Useful chromosome
Testicles were to blame for the aging of mice
But the replacement of the sex chromosome helped to delay this inevitable process
Polina Loseva, "The Attic"
In most animals, females live longer than males, but we still don't know why this happens. Scientists decided to test how the main factors determining sex affect life expectancy: sex chromosomes and sex glands. It turned out that both factors, independently of each other, give the girl mice an advantage, but in the matter of prolonging life, sex change at the level of genes, rather than glands, showed itself better.
Women on average live longer than men. This fact has been confirmed many times in a wide variety of groups, regardless of culture or level of well-being. But there is still no explanation for him. The culprit of the short life of men can be found in two mechanisms that determine the sex of a person – genetic and hormonal. Studies on sex hormones provide us with contradictory data. Thus, a decrease in testosterone levels correlates with an increased risk of cardiovascular diseases, which inevitably hinders longevity. At the same time, castration in some cases is associated with prolongation of life (if you believe, for example, statistics on Korean eunuchs). And the effect of sex chromosome genes on longevity has never been tested before.
The authors of a new study published in the journal Aging cell (Davis et al., Female XX sex chromosomes increase survival and extend lifespan in aging mice) decided to test the effect of genes and hormones separately. To do this, they used the method of four basic genotypes. In one line of mice, the sry gene, which determines the male sex, was transferred to the autosome (asexual chromosome). At the same time, the sex of the animal ceased to depend on its sex chromosomes. Then the mice of this line were crossed with ordinary mice.
There were four variants of offspring, four basic genotypes: XX + ovaries, XY + testes, XY + ovaries, XX + testes. It is known from previous studies that the level of sex hormones even in mice of "non-standard sex" corresponds to the norm.
Scheme for obtaining mice of four genotypes. The top row is the parents, the "normal" and the modified mouse. Circles are hormonal females, squares are males. A is an autosome intact (–Sry) and "male" (+Sry). Drawings from an article in Aging Cell.
Then a long experiment began, which lasted almost three years. The authors of the work closely monitored the life expectancy of experimental mice and built survival curves. According to the graphs, several parameters can be estimated: life expectancy (the time when the survival rate approaches 0%), the onset of aging (the moment when the curve begins to fall) and the rate of aging (the degree of decline of the curve).
The most tenacious were full–fledged females - those who had both the female chromosome XX, and female sex glands, ovaries. If they had a testis instead of an ovary (graph c) or one of the sex chromosomes was male, the Y chromosome (graph a), life expectancy was lower and the rate of aging was higher. As for the males, if they already had a Y chromosome, then replacing the sex gland did not help them (graph d). But if they had testes, but did not have a Y chromosome, then, although they did not live longer, they began to age later (graph b).
Comparison of survival graphs of mice of four genotypes. (a) XX and XY in mice with ovaries, (b) XX and XY in mice with testes, (c) ovaries versus testes in XX-mice, (d) ovaries versus testes in XY-mice.
It is, of course, too early to say that the mechanism of gender's influence on aging has been revealed. For example, it is still unclear which genetic parameter shortens life – the presence of a Y chromosome or the absence of a second X chromosome. In addition, experiments will have to be confirmed on other mouse lines. Nevertheless, it is already clear at this stage that we are dealing with two different mechanisms for shortening/prolonging life and we will have to investigate them independently.
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