29 November 2022

You can live without the Y chromosome

The loss of the Y chromosome did not make Japanese mice female

Anna Novikovskaya, Naked Science

Male from female in mammals is determined by a set of sex chromosomes. The female has two X chromosomes in the genome, the male has one X chromosome and one Y chromosome. The latter contains the genes necessary for the proper development of the male-type reproductive system. In case of her absence, a person is born with the Shereshevsky —Turner syndrome and develops according to the female type. 

However, the absence of the Y chromosome does not lead to developmental disorders in all mammals. In some rodent species, in particular the Ryuk mouse (Tokudaia osimensis) is endemic to a tiny island Oshima off the coast of Japan, — both male and female have 25 chromosomes in the genome, so the development of testes in them is triggered in a different way.


The Ryuk mouse is an endangered species that has been found to have a sex determination mechanism unknown until now / © Asato Kuroiwa

Now a group of Japanese researchers led by Asato Kuroiwa from Hokkaido University managed to find out how mouse embryos "understand" who they become — males or females. After collecting tissue samples from six animals, three males and three females, the scientists studied the genome of each individual and finally identified a doubled DNA section on the third chromosome in all three males. 

It was located near the Sox9 gene, which in other mammals is involved in the development of testes. Usually, a special protein produced by the Y chromosome is needed to start Sox9, but in Ryukian mice, doubled DNA was able to replace it. To test this hypothesis, scientists artificially doubled this region of the genome in the third chromosome of embryos of female laboratory mice, and those earned the gene responsible for the development of testes. 


The Eastern slepushonka is another rodent species whose males do not have a Y chromosome / © Askar Isabekov, askar.birds.watch

Thus, scientists have discovered for the first time in mammals a mechanism for determining sex that does not depend on sex chromosomes. In the future, the authors of the work plan to study the exact mechanisms of the effect of the doubled DNA section on the work of Sox9 and clarify the principle by which sex is determined in other rodent species whose genome lacks a Y chromosome. 

The study is published in the journal Proceedings of the National Academy of Sciences (Terao et al., Reversal of mammalian sex chromosomes in the Sry-deficient Amami spiny rat is due to male-specific upregulation of Sox9).

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