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Abstract

Phenotypic changes in response to environmental change are known as plastic because there are no genetic changes (e.g., new mutations) associated with such rapid phenotypic response. However, the fact that, in many cases, such environmentally- induced phenotypic response differs between individuals suggests an important role for genetic variation in regulating plastic responses. Understanding to what extent the response to environmental change has a genetic basis is fundamentally important to determine the role of plasticity in evolution. Using a large population of outbred Drosophila melanogaster, we set up to identify the genetic basis of lifespan in two conditions: control and high sugar diets. In flies, as well as in mice, humans and other species, high-sugar diets modify many phenotypes including a shortening of lifespan. We sequenced over 10,000 individual flies to track genome-wide allele frequency changes over the lifetime of six replicate populations, recording in real time the changes in the genomic composition of each population as flies aged. The high statistical power of this experimental design allowed us to identify thousands of lifespan-associated alleles whose frequency changed between young and old flies. Remarkably, a third of those lifespan-associated alleles appear cryptic in control diet but play an important role in high sugar conditions. Our results show that phenotypic changes driven by environmental variation within one generation indeed have a genetic basis, suggesting that there is ample opportunity for selection to act on plastic phenotypes.