Transcriptomic analysis reveals the rareness of genetic assimilation of gene expression in environmental adaptations

Genetic assimilation is the evolutionary process by which an environmentally induced phenotype becomes genetically encoded and constitutive. Genetic assimilation has been proposed as a concluding step in environmental adaptation, but its prevalence has not been systematically investigated. Analyzing transcriptomic data collected upon reciprocal transplant, we address this question in the experimental evolution, domestication, or natural evolution of seven diverse species. We find that genetic assimilation of environment-induced gene expression is the exception rather than the rule and that substantially more genes retain than lose their expression plasticity upon organismal adaptations to new environments. The probability of genetic assimilation of gene expression decreases with the expression level and number of transcription factors controlling the gene, suggesting that genetic assimilation results primarily from passive losses of gene regulations that are not mutationally robust. Hence, for gene expression, our findings argue against the purported generality or importance of genetic assimilation to environmental adaptation.