Phenotypic differentiation and diversifying selection in populations of Eruca sativa along an aridity gradient

Background: The aridity gradient in the eastern Mediterranean offers an opportunity to investigate intra-specific genetic differentiation and local adaptation in plant populations. Here we used genetic (F_ST) and quantitative trait (P_ST) differentiation to assess local adaptation among three natural populations of Eruca sativa (Brassicaceae) distributed along a climatic range representing desert, semi-arid and Mediterranean habitats. Results: Amplified fragment length polymorphism (AFLP) analysis revealed high genetic diversity in each population, but low genetic differentiation between populations and relatively high gene flow. Further phenotypic evaluation in a common garden experiment (conduced in a Mediterranean habitat) showed clear differences in phenological traits among populations (day of flowering and duration of the reproductive stage), shoot and root biomass, as well as fitness-related traits (total number of fruits and total seed weight). F_ST–P_ST comparison showed that P_ST values of the phenological traits, as well as below- and above-ground biomass and fitness-related traits, were higher than the F_ST values. Conclusions: Overall, our results support the identification of genotypic and phenotypic differentiation among populations of E. sativa. Furthermore, the F_ST–P_ST comparison supports the hypothesis that these were subjected to past diversifying selection. Thus, the results clearly demonstrate adaptive divergence among populations along an aridity gradient, emphasize the ecological value of early flowering time in arid habitats, and contribute to our understanding of the possible impact of climate change on evolutionary processes in plant populations.