Major advances have recently been made in terms of the genetics of lifespan, particularly using C. elegans and D. melanogaster. For example, the insulin/insulin-like growth factor (IGF-1) signalling pathway has been shown to be a powerful regulator of lifespan in nematodes, flies and probably rodents (Kenyon, 2005). The relationship between the lifespan-determining genes and pathways identified, and those involved in lifespan evolution, remain unclear. One possibility is that at least some of the genes and processes identified by model organism lifespan genetics are the same as those involved in lifespan evolution. In the case of S. ratti, insulin/IGF-1 signalling is a good candidate for a regulator of pheno-typic plasticity of aging. In C. elegans, this pathway controls the differences in lifespan between the long-lived dauer larva and the shorter lived adult (Riddle and Albert, 1997).
In C. elegans, DAF-16, a FOXO-family transcription factor, controls the rate of aging in response to insulin/ IGF-1 signalling. Recently, microarray studies have begun to identify the transcriptional targets of DAF-16, which are predicted to include the ultimate genetic determinants of longevity and aging in C. elegans (Murphy et al., 2003; McElwee et al., 2004). DAF-16 alters the expression of a wide range of genes, involved in many life processes, including defenses against stress, toxins and microbial invasion. To date, these analyses of the role of IIS and DAF-16 have been based on the analysis of mutants, where lifespan differences of up to 2.5 fold can be achieved (Kenyon et al., 1993). While substantial, such differences are much less than evolved, interspecific differences in lifespan, or the 80-fold differences in lifespan between the different adult morphs of S. ratti. Therefore, a plausible hypothesis is that S. ratti lifespan plasticity is determined by differential expression of the same sorts of genes and processes regulated by DAF-16 in C. elegans. Further, one may speculate that this is also the case for lifespan differences between species, such as that between human and chimpanzee. A graphic representation of this hypothesis is presented in Figure 20.5.
Overall, this discussion of the implications of S. ratti aging suggests a view of lifespan as a highly plastic, regulated trait that is consistent with the rapid evolution of interspecific differences in lifespan.
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