X chromosome not the reason for sex differences in lifespan

February 13, 2018, Linköping University
Researchers at Linköping University, Sweden, use fruit flies as a model organism to study sex differences in lifespan. Credit: Karin Söderlund Leifler / Linköping University

The shorter average lifespan of males compared to females appears not to be a result of the fact that males have only one X chromosome. This is the conclusion from a research study on fruit flies at Linköping University, Sweden. The results have been published in the scientific journal Evolution.

It is common that the lifespans of males and females differ. For mammals, including humans, females live on average somewhat longer than males. The difference in humans at a global level is that women live 4.7 years longer than men, according to statistics from WHO for 2015. We do not know why this is the case in so many species, but several theories have been put forward. Researchers at Linköping University (LiU) have now had a close look at one of the theories concerning the role of genetics in sex differences in lifespan.

One obvious genetic difference between the sexes is that they have different numbers of , and what is known as the "unguarded X hypothesis" is based on this. In many species, including humans, females have two X chromosomes while males have an X and a Y chromosome. In birds, and in some species of fish, reptiles and insects, males have two copies of the same sex chromosome, and in such cases it is most often males who have the longer lifespan.

The sex that has two X chromosomes has two variants of each gene on the X chromosome. The gene variants act as back-ups for each other, so if a gene on the X chromosome is not functional, the gene copy on the other chromosome can compensate for this. In the sex that has two different sex chromosomes (one X and one Y), in contrast, only one copy of most genes on the X chromosome is present, and no back-up is available if there is something wrong with them. The theory suggests that this mechanism contributes to making the of males shorter. It is an old theory, but only a few studies have been carried out to test it. The researchers who carried out the new study used fruit flies (Drosophila melanogaster) to test the theory.

"The fruit fly is a useful model organism to test the hypothesis, since females generally live longer, and the X chromosome constitutes as much as one fifth of the genetic material. If the theory is true, the effects should be clear in the fruit fly," says Martin Brengdahl, PhD student at the Department of Physics, Chemistry and Biology at LiU, and one of the principal authors of the study.

The researchers bred fruit flies in which the two X chromosomes in the females were identical, such that neither of the copies was able to compensate for faults in the other. The flies in a second group were instead given two identical copies of one of the other chromosomes (not the sex chromosomes), known as "autosomal ".

"There was no difference in average lifespan between females that had been inbred for the X chromosome and normal females. Inbreeding of one autosomal chromosome, on the other hand, did have a negative effect and the effects were equally large in females and males," says Martin Brengdahl.

The results suggest that having an unguarded X chromosome cannot explain the difference in lifespan between the sexes. At least, not in .

"Genetic mechanisms are often relatively similar in different organisms, but we should be careful about drawing far-going conclusions from this study about how things work in humans. We have investigated the validity of the issue in one model, and further research can build on these results in the future," says Urban Friberg, senior lecturer at the Department of Physics, Chemistry and Biology, who has led the study.

The research group is now continuing to examine why there is a difference in lifespan between the sexes, and will be taking a close look at one of the alternative theories. In many species, competition between males to mate with is intense. The of sexual selection suggests that this may compel the to use more energy in finding partners and less in maintaining bodily functions, which leads to a shorter .

Explore further: XX protection against age-related mutations

More information: Martin Brengdahl et al. Sex differences in life span: Females homozygous for the X chromosome do not suffer the shorter life span predicted by the unguarded X hypothesis, Evolution (2018). DOI: 10.1111/evo.13434

Related Stories

XX protection against age-related mutations

July 20, 2016

Researchers at the University of Valencia's Cavanilles Institute of Biodiversity and Evolutionary Biology have put the 'unguarded X hypothesis' to the test and confirmed that differences in lifespan between the sexes, a widespread ...

New genes on "deteriorating" Y chromosome

October 25, 2017

The Y chromosome, which is found only in males, is difficult to decode even with the latest sequencing technologies. Among evolutionary biologists, the question as to which genes lie on the male sex chromosome and where they ...

DNA which only females have

June 4, 2015

In many animal species, the chromosomes differ between the sexes. The male has a Y chromosome. In some animals, however, for example birds, it is the other way round. In birds, the females have their own sex chromosome, the ...

Inbreeding insects cast light on longer female lifespans

February 6, 2009

Inbreeding can unexpectedly extend male lifespan. Insect experiments described in the open access journal BMC Evolutionary Biology have shown that, in seed beetles, inbreeding causes males to live longer, while shortening ...

Recommended for you

Elephants resist cancer by waking a zombie gene

August 14, 2018

An estimated 17 percent of humans worldwide die from cancer, but less than five percent of captive elephants—who also live for about 70 years, and have about 100 times as many potentially cancerous cells as humans—die ...

Models give synthetic biologists a head start

August 14, 2018

Synthetic biologists have the tools to build complex, computer-like DNA circuits that sense or trigger activities in cells, and thanks to scientists at Rice University and the University of Houston they now they have a way ...

Cancer-fighting drugs also help plants fight disease

August 14, 2018

Cancer-fighting drugs used on humans can help plants fight disease as well. That discovery, by two Washington State University plant pathologists, could help scientists develop new pathways for plants to battle infection, ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.