Supplementary material for "Gene expression and drought response in an invasive thistle." To complement studies of phenotypic variation, we investigate gene expression during drought response that might underlie variation in drought tolerance between native and introduced populations of diffuse knapweed (Centaurea diffusa, Asteraceae), and possibly contribute to invasion success. Using species-specific microarrays and tissue sampled under drought and control conditions at three time points, we identified genes whose expression either varied constitutively or responded to drought stress differently between ranges. Further, we functionally investigate these genes. Supplementary Material 1 contains the following tables and figures: Figure S1: Putative climatic niche expansion between native and invasive ranges of Centaurea diffusa. Figure S2: Heat map of genes with drought-induced expression differences between native and invasive Centaurea diffusa for the drought treatment from time point 2. Figure S3: Heat map of genes with drought induced expression differences between native and invasive Centaurea diffusa for the control treatment from time point 2. Table S1: Centaurea diffusa experimental population information. Table S2: Abiotic environmental data variables used in principal component analysis. Table S3: Results from likelihood ratio tests for genes with significant model terms. [Selection only, full table available at http://dx.doi.org/10.6084/m9.figshare.1372537 (Turner 2015), or in associated Dryad repository.] Table S4: Gene Ontology terms for gene with a significant effect of seed collection environment. Table S5: Gene Ontology terms for genes with a significant effect of drought treatment. [Selection only, full table available at http://dx.doi.org/10.6084/m9.figshare.1372537 (Turner 2015), or in associated Dryad repository.] Table S6: Genes previously identified as part of the drought response gene regulatory network in domesticated sunflower (Helianthus annuus) and also identified as responding to drought in Centaurea diffusa.