Environmental conditions and genetic differentiation: what drives the divergence of coexisting Leymus chinensis ecotypes in a large-scale longitudinal gradient?

Aims The two coexisting Leymus chinensis ecotypes exhibit remarkable divergences in adaptive strategies under drought and salinity in semi-humid meadows and semi-arid steppes. In order to detect the major genetic and environmental factors dominating the intraspecific phenotype variations and ecotype formation, the questions regarding the two distinct phenotypic forms (ecotypes) in L. chinensis were addressed: (i) did environments drive the L. chinensis ecotype formation? (ii) was there a molecular basis for the morphological divergence between the two ecotypes? (iii) which driving force dominated the intraspecies divergence, divergent natural selection, genetic drift or stabilizing selection?. Methods We applied a series experiments on demographical, morphological and physiological traits of two Leymus chinensis ecotypes with gray green (GG) and yellow green (YG) leaf color in nine wild sites along a longitudinal gradient from 114° to 124°E in northeast China. The environmental data including mean annual precipitation, mean annual temperature, elevation and soil properties were collected. We compared the differences of morphological, physiological and genetic differentiations between the two ecotypes. Important Findings The GG type exhibited stronger fitness than YG type from the population densities, morphological traits (e.g. shoot height, leaf area, leaf and seed weights et al.), leaf mass per area (LMA) and physiological traits [relative water content (RWC), proline, soluble sugar contents]. Most of above phenotypes (e.g. total shoot densities, spike length et al.) were significantly correlated with mean annual precipitation, mean annual temperature and soil water content (SWC), rarely a correlated with soil pH and soil nutrient. Transplanted populations showed convergence trend by their leaf chlorophyll contents and osmotic adjustments (proline and soluble sugar contents) in the greenhouse, but still exhibited their divergences between two ecotypes in the outdoor transplantation, suggesting that whether L. chinensis ecotype differentiated could be largely affected by the environmental conditions. Furthermore, by the comparison result of quantitative genetic variation (Q_ST) values from phenotypes with theoretical neutral genetic differentiation (F_ST), differentiation in phenotypic traits greatly surpassed neutral predictions, implying that directional natural selection played a crucial role in L. chinensis ecotype differentiation. In addition, microsatellite analysis from Neighbor-joining tree and Bayesian assignment generated into two groups according to ecotypes, indicating molecular genetic differentiation also propelled the two ecotypes divergence. We conclude that L. chinensis population variations were driven by combing divergent natural selection (precipitation, temperature and SWCs) along the large-scale gradient and significantly intrinsic genetic differentiation. [ABSTRACT FROM AUTHOR]