Helianthus winteri: A new species of sunflower?

Background/Question/Methods

Helianthus winteri is an ecotype and putative new sister species of the common sunflower Helianthus annuus, with pronounced differences in habitat, morphology, and life history. While H. winteri and H. annuus have overlapping geographic ranges, H. winteri is found only on hillsides with southern exposure and poor, fast-draining soil, and its phenotypic differences from H. annuus are consistent with adaptations to water scarcity, suggesting ecological speciation may be occurring. In this analysis, we investigate genetic differentiation among populations of H. winteri and both sympatric and allopatric populations of H. annuus. We also use common-garden experiments to test for the formation of reproductive barriers between the two ecotypes.

Results/Conclusions

Our results show that H. winteri populations are genetically distinct from H. annuus. However, considerable gene flow is still occurring between the ecotypes, suggesting that H. winteri is not currently a separate species. This is consistent with the results of the common garden experiments, which largely did not show the formation of reproductive barriers between the ecotypes. Crosses between H. winteri and H. annuus did not have reduced seedset, F1 growth, survival, or pollen production relative to crosses within ecotypes. However, there was a significant effect of parent population on F1 seed germination, which our model indicated is partially due to reproductive incompatibility between the ecotypes. Our results highlight the importance of conserving genetic diversity at levels below that of the species, particularly as the H. winteri phenotype may exhibit increased drought tolerance.