Suggesting that the Unguarded X Chromosome is not Important in Gender Longevity Differences
Researchers here discuss the unguarded X hypothesis in the context of gender differences in life span. That these differences exist across species strongly suggests evolutionary, biological origins, rather than the lifestyle and behavioral origins sometimes suggested to explain life span differences in our species. It seems likely that the interaction between evolutionary pressures and mating strategies drives a great deal of the differences between genders, and life span may be included in that list, but exactly how that difference in longevity is produced at the level of cellular biochemistry remains up for discussion, as illustrated here.
Females and males often have markedly different mortality rates and life spans, but it is unclear why these forms of sexual dimorphism evolve. The unguarded X hypothesis contends that dimorphic life spans arise from sex differences in X or Z chromosome copy number (i.e., one copy in the "heterogametic" sex; two copies in the "homogametic" sex), which leads to a disproportionate expression of deleterious mutations by the heterogametic sex (e.g., mammalian males; avian females). Although data on adult sex ratios and sex-specific longevity are consistent with predictions of the unguarded X hypothesis, direct experimental evidence remains scant, and alternative explanations are difficult to rule out.
Using a simple population genetic model, we show that the unguarded X effect on sex differential mortality is a function of several reasonably well-studied evolutionary parameters, including the proportion of the genome that is sex linked, the genomic deleterious mutation rate, the mean dominance of deleterious mutations, the relative rates of mutation and strengths of selection in each sex, and the average effect of mutations on survival and longevity relative to their effects on fitness. We review published estimates of these parameters, parameterize our model with them, and show that unguarded X effects are too small to explain observed sex differences in life span across species. For example, sex differences in mean life span are known to often exceed 20% (e.g., in mammals), whereas our parameterized models predict unguarded X effects of a few percent (e.g., 1-3% in Drosophila and mammals). Indeed, these predicted unguarded X effects fall below statistical thresholds of detectability in most experiments, potentially explaining why direct tests of the hypothesis have generated little support for it.
Our results suggest that evolution of sexually dimorphic life spans is predominantly attributable to other mechanisms, potentially including "toxic Y" effects and sexual dimorphism for optimal investment in survival versus reproduction.