Effects of climatic gradients on genetic differentiation of Caragana on the Ordos Plateau, China

Abstract

The genus Caragana (Fabr.) in the Ordos Plateau of Inner Mongolia, China, provides a strong opportunity to investigate patterns of genetic differentiation along steep climatic gradients, and to identify the environmental factors most likely to be responsible for driving the radiation. This study used a factorial, multi-model approach to evaluate alternative hypotheses and identify the combination of environmental factors that appear to drive genetic divergence of Caragana in the Ordos Plateau. We had three specific hypotheses. First, we expected that gradients of changing climate would act as resistant factors limiting gene flow, and would provide stronger prediction of genetic differentiation than isolation by distance. Second, we expected that variation in precipitation would be a stronger predictor of genetic differentiation among populations than variation in temperature. Third, we expected that the pattern of phylogenetic differences, in terms of derived versus ancestral states of rachis and leaf shape, would be highly correlated with these gradients of changing precipitation, reflecting adaptive radiation along gradients of changing precipitation driven by reduced gene flow and differential patterns of directional selection. As we expected, variation in precipitation was a much stronger predictor of genetic differentiation than were other climatic variables or isolation by distance. The pattern of phylogenetic differentiation among Caragana species is also closely associated with gradients of changing patterns of precipitation, suggesting that differential precipitation plays a major role in driving the genetic differentiation and adaptive radiation of the Caragana genus in the region of the Ordos Plateau.