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Counting X Chromosomes To Determine Sex

November 12, 1998—In mammals, the developmental decision for a fetus to become male or remain female rests with the Y chromosome—if the Y chromosome is present, the fetus becomes male. But many species lack a Y chromosome, so the choice must be made by counting X chromosomes instead, which is not an easy task.

Barbara Meyer, a Howard Hughes Medical Institute investigator, and her colleagues at the University of California, Berkeley, have discovered a signal, which they call SEX-1, that helps the nematode worm C. elegans tally its X chromosomes. SEX-1, a hormone receptor found in the cell nucleus, belongs to a family of proteins widely used by animals to translate environmental cues into decisions on how to regulate gene expression. A similar molecule may be involved in the first steps of mammalian sex determination.

"The process of sex determination may differ from organism to organism, but the underlying mechanisms may be much more similar," says Meyer. She and her colleagues report their findings in the November 12, 1998 issue of the journal Nature. The work was partly supported by grants from the National Institutes of Health.

Among C. elegans, worms with two X chromosomes, or XX animals, usually develop as hermaphrodites, while those with a single X chromosome, known as XO animals, develop as males. Hermaphrodites, given a choice, will mate with males, but unlike females of other species, hermaphrodites produce sperm that they can use to fertilize their own eggs in the absence of males.

The X chromosome in C. elegans, says Meyer, "has at least four genes, probably five or six," that signal the chromosome's presence All those X signals are believed to dampen the activity of a pivotal protein, called XOL-1, that processes the incoming X signals and steers organisms' sexual fates. When the X signals are abundant enough to overwh
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Contact: Jim Keeley
keeleyj@hhmi.org
301-215-8858
Howard Hughes Medical Institute
12-Nov-1998


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