How sex differences develop between organs

How sex differences develop between organs

Development of gender-specific organ differences in different animal species. © Leticia Rodríguez-Montes

Depending on their gender, mammals have different characteristics, including at the level of internal organs. A study now shows that these sex differences vary greatly in different animals depending on the species, organ and stage of development. Accordingly, most gender-specific differences only become apparent when sexual maturity is reached. Genetic analyzes also show that the genes involved differ from animal species to animal species, indicating rapid evolution.

Female and male individuals differ physically not only based on their sexual organs. Other characteristics that are not directly related to reproduction are also different – from the antlers of male deer to the magnificent plumage of many male birds. Depending on the animal species, this so-called sexual dimorphism can also extend to internal organs such as the liver, kidneys, heart and brain. In humans, for example, the male liver breaks down drugs and toxins more quickly, and when a heart attack occurs, women usually experience different symptoms than men.

Thoroughly tested

A team led by Leticia Rodríguez-Montes from the University of Heidelberg has now gotten to the bottom of the genetic basis of such gender differences. “Until now, it was largely unknown how genetic programs differ between female and male individuals during development and what effects these differences have on the function and cellular composition of organs in adult mammals,” says Rodríguez-Montes.

“In our study, we examined the extent, temporal dynamics and development of sex-specific gene expression during organ development in humans, mice, rats, rabbits, opossums and chickens,” report the researchers. They focused on the brain, cerebellum, heart, kidney and liver. In animals, the samples covered the developmental stages from the beginning of organ development in the womb to adulthood; in humans, the team examined samples from various embryonic stages up to birth.

Similar development until sexual maturity

The result: In all mammal species examined, there were hardly any gender-specific differences between the organs until they reached sexual maturity. “Almost all differences in gene expression only arise suddenly with sexual maturity. This means that the genetic programs that are responsible for the development of gender-specific organ characteristics are almost exclusively switched on late in the development of the organs, triggered by female or male hormones,” reports Rodríguez-Monte’s colleague Henrik Kaessmann.

“This result was surprising to us,” explains co-author Margarida Cardoso Moreira from the Francis Crick Institute in London. “Although we had expected that most differences would occur in adulthood because that is when sex differences are most clearly visible, we suspected that sex differences increase gradually during organ development rather than abruptly around sexual maturity.” However, in chickens it showed The pattern was reversed: around two thirds of all sexually dimorphic genes showed different activity at the beginning of organ development. In only ten percent did the differences only become apparent in adulthood.

Differences vary depending on animal species and organ

In addition, the analysis showed that gender differences are particularly pronounced in different organs depending on the animal species. In rabbits, the greatest differences were found in the heart, in rats and mice in the kidneys, in opossums in the liver and in chickens in the brain. The genetic analyzes also revealed that although the differences affect the same cell types, they are caused by different genes. The researchers conclude from this that sex differences developed quickly and in a species-specific manner on a genetic basis over the course of evolution. Only on the X and Y chromosomes do some genes exhibit sex differences across all mammals. “These likely serve as fundamental genetic triggers for the evolution of sex-specific traits in all mammals,” says Rodríguez-Montes.

On the one hand, the results help to better understand differences between female and male individuals and, for example, to take greater account of them in the medical treatment of women and men. “They also have implications for how we use animal models to understand sex differences in humans, because it is helpful to know that a certain cell type is sexually dimorphic in different species, even if there are other differences,” says Cardoso Moreira. “This research is another piece of the puzzle to understand why we are sexually dimorphic and what effects this has on us.”

Source: Leticia Rodríguez-Montes (Heidelberg University) et al., Science, doi: 10.1126/science.adf1046

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