How different are the organs of women and men really? A study shows that a binary division into male and female falls too short for most of our organs. Because in the heart, brain and the like, both male and female genes are active and ensure that the characteristics between the sexes overlap. Analyzes on mice also indicate that this gender -specific gene expression outside of the sexual organs quickly changes.
Gender -specific genes are also active outside the sexual organs. Among other things, they determine how big we become, how our organs develop and how our immune system works. But previous studies have suggested that a clear distinction between male and female is not possible in most parts of our body. For example, men are larger than women on average, but we cannot infer the gender of a person solely based on the size. The situation is similar with alleged gender differences in the brain. Here, too, the overlaps between men and women are so great that no clearly “male” or “female” brain can be defined.
Mixing of male and female
A team around Chen Xie from the Max Planck Institute for Evolutionary Biology in Plön has now examined how much male and female genes are active in the different parts of the body. “We have developed a sex bias gene expression index that reflects the masculinity or femininity of every organ,” report the researchers. Xie and his colleagues applied this index in four different types of mice, in which they examined how strongly male and female genes are read in every organ. They then transferred the results to gene expression in humans.
The result: Both male and female genes were active in most organs. The overlaps between the sexes were so great that it could not be determined on the basis of gene expression whether the respective organ came from a male or a female mouse. The difference was only clear with the testicles and ovaries. In the case of liver and kidneys, the researchers found at least gender tendencies. Many other organs – including the brain – there were hardly any differences between the sexes. In some organs of male animals, even more female than male genes were active and vice versa.
In humans, the differences were even lower than with the mice. “Overall, there are fewer gender genes in humans than with mice and the distributions among the somatic organs overlap,” report Xie and his colleagues. For example, the heart of some men can be more female than that of some women. At the same time, other organs can have male gene expression in the same person. “With a view to gene expression, there is both male and rather female organs within the same individual,” explain the researchers.
Spectrum instead of binary classification
The results also show that the activity of gender -specific genes in the different parts of the body changes quickly in the course of evolution – much faster than with gender -independent genes. Even with closely related mouse species, which evolutionarily only separated less than two million years ago, there are only a few gender -specific genes that play the same role in both species. The mice have hardly any gender -specific genes in common with humans.
From the point of view of the researchers, these results show that mice can only serve as model animals for gender -specific medicine in humans. On the other hand, they suggest that the gender -specific genes are subject to strong sexual selection and therefore change quickly. “We come to the conclusion that adult individuals have a mosaic -like spectrum of gender features in their somatic tissues, which should not be summarized in a simple binary classification,” conclude Xie and his team.
Source: Chen Xie (Max Planck Institute for Evolutionary Biology) et al., Elife, DOI: 10.7554/Elife.99602.4