[S]o far, researchers have uncovered a dizzying array of exceptions to the XX/XY “rule:” birds and butterflies that use the ZW system, in which males have a matching pair of sex chromosomes (ZZ) and females have a mismatched pair (ZW), a monotreme with ten sex chromosomes, and animals for which maleness or femaleness can each be encoded by multiple genotypes. There are species whose sex is not written into the genome at all: Some turtles have temperature-dependent sex determination and some fish can change from one sex to another in a matter of days in response to social signals. When it comes to sex determination, “The only rule is that there are no rules,” said Judith Mank, a comparative genomics researcher at the University of British Columbia.
Follow the latest news and policy debates on sustainable agriculture, biomedicine, and other ‘disruptive’ innovations. Subscribe to our newsletter.
While researchers once assumed that [mammalian] gonadal fates were set in stone once animals reached adulthood, some studies now suggest that a structure’s identity as an ovary or testis may need to be actively maintained throughout the lifespan. When researchers deleted Foxl2 in adult female mice, for example, their ovaries quickly began to change: In a matter of weeks, they became testes-like in terms of both structure and gene expression.
“There aren’t quite the barriers that we previously thought there were, even in groups like mammals,” remarked Godwin.
