A mammalian egg sits on the precipice of sex determination. Depending on the genetic makeup of the sperm that fertilizes the egg, the embryo produced will develop into an individual with either male or female sex organs. The cellular environment surrounding the embryo has little to no effect on sex determination—or at least that’s what scientists thought until now. In a new study published in Nature, researchers at Osaka University, led by Makoto Tachibana, have shown that iron deficiency in the mother can have profound effects on the development of testes in genetically male mouse embryos, even causing male-to-female sex reversal.1 These findings lay the foundation for investigating the effects of other metabolic factors on fetal development.
Sex determination in mammals relies on the X and Y chromosomes.2 Activation of the Sry gene on the Y chromosome triggers a series of events that lead to the formation of testes. In the absence of this gene, ovaries form. Early during embryonic development, the Sry gene has epigenetic methyl markers that keep it repressed. But during the crucial period of sex determination, certain enzymes remove these blocks to activate the gene. Tachibana and his team had previously shown that the enzyme KDM3A facilitates the removal of these methyl groups from the Sry gene.3 Since iron is crucial the enzymatic activity of KDM3A, Tachibana wanted to decipher the relationship between iron metabolism and sex determination.4
The team first created mice that lacked a key iron accumulation gene in the gonadal cells of XY embryos during the period of sex determination. These embryos had higher levels of DNA methylation, specifically at the Sry gene, as compared to control mice. The elevated repression of Sry gene resulted in half the levels of the SRY protein, as compared to mice that had normal levels of iron. When the team took a closer look at the embryos’ sex organs, they observed that seven out of the 39 XY embryos with compromised iron accumulation had ovaries in place of testes.
Next, Tachibana and his team fed pregnant mice oral iron chelators, which led to iron deficiency, during the sex determination period and observed a male-to-female sex reversal in three out of 72 XY embryos. Maternal iron deficiency caused a 60 percent reduction in the expression of SRY in XY embryos, which resulted in the development of ovotestes—gonads with both ovarian and testicular characteristics—that had both male and female somatic cells. The team observed a reduction in the proportion of female somatic cells by overexpressing Sry in XY embryos of iron-deficient mice. However, when the team replicated maternal iron deficiency through diet, they noticed a sex change only in XY embryos that lacked one copy of the Kdm3a gene. This mutation did not cause a change in sex until iron availability was low.
These findings set the stage to investigate the influence of other environment factors in the development of embryos and to explore if the link between maternal iron deficiency and fetal sex determination translates to humans.
- Okashita N, et al. Maternal iron deficiency causes male-to female sex reversal in mouse embryos. Nature. 2025.
- Livernois AM, et al. The origin and evolution of vertebrate sex chromosomes and dosage compensation. Heredity. 2012;108(1):50-58.
- Kuroki S, et al. Epigenetic regulation of mouse sex determination by the histone demethylase Jmjd1a. Science. 2013;341(6150):1106-1109.
- Galy B, et al. Mechanisms controlling cellular and systemic iron homeostasis. Nat Rev Mol Cell Biol. 2024;25(2):133-155.
