Polygenic routes lead to parallel altitudinal adaptation in Heliosperma pusillum (Caryophyllaceae)

Understanding how organisms adapt to the environment is a major goal of modern biology. Parallel evolution - the independent evolution of similar phenotypes in different populations - provides a powerful framework to explore this question. Here, we quantified the degree of gene expression and functional parallelism across replicated ecotype formation in Heliosperma pusillum (Caryophyllaceae) and gained insights into the architecture of adaptive traits. Population structure analyses and demographic modelling confirm the previously formulated hypothesis of parallel polytopic divergence of montane and alpine ecotypes. We detect a large proportion of differentially expressed genes (DEGs) underlying adaptation of each replicate ecotype pair, with a strikingly low amount of shared DEGs across pairs. Functional enrichment of DEGs reveals that the traits affected by divergent gene expression are the same across ecotype pairs, in strong contrast to the non-shared genetic basis. The remarkable redundancy of differential gene expression indicates that diverged adaptive traits are highly polygenic. We conclude that polygenic traits appear key to opening multiple routes for adaptation, widening the adaptive potential of organisms.