It has been a widespread belief that men live a shorter life due to lifestyle choices, such as smoking and drinking alcohol. But a new study shows that men are genetically predisposed to dying younger because their Y chromosome is unable to protect an unhealthy X chromosome.
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A team of researchers at the University of New South Wales in Sydney has found a potential reason why women live longer than men, and it has something to do with their genetic makeup.
Published in the journal Biology Letters, the researchers looked at all available research on sex chromosomes and lifespan, trying to establish if there was a specific pattern of one sex living longer than the other, that was repeated across the animal kingdom.
What is the possible reason?
The researchers believe that those with XY (male) sex chromosomes rather than XX (female) sex chromosomes – are less able to protect an individual from harmful genes expressed on the X chromosome
This means that a healthy X can stand in and protect another X that has faulty genes to make sure this disease-causing is not expressed, improving the length of life for the organism. The scientists dubbed the theory as the "unguarded X hypothesis," which has been seen in a broad range of species.
The team examined the lifespan data from many species of animals to see if the unguarded X hypothesis is right across the animal kingdom. For the first time, researchers have tested the hypothesis across many animals, which has been previously tested within a small group of animals.
"We found that across that broad range of species, the heterogametic sex does tend to die earlier than the homogametic sex, and it's 17.6 per cent earlier on average," Zoe Xirocostas, first author and a Ph.D. student, said.
Studying heterogametic chromosomes
The team also found similar patterns in animals with XY chromosomes. For instance, in moths, butterflies, and birds, the male species have the ZZ chromosomes, while their female counterparts have heterogametic chromosomes (ZW). These female species die earlier than males, supporting the new theory.
"Our findings demonstrate the importance of considering chromosome morphology in addition to sexual selection and environment as potential drivers of sexual dimorphism, and advance our fundamental understanding of the mechanisms that shape an organism's lifespan," the researchers wrote in the paper.
Further, they found a smaller difference in lifespan between the males and females in the female heterogametic species than to males and females in the male heterogametic species. This means that heterogametic males die sooner than heterogametic females, or a male XY has a shorter lifespan than ZW female.
"I was only expecting to see a pattern of the homogametic sex (XX or ZZ) living longer, so it came as an interesting surprise to see that the type of sex determination system (XX/XY or ZZ/ZW) could also play a role in an organism's longevity," Xirocostas added.
The researchers urge others to test the hypothesis to help further understand certain factors that affect aging. The new study can someday help in developing therapies and measures to improve lifespan, especially among males.
Xirocostas, Z., Everingham, S., and Moles, A. (2020). The sex with the reduced sex chromosome dies earlier: a comparison across the tree of life. Biology Letters. https://royalsocietypublishing.org/doi/10.1098/rsbl.2019.0867