Maternal iron levels: An immense influence on sex determination in embryos

Osaka, Japan – Developmentally expressed genes play a fundamental role in the formation of a wide range of organs. The Y chromosome-located mammalian sex-determining gene Sry, required for the formation of testis, is activated only within a narrow time window during embryogenesis. However, the processes behind the activation of Sry remain uncertain.

Now, in a recent publication from Nature, a team from The University of Osaka has discovered that iron plays a crucial role in male mammalian sex development.

The Sry gene directs sexually undifferentiated gonads to differentiate into testes rather than ovaries. The Sry gene is initially inactive because of a modification added to DNA-wrapping proteins called histone methylation. In males, a protein called KDM3A is responsible for removing this histone methylation, allowing the Sry gene to become active.

“We already knew that KDM3A requires iron for its enzymatic activity,” says lead author, Naoki Okashita. “Therefore, we wanted to investigate the extent and nature of iron metabolism and histone demethylation in sex determination.”

The researchers established mice lacking Tfrc, a gene crucial for cellular iron incorporation. Loss of Tfrc prevented the removal of the histone methylation, leading to reduced Sry expression. When inspecting the genitals of these mice, it was found that some XY mice developed as female, despite being genetically male.

The team also established an organ culture system for gonadal cells in which they could manipulate the levels of available iron. This confirmed that iron deficiency reduces the activity of KDM3A and activation of Sry. Therefore, the reduced Sry suppression under iron-deficient conditions was caused by the failure of KDM3A to remove histone methylation.

Perhaps most significantly, they then showed that maternal iron deficiency – whether induced by diet or by pharmacological intervention – could lead to altered histone methylation of the Sry gene because of reduced KDM3A activity. Some population of XY offspring derived from iron-deficient pregnant females were shown to undergo male-to-female sex reversal, potentially due to the impaired Sry activation.

“Our findings indicate that iron deficiency leads to disorders of male development, at least in mice. Our study emphasizes the importance of maintaining maternal iron levels,” explains senior author, Makoto Tachibana.

It is known that in humans, Fanconi anemia and Diamond–Blackfan anemia are risk factors for disorders of development. Understanding how iron supports embryonic development underscores the importance of maternal iron intake for the healthy development of human embryos.

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The article, “Maternal iron deficiency causes male-to-female sex reversal in mouse embryos”, was published in Nature at DOI: https://doi.org/10.1038/s41586-025-09063-2.

About The University of Osaka

The University of Osaka was founded in 1931 as one of the seven imperial universities of Japan and is now one of Japan's leading comprehensive universities with a broad disciplinary spectrum. This strength is coupled with a singular drive for innovation that extends throughout the scientific process, from fundamental research to the creation of applied technology with positive economic impacts. Its commitment to innovation has been recognized in Japan and around the world, being named Japan's most innovative university in 2015 (Reuters 2015 Top 100) and one of the most innovative institutions in the world in 2017 (Innovative Universities and the Nature Index Innovation 2017). Now, Osaka University is leveraging its role as a Designated National University Corporation selected by the Ministry of Education, Culture, Sports, Science and Technology to contribute to innovation for human welfare, sustainable development of society, and social transformation.

Website: https://resou.osaka-u.ac.jp/en