Role of plasticity in adaptation

When environmental change is rapid or unpredictable, phenotypic plasticity can facilitate adaptation to new or stressful environments to promote population persistence long enough for adaptive evolution to occur. However, the underlying genetic mechanisms that contribute to plasticity and its role in adaptive evolution are controversial. Two main opposing hypotheses dominate - genetic compensation and genetic assimilation. Here we predominately find evidence for genetic compensation over assimilation in adapting the freshwater algae Chlamydomonas reinhardtii to 36g/L salt environments over 500 generations. More canalized genes in the salt-adapted lines displayed a pattern of genetic compensation fixing at the ancestral native expression level, rather than genetic assimilation of the salt-induced level, suggesting that compensation plays more of a role in salt adaptation. In addition, whole-transcriptome similarity amongst ancestral and salt-adapted lines displayed the evolution of a similar plastic response to salt conditions in independently reared salt-adapted lines. We also found an enrichment of mutations in differentially expressed genes, and genes involved in stress responses and the photosystem were positively correlated with the salt-adapted lines. Thus, the expression changes induced via plasticity offer temporary relief, but downstream changes are required for a sustainable solution during the evolutionary process.