Study Reveals Environmental Influence on Sex Determination in Mice
All living humans are initially both male and female for about six weeks. While our sex is determined at fertilization, the biological programming to develop either testes or ovaries kicks in about a month and a half later.
Traditionally, it’s been taught that genetics dictate this process completely: males have one X and one Y chromosome (XY), while females have two X chromosomes (XX). However, recent research involving mice has highlighted some unexpected findings. It turns out that low iron levels in a mother can lead to male offspring developing female characteristics, regardless of their genetic makeup.
“This is, to our knowledge, the first demonstration that an environmental factor can influence sex determination in a mammal,” said Makoto Tachibana, a biologist at Osaka University. He led the study published in the journal Nature. Tachibana emphasized that this discovery shows how environmental and metabolic conditions can impact fundamental developmental processes that were previously believed to be solely genetic.
Nearly 40 years ago, researchers studying individuals with atypical sexual development discovered the SRY gene, which resides on the male Y chromosome and is crucial for sexual differentiation. The presence of this gene prompts the formation of testes at around six weeks; in its absence, ovaries develop instead.
This genetic process also triggers the production of sex hormones and subsequent physical traits associated with each sex. Mammals typically experience this development in a protected uterine environment, unlike many other species that determine sex based on external factors like temperature or social structure.
The researchers found that a 60% reduction in iron concentration at the cellular level can deactivate the gene responsible for testis formation. In experiments where this deficiency was induced in pregnant female mice carrying males, six out of 39 XY offspring were born with two ovaries, indicating a complete sex reversal. Moreover, one mouse was born with both an ovary and a testicle.
Another set of experiments confirmed these results when scientists treated pregnant mice with a drug that removes iron from the body. Out of 72 male embryos studied, five developed female sexual organs. Although the sex reversal numbers appear small, the findings challenge the previous understanding of developmental mechanisms believed to be shielded from environmental influences.
The study illustrates that external factors can affect genetic expression through epigenetics, which involves chemical markers sensitive to environmental conditions that modify gene functioning.
In this case, extremely low iron levels impact the enzyme KDM3A, which is essential for activating the SRY gene during sex determination. The resulting genetically male mice with two ovaries appeared healthy for up to eight weeks, but it remains unclear if they could reproduce; typically, sex reversal in humans leads to infertility.
Iron deficiency poses health risks for both the mother and the baby, like anemia. While mild deficiencies can often be remedied by diet, severe cases require treatment to mitigate risks such as miscarriage or increased susceptibility to infections.
Tachibana acknowledges that it’s uncertain if a similar process occurs in humans. While finding such low iron levels naturally might be a challenge, he believes it is a vital area for future exploration.
Francisco Javier Barrionuevo, a genetics professor at the University of Granada, who didn’t take part in the study, highlights its significance, noting that discovering how something as ordinary as iron levels can influence sex development is remarkable.
Barrionuevo’s team is examining other biological elements that could impact this process, including tiny RNA sequences that might cause sex reversal despite not being linked to genetic coding. He suspects that extreme malnutrition could have led to similar unnoticed cases in humans.
Two cellular biologists, Shannon Dupont and Blanche Capel from Duke University, praised the experiments, stating that the study provides solid evidence of how metabolic variations can interact with genetic cascades during sex determination.
They noted that iron deficiency is not only a health concern but might also influence neurological development in both mother and child. This line of inquiry raises questions about other subtle effects of nutritional deficiencies, suggesting that a mother’s diet could impact not just testicular formation but other masculine traits as well.
