ESPE Yearbook of Paediatric Endocrinology

To read the full abstract: Biology Letters 2020;16:20190867

Why do men die younger than women? It is believed that this is because they take bigger risks, have more dangerous jobs, drink, and smoke more, and are poorer at seeking advice from doctors. But maybe it also had something to do with the sex chromosomes that men have. These authors studied sex chromosomes and lifespans in a wide range of animal species and found shorter lifespan in the sex with reduced or absent chromosome in the heterogametic sex (e.g. the Y chromosome in mammals and the W chromosome in birds).

Sexual dimorphism is seen in many traits and behaviors among animals, including the most fundamental of life-history traits - lifespan. Previous work had showed shorter lifespans of mammalian males and avian females. Here, they took a broader view across the tree of life. They asked whether the heterogametic sex tends to have reduced longevity relative to the homogametic sex. The increased phylogenetic span of this study relative to previous work is important, as it gives greater power to unravel the importance of sex chromosomes.

Across a broad range of species, the heterogametic sex dies 17.6% earlier on average. In birds, butterflies and moths, the males have the homogametic sex chromosomes (usually ZZ) while the females have the heterogametic chromosomes (ZW). In species where males are heterogametic (XY), females live almost 21% longer than males. But in the species where females are heterogametic (ZW), males outlive females by only 7%.

The ‘unguarded X hypothesis’ suggests that the Y chromosome in male is less able to protect an individual from harmful genes expressed on the X chromosome, which may later expose the individual to health threats.While in an XX individual, a healthy X chromosome can stand in for another X that has deleterious genetic mutations. Another reason might be that the shorter Y is more easily lost with age/environmental damage (1). Some researchers suggested that lifespan differences between sexes are not solely genetic, but are also influenced by a combination of parental investment, exposure to predators, sexual selection and other biotic factors.

The authors suggest three possible explanations for this trend: the degree of degradation of the Y chromosome, telomere dynamics, and side effects of sexual selection.

Reference:

1. Thompson DJ, et al. Genetic predisposition to mosaic Y chromosome loss in blood. Nature. 2019 Nov; 575(7784):652-657.