No Default Sex

Male and female embryos start out roughly the same, but as development progresses, the tissues that will ultimately become sperm- or egg-producing structures are either retained or eliminated, depending on sex. It was thought that genes present only on the Y chromosome controlled the development of male-specific structures, and that the lack of a Y chromosome resulted in female-specific structures. In other words, that male development was an active process and female development the default. It is now clear, however, that the development of female sex organs is active too. Researchers have discovered a factor produced in female mice necessary for the elimination of male-specific tissues. The mutant embryo (bottom) lacks this factor and thus retains both female and male precursor tissues (pink and blue). This new discovery should help to inform studies into both intersex development and certain reproductive problems.

Written by Ruth Williams

Ruth Williams

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Ruth is a freelance science journalist based in the US. She's a regular correspondent for The Scientist, and her work has appeared in The Lancet, Nature, Scientific American Mind, BBC Focus and elsewhere. Twitter @rooph

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• Image from work by Fei Zhao and colleagues
• Reproductive Developmental Biology Group, National Institute of Environmental Health Sciences, Durham, NC, USA
• Image copyright held by the original authors
• Research published in Science, August 2017