The genetic variation in response to drought in Tunisian populations of Brachypodium hybridum (Poaceae): an interplay between natural selection and phenotypic plasticity

With the ongoing climate change, widespread changes could be noticed in both the extent of the geographical distribution as well the patterns of phenotypic variation of plant species in response to environmental heterogeneity. Whether these changes are a direct consequence of global climate change, and whether they result from phenotypic plasticity or evolutionary forces, are currently main areas of interest. In the present study, 130 lines belonging to ten Tunisian Brachypodium hybridum populations differing greatly in climatic conditions were exposed to water stress and the pattern of their genetic variation was analyzed using sixteen functional traits in order to unravel the main drivers of their response to drought. Our results showed that B. hybridum populations exhibited an extensive intra-population genetic variation in control and water stress, whereas principal component analysis did not show any clear populations grouping pattern. In addition, ANOVA showed that most of analyzed traits revealed a significant population and line effects, indicating that their response to drought was, in part, controlled at the genetic level. These results suggest that the natural selection on standing intra-population genetic variation plays a major role in dealing with drought in B. hybridum. On the other hand, we found that many traits exhibited phenotypic plasticity in response to water stress. The plasticity index showed that the populations responded differently to water stress, with Ain Drahem and Djerba populations showed the highest and the lowest plasticity levels, respectively. However, plasticity of most analyzed traits seems to be unrelated to climatic factors, except for the precipitation of wettest month, which showed significant correlation with five traits related to biomass production and the mean plasticity index. In summary, the present study revealed that the genetic variation in response to drought in the natural populations of B. hybridum was most likely governed by an interplay between two major evolutionary and ecological processes: natural selection and phenotypic plasticity.