Gravity tested on cosmic scales

lishes its sexual identity at the same time that X-chromosome dosage is regulated. However, even in Drosophila, a cell’s sexual identity can be overridden by secreted signals. How should Zhao and colleagues’ results be reconciled with the substantial body of work involving hormone-induced sex reversal in chickens? Seventy-five years of research have definitively demonstrated that oestrogen is necessary and sufficient for female development: ZZ (male) embryos exposed to oestrogen develop as females, whereas ZW (female) embryos depleted of oestrogen develop as males. ZZ ‘females’ revert to a male phenotype at puberty if the hormone treatment is discontinued, possibly because ZZ cells cannot produce enough oestrogen to maintain ovarian structures. By contrast, adult ZW ‘males’ have fairly normal-looking testes, indicating that, when oestrogen is absent, ZW cells can become Sertoli cells and organize into male patterns. Do these hormone treatments simply override cell-autonomous sexual identity? Investigation into the adult phenotypes of castrated chick embryos should show whether an underlying sexual identity does indeed exist in the absence of hormones. Although this procedure was previously attempted using irradiation, the animals did not survive to hatching. A final question is whether cell-autonomous sexual identity will turn out to be a common element in the arsenal of sex-determining systems in vertebrates, with variable influence on the outcome of sexual differentiation. Perhaps it will. These funky chickens, oddities of nature that they are, may well provide new perspectives on questions of sexual identity long thought to have been resolved. ■