Plasticity of lifespan: a reaction norm perspective

Abstract

It is a well-appreciated fact that in many organisms the process of ageing reacts highly plastically, so that lifespan increases or decreases when the environment changes. The perhaps best-known example of such lifespan plasticity is dietary restriction (DR), a phenomenon whereby reduced food intake without malnutrition extends lifespan (typically at the expense of reduced fecundity) and which has been documented in numerous species, from invertebrates to mammals. For the evolutionary biologist, DR and other cases of lifespan plasticity are examples of a more general phenomenon called phenotypic plasticity, the ability of a single genotype to produce different phenotypes (e.g. lifespan) in response to changes in the environment (e.g. changes in diet). To analyse phenotypic plasticity, evolutionary biologists (and epidemiologists) often use a conceptual and statistical framework based on reaction norms (genotype-specific response curves) and genotype × environment interactions (G × E; differences in the plastic response among genotypes), concepts that biologists who are working on molecular aspects of ageing are usually not familiar with. Here I briefly discuss what has been learned about lifespan plasticity or, more generally, about plasticity of somatic maintenance and survival ability. In particular, I argue that adopting the conceptual framework of reaction norms and G × E interactions, as used by evolutionary biologists, is crucially important for our understanding of the mechanisms underlying DR and other forms of lifespan or survival plasticity.

Life-History Evolution and the Genetics of Fitness Components inDrosophila melanogaster

“…Min et al 2007;Tatar 2011); careful studies that manipulated yeast and sugar in different combinations discovered that DR extends life span because of the reduced amount of dietary yeast (relative to sugar), the major source of protein in the fly diet, suggesting that the main determinant is the ratio of protein to carbohydrate (Lee et al 2008;Skorupa et al 2008;Bruce et al 2013;Lee 2015;Tatar et al 2014). These insights were aided by the advent of the "nutritional geometry" framework, providing a quantitative method for examining multidimensional dietary responses (Lee et al 2008;Simpson and Raubenheimer 2012;Flatt 2014. The role of yeast is also underscored by observations showing that the quality and species of dietary yeast fungi has profound effects on fly life history (Begon 1982;Bass et al 2007;Anagnostou et al 2010;Grangeteau et al 2018).…”

“…discussion in Flatt (2009), Hoedjes et al (2017), and Flatt and Partridge 2018]. However, there is still little evidence for any specific gene to be functionally required for the life span increase under DR (Tatar 2007(Tatar , 2011Flatt 2014;Tatar et al 2014).…”

The relationship between longevity and diet is genotype dependent and sensitive to desiccation

“…The generality of the DR response has been questioned, however, by reports that DR does not extend lifespan under certain experimental conditions (Austad, 2012;Ja et al, 2009;Piper et al, 2010) or in some genotypes (Liao et al, 2010;Tatar, 2011). These conclusions are routinely based on experiments using two diets (dietary dyad) alone, whereas it is recognised that a change in the continuous relationship between diet and lifespan (reaction norm) can obscure lifespan extension by DR (Flatt, 2014;Tatar, 2011). The bell-shaped nature of the dietary reaction norm dictates that one particular diet concentration, in one genotype or environment, will result in the longest lifespan; with lower or higher diet concentrations inducing a shortened lifespan due to overfeeding or malnutrition.…”

How the effects of aging and stresses of life are integrated in mortality rates: insights for genetic studies of human health and longevity

“…For each given genotype, there is a curve that relates the contribution of environmental variation to observed phenotypic variation. The concept of the norm of reaction also clarifies how carriers of two different genotypes may produce different phenotypic responses to the same environmental conditions (Flatt 2014). They also may have different responses to the same changes in these conditions over time.…”