) autosomes, giving an X:AA expression ratio of 1.

Xiong et al. have used publicly available RNA sequencing data, which they demonstrate is more sensitive than equivalent microarray-based analyses, to measure the X:AA expression ratio across several mouse and human tissues. They find that the ratio is consistently lower than 1, and the average falls close to 0.5, which is inconsistent with Ohno's hypothesis. Proteomic data also support an expression ratio of ∼0.5 in mice and worms. The apparent absence of expression equalization between the sex chromosomes and autosomes suggests that new models are needed to explain dosage compensation.

Nat. Genet. 42, 1043 (2010).

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