Ecological genomics aims to understand the genetic basis of ecologically important traits which constitutes a long-term goal for evolutionary biologists. Although recent advances in genomics have revealed huge amount of segregating variation, we have little understanding of why genes segregate for polymorphisms that influence ecologically important quantitative traits. Plausible evolutionary explanations include deleterious polymorphisms, local adaptation, ongoing positive selection, and other evolutionary processes. Particularly, local adaptation is widely recognized as one of the most important influence on evolution in many plant species. Water stress is one of the major acting selective forces that promote plant adaptation. Understanding the mechanisms of how plants tolerate drought is, thus, a central topic in plant ecology and evolution. This proposed research will provide the first comprehensive evolutionary analysis of the genetic basis of the wild grass Brachypodium distachyon, a new model for functional genomics of temperate cereals, adaptation to dry environments. Specifically, we will address the following major goals: a) to examine the spectrum of genetic variation in traits determining the acquisition and allocation of resources to growth and reproduction in B. distachyon populations adapted to differing drought regimes from multiple genotypes across a naturally occurring water-stress gradient in the Iberian Peninsula and southern France; b) to infer what evolutionary forces shaping natural genetic variation in this ecologically important trait, with special emphasis on local adaptation processes. The major hypothesis to be tested in this project is that local adaptation will determine in a major extent nuclear polymorphisms for drought tolerance in B. dystachyon.