Habitat divergence between a homoploid hybrid sunflower species, Helianthus paradoxus (Asteraceae), and its progenitors.

The diploid hybrid species Helianthus paradoxus is restricted to salt marshes with sodium concentrations that exceed those found in the habitats of its progenitors, H. annuus and H. petiolaris. The observed association with saline habitats has led to the hypothesis that H. paradoxus is more salt tolerant than its progenitors. This hypothesis was tested by growing all three species in three NaCl treatments (0 mmol/L, 100 mmol/L, and 200 mmol/L). Helianthus paradoxus treated with NaCl was found to be more than five times as fit, in terms of biomass and survivorship, than its progenitors. Selection for salt tolerance in early generation hybrids may have contributed to the formation of H. paradoxus because theory predicts that homoploid hybrid speciation is feasible even when selection favoring hybrid genotypes is much weaker. Additionally, we show that H. paradoxus is significantly different from its parental species for several traits that often distinguish salt-tolerant species and suggest a mechanistic basis for the elevated salt tolerance expressed by H. paradoxus.