Ecological divergence between a homoploid hybrid species, Helianthus paradoxus, and its parental species.
WELCH, M.E.* and L.H.RIESEBERG
Indiana University, Bloomington, IN 47405 USA 1
Theory suggests that ecological divergence between an incipient hybrid lineage and its parental species may facilitate the process of diploid or homoploid hybrid speciation. Concordant with the predictions of theory, all diploid species of demonstrated hybrid origin are associated with habitats outside the described habitat ranges of the parental species. The puzzle sunflower, Helianthus paradoxus, is a species of purported hybrid origin and occurs exclusively in brackish marshes with sodium ion concentrations (10-14 ppt) well above those found in the habitats of the parental species, the common sunflower, H. annuus, and the prairie sunflower, H. petiolaris. Previous work reports that H. paradoxus is more salt tolerant than H. annuus. Further, we have found that sodium ion concentrations in H. paradoxus exceed those in H. annuus by an order of magnitude, suggesting that these species have different means of processing sodium. However, these works fail to demonstrate a fitness advantage for H. paradoxus under sodium ion concentrations found in natural H. paradoxus habitat. Here, we ask how the relative fitness of H. paradoxus and its parental species are affected by sodium ion concentration in a controlled environment. Individuals from all three species were grown in the greenhouse under three experimental treatments, no salt (0mM NaCl), low salt (100mM NaCl) and high salt (200mM NaCl). After four weeks under these conditions, plants were harvested and fresh and dry biomass was measured for each individual in the study. Preliminarily, these data imply that the relative biomass of H. paradoxus actually increases in response to increasing salinity while H. annuus. and H. petiolaris both show a relative biomass decrease in response to increasing salinity. This finding tentatively supports a habitat specific fitness advantage for H. paradoxus resulting from higher salt tolerance.
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