Print PageRecent studies have suggested a causal link between hybridization and invasion. Hybridization has the potential to generate increased genetic variance and evolutionary novelty, which may confer an advantage to organisms in an introduced range where they experience a novel habitat and selection regime. Previous work has shown that the invasive plant, diffuse knapweed (Centaurea diffusa Lam.), was likely introduced to North America with admixture from a closely related species, spotted knapweed (Centaurea stoebe subsp. stoebe L.). In this study we experimentally examined whether hybridization may have contributed to the successful invasion of diffuse knapweed through a greenhouse common garden study with artificially created Backcross 1 (BC1) diffuse knapweed hybrids. As the environment is critical to evaluating traits linked to fitness, we varied competition, which has been shown to influence diffuse knapweed invasion. We compared BC1 hybrids to pure European diffuse knapweed to examine whether early generation hybrids exhibit increased performance and/or increased phenotypic variance. Additionally, we compared North American diffuse knapweed, which contains introgression from spotted knapweed, to pure European diffuse knapweed to evaluate evidence for a response to selection towards increased performance in North America, which might be attributed to the presence of hybrids in the incipient invasion.
Results/Conclusions
In comparison with diffuse knapweed plants of native (European) origin, BC1 plants exhibited increased variance for five of the examined traits, and greater leaf and reproductive shoot production. However, BC1 and European diffuse knapweed did not respond differently to competition. Individual BC1 lines differed for several traits, including their effect on the competitor, suggesting that the identity of the specific cross could be critical in determining how hybridization affects invasions, making drawing general conclusions from such comparisons more challenging. When compared to the parental species (diffuse and spotted knapweed), the BC1 plants were not transgressive for any of the measured traits. Surprisingly, BC1 plants were more spotted-like for a number of traits, even though F1 hybrids were back-crossed into diffuse knapweed. While the increased variance and robustness suggest that hybridization could have contributed to this successful invasion, such differences were not apparent in comparisons between North American diffuse knapweed (including hybrid phenotypes) and European diffuse knapweed. Because North American hybrids resulted from a pre-invasion hybridization event, they may have lost this additional phenotypic variation prior to invasion. We conclude that hybridization may be more likely to contribute to invasion success if the hybridization event happens post- rather than pre-introduction.
