Crossbreeding reveals the scale and genetic architecture of local adaptation in a predominantly selfing Hordeum spontaneum

Environmental variation can be large across a wide range of spatial scales resulting in complex patterns of local adaptation across species ranges. We analyzed the scale, genetic mechanism and direct climatic causes of local adaptation in a widely distributed grass Hordeum spontaneum. We performed artificial crosses of maternal plants representing the same Negev desert population with plants originating elsewhere. Pollen donors were plants from other Negev desert populations, non-desert Israeli populations sampled along an aridity gradient, and accessions covering the entire species range. Our study included planting of inter-population hybrids under favorable and simulated desert experimental conditions, followed by analysis of their performance, variation in adaptive traits and relationship with climatic parameters at sampling locations. The combined results of parental phenotypic variation and performance of hybrids were consistent with local selection, reflecting the importance of both regional and local climates. The adaptive genetic differentiation of barley desert populations had a complex architecture. None of the three effects (additive, dominance and epistasis) were fully responsible for this differentiation. Although genetic effects not related to extrinsic selection appear to contribute to genetic differentiation in barley, epistatic effects arising from local selection clearly predominated. The short-term effect of gene flow by pollen was generally negative, indicating that a majority of the new allele combinations created by recombination were maladaptive. However, the long-term effect of occasional pollen flow from other desert populations appears to be positive, as some new recombined genotypes were superior in fitness to the maternal plants even in the F2 generation.