Lifespan Predicts Mitochondrial Substitution Rates across Vertebrates, but Methodology Matters.

Why do some species live for mere months, while others persist for centuries? A leading explanation implicates mitochondria. The mitochondrial theory of aging predicts that mitochondrial efficiency diminishes with age due to the accumulation of mutations within mitochondrial DNA (mtDNA). While experimental evidence for this theory is mixed, evolutionary analyses offer an ideal opportunity to determine if mitochondrial substitution rates are linked to longevity. Here, we explored the relationship between mtDNA evolution and species' lifespans across four clades-Aves, Actinopterygii, Bivalvia, and Sebastidae-using five normalization strategies. Across most methods, long-lived vertebrates showed reduced synonymous and nonsynonymous substitution rates, suggesting lower mtDNA mutation. However, we found that the strength and direction of these relationships varied drastically depending on the normalization approach used (ie correcting for divergence, generation time, and phylogeny). We also analyzed mtDNA mutation spectra and found similar patterns in long- and short-lived species, suggesting decreased rates of mtDNA mutations in long-lived species are not due to suppression of specific mutation processes, as predicted from the free radical theory of aging. We also find little evidence for a relationship between selection on mitochondrial protein-coding genes and lifespan. Our results align with the idea that decreased mutation rates may help preserve mitochondrial integrity in long-lived vertebrate species, but that these species have not been selected to have particularly efficient OXPHOS or protection against a specific mitochondrial mutation process. Together, these findings underscore the critical link between mitochondrial stability and lifespan, and highlight the power of natural systems in this field.