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Trends Genet. 2000 Jun;16(6):247-53.

Sex in the wormcounting and compensating X-chromosome dose.

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Howard Hughes Medical Institute, Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720-3204, USA.


The nematode Caenorhabditis elegans counts its X chromosomes to determine sex and to activate the process of dosage compensation, which ensures that males (XO) and hermaphrodites (XX) express equal levels of most X-chromosome products. The number of X chromosomes is communicated by a set of X-linked genes called X-signal elements, which repress the master sex-determination switch gene xol-1 via two distinct, dose-dependent molecular mechanisms in XX embryos. X-chromosome gene dosage is compensated by a specialized protein complex that includes evolutionarily conserved components of mitotic and meiotic machinery. This complex assembles on both X chromosomes of hermaphrodites to repress transcription by half. The recruitment of chromosome segregation proteins to the new task of regulating X-chromosome-wide gene expression points to the evolutionary origin of nematode dosage compensation.

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