Plastic responses to past environments shape adaptation to novel selection pressures

Phenotypic plasticity may pave the way for rapid adaptation to newly encountered environments. There is growing evidence that initial plastic responses of ancestral populations to new environmental cues may promote subsequent adaptation. However, we do not know whether plasticity to cues present in the ancestral habitat (past-cue plasticity) can facilitate adaptation to novel cues. Conceivably, this could occur if plastic responses are coincidentally optimal to both past and novel cues (i.e., are pre-adaptive) or if they are transferred to novel cues during adaptation. Past plastic phenotype values could also become fixed and genetically co-opted during adaptation to the new environment. To uncover the role of past-cue plasticity in adaptation, we tested gene expression plasticity responses of two parallel mine-waste adapted Sea Campion (Silene uniflora) populations and their closest coastal relatives to past and novel-cues of salt and zinc. During adaptation to mine-waste in these plants, plasticity to salt diminishes. Despite this, our results show that ancestral plasticity to salt has a substantial impact on subsequent adaptation to zinc. For a third of genes that have evolved zinc plasticity in mine populations, salt plasticity has been transferred to the zinc response. Furthermore, a quarter of fixed expression differences between mine and coastal populations were similar to ancestral salt responses. Alongside a broader evidence base that ancestral plasticity to novel cues can facilitate adaptation, our results provide a clear indication that ancestral past-cue plasticity can also play a key role in rapid, parallel adaptation to novel habitats. This entry is for salt dataset only, as the zinc transcriptome data is already present.