| The origin of new diploid species through hybridization is theoretically difficult because reproductive isolation must arise in the presence of gene flow, and yet empirical evidence suggests the process is repeatable. Mathematical models indicate that this process may be facilitated if hybrids can adapt to extreme environments by combining genes from the parental species. Here, I test these models by identifying the genetic and ecological changes required for the origin of the Pecos sunflower, Helianthus paradoxus. I show that H. paradoxus is more salt tolerant in a laboratory environment, and has greater fitness than its parental species, H. annuus and H. petiolaris, in a desert salt marsh where H. paradoxus naturally occurs. In these experiments, values of traits associated with general salt tolerance mechanisms measured in H. paradoxus were shown to be extreme relative to those of its progenitors. The number, location, and magnitudes of effects of quantitative trait loci (QTLs) for uptake of calcium and sodium, traits correlated with salt tolerance, were estimated in a H. annuus × H. paradoxus hybrid population. Two independent QTLs were discovered and both were found to have strong influences on the salt tolerance correlated traits. However, contrary to findings for other hybrid species, analysis of the genetic relatedness of individuals across the geographic range of H. paradoxus, suggest that this species results from a single ancient hybridization event. |
