Flatworms can either regrow lost heads or reproduce sexually, not both

Planarians, flatworms known for their goofy cross-eyed gazes, may have to make an evolutionary trade-off between regeneration and sex – across different species of planarians, the same gene signalling pathway facilitates either the ability to regrow heads or to produce eggs, depending on whether it’s turned on or off. This could help answer the long-standing mystery of why some animals can regenerate lost body parts while others cannot.

This is “actually a really old question in the regeneration field,” says Jochen Rink …