Submitted to: International Journal of Plant Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 1, 2013
Publication Date: May 1, 2014
Citation: Reeves, P.A., Richards, C.M. 2014. Effect of a geographic barrier on adaptation in the dwarf sunflower (Helianthus pumilus Nutt.). International Journal of Plant Sciences. 175:688-701. Interpretive Summary: Adaptations from wild relatives of crop species are critical for the development of new agricultural products and continued increase of agricultural yields. Evolutionary theory predicts that isolated populations should harbor unique adaptations as a result of a process termed “local adaptation”. Helianthus pumilus, the dwarf sunflower, is a source of genes for improvement of the cultivated sunflower because of its ability to produce desirable seed fatty acid profiles under drought conditions. We show that two geographically isolated populations of Helianthus pumilus have diverged genetically and exhibit different adaptations related to seed production. These populations should be treated as separate gene pools. Pikes Peak prevents the movement of genes between the two populations because it contains little suitable habitat. Individuals north of Pikes Peak are less well adapted to their local environment, but the genes necessary to improve their performance are present in the southern part of the species range. We argue that crops whose wild relatives persist as a series of isolated populations in nature will be more easily improved using wild germplasm than those whose relatives are uniformly distributed. This is because geographic isolation increases the opportunities for specific adaptations to arise.
Technical Abstract: Premise of research. Geographic isolation promotes local adaptation but may also prevent the movement of advantageous gene complexes throughout a species distribution. We identify an ecogeographic barrier that inhibits gene flow and adaptation across the disjunct species range of the Rocky Mountain endemic sunflower Helianthus pumilus. Methodology. Habitat suitability modeling and least cost path analysis were used to characterize environmental preferences across the species range, and identify ecogeographic barriers to gene flow. Simple sequence repeat data were used to assess genetic structure and estimate recent levels of gene flow across the distributional disjunction. A common garden was used to study phenotypic differentiation and local adaptation. Controlled crosses tested for intrinsic barriers to reproduction. Pivotal Results. Despite significant differences in available habitat, individuals in northern and southern regions occupy similar sites, suggesting a lack of adaptive differentiation in environmental preferences. A region of low suitability habitat separates the two regions. Northern and southern plants are genetically distinct. There has been little recent gene flow. Southern plants had higher fitness (measured as seed set) in the northern region than northern plants. A suite of morphological characters, present in half of southern plants, conveyed a two fold fitness advantage in the north, but this adaptive trait is virtually non existent in the northern range. Between region crosses were equal to or more productive than within region crosses. Conclusions. The Pikes Peak massif, a high altitude eastward projection of the Rocky Mountains, forms an ecogeographic barrier to gene flow in Helianthus pumilus, a lowland foothills species. No intrinsic barriers to gene exchange between regions have yet evolved. Therefore the ecogeographic barrier alone appears to impede the movement of the adaptive trait found in the southern region, resulting in a northern group that is less fit than it might be, contrary to the expectation of local adaptation.