On average, women live longer than men, at least in places where the dangers of childbirth have been tamed. There are memes offering explanations, and undoubtedly many factors are involved, but a new study suggests part of the answer lies in additional X chromosomes.
Introductory genetics courses talk about women having two X chromosomes, while men have an X and a Y. The truth is far more complex. Some people have extra chromosomes, such as XXY, while women with complete androgen insensitivity syndrome have an XY combination but female genitalia. That's all before considering the complexity transgender people add to the story. Nevertheless, XX women and XY men represent the most common combinations.
Some advantages of having two X chromosomes are well known. Many genetic diseases, most famously hemophilia, are caused by recessive X chromosome mutations. Boys with the defective version of the gene stand a high chance of dying as children. Girls are only affected in rare cases where both their X chromosomes carry the mutated version.
Dr Dena Dubal of the University of California San Francisco has shown that this cover against recessive genes is not the only benefit X chromosomes offer, at least in rodents, where females also live longer. Dubal engineered some mice to have XX chromosomes but male reproductive organs and others to have XY genetics but female organs. These were compared in Aging Cell with mice that had the conventional mammalian chromosomes and sex organ combinations.
Unusually for lab mice, the ones Dubal was studying were allowed to survive until they died of natural causes. Dubal found that while female sex organs were associated with longer lives, the chromosomes were still more important. The XX mice, whether they had ovaries or testes, outlived their XY counterparts on average.
Specifically, the XX mice were less likely to die before they reached 23 months old, irrespective of their genitalia. Those that survived to this venerable age (by mouse standards) did best if they combined XX chromosomes and ovaries.
"One can imagine nature has driven females to evolve this way. When you're living longer, you can really ensure the well-being of your offspring, and maybe even their offspring," Dubal said in a statement. This assumes the females are providing more parental care than the males, something that is certainly not true for every species.
Whatever the evolutionary explanation, the mechanism remains unknown. Dubal has floated the theory that the Y chromosome may be toxic, but also notes that extra X chromosomes are expressed in female cells, possibly providing some unidentified protective effect. Dubal's specialization is in neurodegenerative diseases like Alzheimer's, and she hopes to learn if the protective effects of X chromosomes can be replicated, possibly with protective proteins to keep our brains healthy longer.