Revisiting Bivalves for the Study of the Determinants of Species Lifespan

Mammals have a very wide range in life spans, and the study of mammals might be thought to be more relevant to efforts to extend human longevity than the study of other taxonomic classes. It is quite unclear at this time whether moving genes and mechanisms between mammalian species is likely to produce meaningful gains cost-effectively in the near future, but we certainly won't know if we don't try. Meanwhile, bivalves are another class with a large range in species life span. There is much to be said for undertaking the same sort of search for the determinants of species life span in bivalves as is presently ongoing in mammals. Having results for two quite different classes is likely to be more illuminating of the mechanisms of aging than the results for mammals alone.

Among Metazoa, bivalves have the highest lifespan disparity, ranging from 1 to 500+ years, making them an exceptional testing ground to understand mechanisms underlying aging and the evolution of extended longevity. Nevertheless, comparative molecular evolution has been an overlooked approach in this instance. Here we leveraged transcriptomic resources spanning thirty bivalve species to unravel the signatures of convergent molecular evolution in four long-lived species: Margaritifera margaritifera, Elliptio complanata, Lampsilis siliquoidea, and Arctica islandica (the latter represents the longest-lived non-colonial metazoan known so far). We applied a comprehensive approach - which included inference of convergent dN/dS, convergent positive selection, and convergent amino acid substitution - with a strong focus on the reduction of false positives.

Genes with convergent evolution in long-lived bivalves show more physical and functional interactions to each other than expected, suggesting that they are biologically connected; this interaction network is enriched in genes for which a role in longevity has been experimentally supported in other species. This suggests that genes in the network are involved in extended longevity in bivalves and, consequently, that the mechanisms underlying extended longevity are - at least partially - shared across Metazoa. Although we believe that an integration of different genes and pathways is required for the extended longevity phenotype, we highlight the potential central roles of genes involved in cell proliferation control, translational machinery, and response to hypoxia, in lifespan extension.