Loss of Enemy Resistance Among Introduced Populations of St. John's Wort (Hypericum Perforatum)

The Evolution of Increased Competitive Ability (EICA) hypothesis predicts that introduced plants should lose enemy resistance and in turn evolve increased size or fecundity. We tested the first prediction of this hypothesis by growing introduced North American and native European genotypes of St. John’s Wort (Hypericum perforatum) in common gardens in the state of Washington, USA, and in Girona, Spain. In both gardens we measured levels of hypericin and pseudohypericin (and in Washington, hypericide)— compounds known to be toxic to generalist pathogens and herbivores. In a third common garden, in Spain, we experimentally manipulated native pathogen pressure (by treating plants with fungicides) and quantified how pathogen resistance varied between North American and European genotypes. North American St. John’s Wort had lower levels of hypericin than European conspecifics in common gardens in Washington and Spain. North American plants also produced less hypericide (in Washington) and pseudohypericin (in Spain) than did European plants. In Spain, individuals were attacked by three generalist pathogens: Colletotrichum sp. (Coelomycetes), Alternaria sp. (Hyphomycetes), and Fusarium oxysporum (Hyphomycetes). A higher percentage of individuals from North American populations were infected by pathogens and died from pathogen attack compared to European genotypes. Infection also appeared to reduce plant size and fecundity; these negative effects were similar in magnitude for North American and European genotypes. Taken together, results indicate that introduced St. John’s Wort has lost enemy resistance. However, contrary to EICA, current and previous results indicate that these changes have not been associated with an increase in plant size or fecundity.