How much does the unguarded X contribute to sex differences in life span?
Collaboration with Tim Connallon, Yasmine McDonough & Filip Ruzicka
Figure 1: Curated published estimates of species-specific sex-biased mutation rates, expressed as the ratio of male to female rates
This project was written into a open access journal article published in Evolution Letters in 2022.
Somewhat less technical summary of this ‘Unguarded X’ effect project:
In many species, females and males often have different life expectancies. The ‘Unguarded-X’ effect is one popular explanation for this phenomenon: differences in lifespan arise from sex differences in chromosomal sets. Specifically, by having a ‘backup’ copy of the chromosome, the sex with two identical sex chromosomes (either X or Z) is more protected from harmful genetic mutations. While the ‘Unguarded-X’ effect likely explains some part of observed life span differences, researchers still debate whether or not it explains a meaningful proportion of this difference.
My collaborators and I explored this question in this project by adapting a previously published mathematical model. To test what sex differences in lifespan would be consistent if only Unguarded X effects contributed to this difference by harnessing prior experimental work to develop plausible values for all known contributing factors to this effect. Not only were our predicted range/s of likely ‘Unguarded X effects’ much smaller than observed life span differences - these predicted values were even smaller than most studies could detect. For example, sex differences in mean life span are often known to exceed 20% (e.g., in mammals). In contrast, our models predict unguarded X effects of a few percent (e.g., 1–3% in Drosophila and mammals).
