American scientists have put forward a new hypothesis that aims to explain why males are smaller than females — in this case flies. They noticed that the Y chromosome of Drosophila is more “meaningless” repetitions, than in the X chromosome, and these regions of DNA more twisted in young animals. With age, epigenetic marks disappear, the DNA unfolds and “free” go mobile elements hidden inside these repeats. Creating a line of females with an extra Y chromosome and males without the Y chromosome, researchers have confirmed that life flies reduces it. Work published in the journal Nature Ecology & Evolution.
In many species females live on average longer than males. What caused this disparity, it is still not clear. Some researchers believe that to blame the sex chromosomes: those animals who have sex is determined by chromosomes, gender heterogametic (i.e. with different types of chromosomes) live on less. Other scientists believe that it is not in genetic factors, at least in mammals, and in the influence of the environment: one gender can often put at risk or have poorer health.
At the same time, we know that one of the engines of aging at the cellular level is the unwinding of DNA in “meaningless” sites. In such regions of the genome are often found in mobile elements, which after unwinding of the “go free” and have the opportunity to move around the genome and integrate at random locations and disrupt individual genes. Emily brown (Emily Brown) and her colleagues from the University of California at Berkeley suggested that the difference in lifespan may be due to the different structure of the sex chromosomes, as in fruit flies, the Y chromosome carries more “meaningless” sequence-repeats than the X chromosome.
For a start, the researchers measured the amount of heterochromatin (“twisted” sections of DNA) in the cells of young fruit flies and found that, as previously thought, in females it is about 20 MB smaller than the males. Counting the life expectancy of both sexes, scientists have confirmed that females live about 10 days longer.
The researchers then compared the amount of heterochromatin in flies in 8 days and 64 days of life (it is almost the peak of life expectancy in males). It turned out that in old age the males derepression (“promoted”) become one and a half times more sites than females. The majority of these sites are located on the Y chromosome. Among them were many of the potentially mobile elements. Females with age, the increased expression of mobile elements 6 and 14 declined. Males have also decreased the expression of only 4 of the mobile elements and has increased 32.
To check how the unpacking of sex chromosomes affects life expectancy, the authors have created two lines of flies with non-classical set of chromosomes. In fruit flies, sex is determined by the ratio of the number of X chromosomes to the number of somatic (asexual) chromosomes in the presence of the Y chromosome. It is therefore possible to obtain males with the genotype XO (no Y chromosome) or XYY and XXY females.
As expected, males XYY lived a less than ordinary males are about 40 days instead of 60. The life of XXY females were slightly shorter than in normal females. But males HO turned centenarians, overtaking and other males, and females, and after 80-90 days. Thus the authors confirmed that the Y-chromosome shortens the life of males, and getting rid of it, on the contrary, prolongs their days.