Gene regulatory evolution in cold-adapted fly populations neutralizes plasticity and may undermine genetic canalization

Abstract

AbstractThe relationships between adaptive evolution, phenotypic plasticity, and canalization remain incompletely understood. Theoretical and empirical studies have made conflicting arguments on whether adaptive evolution may enhance or oppose the plastic response. Gene regulatory traits offer excellent potential to study the relationship between plasticity and adaptation, and they can now be studied at the transcriptomic level. Here we take advantage of three closely-related pairs of natural populations of Drosophila melanogaster from contrasting thermal environments that reflect three separate instances of cold tolerance evolution. We measure the transcriptome-wide plasticity in gene expression levels and alternative splicing (intron usage) between warm and cold laboratory environments. We find that suspected adaptive changes in both gene expression and alternative splicing tend to neutralize the ancestral plastic response. Further, we investigate the hypothesis that adaptive evolution can lead to decanalization of selected gene regulatory traits. We find strong evidence that suspected adaptive gene expression (but not splicing) changes in cold-adapted populations are more vulnerable to the genetic perturbation of inbreeding than putatively neutral changes. We find some evidence that these patterns may reflect a loss of genetic canalization accompanying adaptation, although other processes including hitchhiking recessive deleterious variants may contribute as well. Our findings augment our understanding of genetic and environmental effects on gene regulation in the context of adaptive evolution.Significance StatementIt is unclear whether adaptive evolution is concordant or discordant with regulatory plasticity, especially for splicing plasticity which is rarely studied. Here we analyzed RNA-seq data from three pairs of natural fly populations that represent separate adaptive evolution to cold climate. We found that adaptive evolution is generally discordant with the ancestral plasticity between cold and warm temperatures for gene expression abundance and splicing. We also investigate the hypothesis that adaptation leads to decanalization of the selected traits. By comparing the expression variance between inbred and outbred samples, we found evidence that adaptation may lead to genetic decanalization for expression abundance but not for splicing. Our study reveals the relationship between adaptation, plasticity and canalization in three instances in nature.