Contents lists available at ScienceDirect Environmental and Experimental Botany journal homepage: www.elsevier.com/locate/envexpbot The genetic variation in response to drought in Tunisian populations of Brachypodium hybridum (Poaceae): an interplay between natural selection and phenotypic plasticity Yosra Ibrahim a,1 , Mohamed Neji a,d, * ,1 , Wael Taamalli c , Chedly Abdelly a , Mhemmed Gandour a,b a Laboratory of Extremophile Plants, Centre of Biotechnology of Borj Cedria, BP 901 Hammam Lif 2050, Tunisia b Faculty of Sciences and Technology of Sidi Bouzid, 9100 Sidi Bouzid, Tunisia c Laboratory of Olive Biotechnology, Centre of Biotechnology of Borj Cedria, BP 901 Hammam Lif 2050, Tunisia d Laboratoire des Ecosystèmes pastoraux et valorisation des plantes spontanées et des microorganismes associés, Institut des Régions Arides, 4119 Medenine, University of Gabès, Tunisia ARTICLE INFO Keywords: Brachypodium hybridum genetic variation drought phenotypic plasticity natural selection climatic factors ABSTRACT 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 hetero- geneity. 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 ex- posed 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 com- ponent 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. 1. Introduction Ongoing and projected global climate change points towards the intensification of various extreme environmental events such as fre- quent and severe drought, resulting from the continuous rise of global mean temperatures and the variability in precipitation levels (Kong et al., 2020). These environmental changes may alter the geographical distribution of natural plant species and the pattern of their genetic variation, and consequently may deeply restrict their ecological adaptation (Blackman et al., 2017). However, in response to novel environmental conditions, new phenotypes may be favored either via selection on standing intra-population variability or via phenotypic plasticity (de Villemereuil et al., 2018; Glenny et al., 2018). The latter, defined as the capacity of genotypes to express diverse phenotypes in response environmental variability, is recognized as an important pro- cess through which plant species efficiently resist to grave environ- mental fluctuations resulting from climate change (Nicotra et al., 2010; Richter et al., 2012; Valladares et al., 2014). Importantly, it has been https://doi.org/10.1016/j.envexpbot.2020.104234 Received 25 April 2020; Received in revised form 28 July 2020; Accepted 7 August 2020 ⁎ Corresponding author. E-mail address: mnmedneji@gmail.com (M. Neji). 1 Mohamed Neji and Yosra Ibrahim contributed equally to this work. Environmental and Experimental Botany 179 (2020) 104234 Available online 14 August 2020 0098-8472/ © 2020 Elsevier B.V. All rights reserved. T