In many species including humans, females have two X sex chromosomes, while males have an X and a Y. Females have two variants of each gene on on the X chromosomes (one on each X), so if one doesn't function properly, they essentially have a backup that can compensate; whereas because the males only have one, they're SOL. This has lead to the hypothesis called the "unguarded-X hypothesis", which supposes that the longer average lifespan seen in females is connected to their extra X.
Two recent studies in Evolution shed new light on that hypothesis:
The “unguarded‐X” and the genetic architecture of lifespan: Inbreeding results in a potentially maladaptive sex‐specific reduction of female lifespan in Drosophila melanogaster
Somewhat related to the issues we discussed that can arise in endogamous communities... If you have a harmful recessive trait continuously passed on via the X chromosome, it's more likely to start showing up on both variants (so even in the females). This study shows that indeed, inbreeding, and passing down those traits, resulted in shorter life spans according to the U-x Hypothesis.
Sex differences in life span: Females homozygous for the X chromosome do not suffer the shorter life span predicted by the unguarded X hypothesis
In this study (also using fruit flies), on the other hand, the authors conclude, "The logic of the unguarded X hypothesis is indisputable, but our results suggest that the degree to which recessive deleterious X‐linked alleles depress performance in the heterogametic sex appears too small to explain general sex differences in life span."
-Courtney
Third post from Evolution.
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