Gene expression plasticity in the antibiotic environment is predominantly adaptive in the budding yeast

Phenotypic plasticity plays a key role in adaptation to fluctuating environments. However, its evolutionary significance remains debated, with conflicting views on whether it is actively maintained by natural selection or a neutral consequence of molecular constraints. In this study, we investigate the evolutionary role of gene expression plasticity in yeast populations exposed to antibiotic and osmotic stress. Using mutation accumulation (MA) lines to separate the effects of natural selection from genetic drift, we compare gene expression responses (referred to as plastic change) between 22 natural strains, 21 MA lines, and their progenitor under non-stressed and stressed conditions. Our results show that, in the antibiotic environment, gene expression plasticity is selectively maintained, as indicated by its reduction in magnititude, reversal in directionality, and loss of stress-responsive pathways in MA lines. In contrast, plasticity in the osmotic stress condition appears neutral, with random variation across MA lines. This study provides direct evidence for the adaptive role of gene expression plasticity in the antibiotic environment and sheds light on its molecular mechanisms.