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A genome sequence for the flatworm Schmidtea mediterranea reveals a chromosome that might be primed to become a sex chromosome. The finding offers a remarkable chance to study the evolution of sex determination.
Judith E. Mank is in the Department of Zoology and Biodiversity Research Centre, University of British Columbia, Vancouver V6T 1Z4, Canada, and in the Centre for Ecology and Conservation, University of Exeter, Penryn, UK.
There is an interesting dichotomy in the fact that sperm and eggs are broadly similar across animals, and yet the mechanism by which sex is determined varies enormously1. For instance, some animals are hermaphroditic, producing both eggs and sperm, either simultaneously or sequentially. Many other animals spend their entire lives as either females or males. In this case, sex can be determined by ecological factors such as temperature or demography, or by sex chromosomes. This diversity makes sex determination challenging to understand – a problem compounded by the fact that sex-determination mechanisms often evolve rapidly, and can vary even between closely related species2. Writing in Nature, Guo et al.3 offer clues to how animals might transition from hermaphroditism to having separate sexes using sex chromosomes, with an unexpected twist to events.