|About the Book|
Hybridization is an evolutionary force that has the potential to alter invasion dynamics. Centaurea diffusa Lam. (diffuse knapweed) and C. stoebe L. (spotted knapweed) are two problematic invasive weeds in western North America. Anecdotal informationMoreHybridization is an evolutionary force that has the potential to alter invasion dynamics. Centaurea diffusa Lam. (diffuse knapweed) and C. stoebe L. (spotted knapweed) are two problematic invasive weeds in western North America. Anecdotal information suggested these two plants were hybridizing in the introduced range. The overall goal of my dissertation was to examine the patterns of hybridization in the introduced and native ranges in the field and at the molecular level, and determine if hybridization was altering the invasion of either species.-In the field in North America, I detected intermediate hybrid plants in 39 out of 40 diffuse knapweed sites- hybrid-like plants were taller and more often exhibited polycarpy than diffuse-like plants. Hybrid-like plants were not detected in North American spotted knapweed sites. In Europe in most countries surveyed, diffuse and spotted knapweed existed as distinct, non-hybridizing species. However, in the Ukraine, the two species frequently co-existed within a site, resulting in hybrid swarms. It has recently been confirmed that the spotted knapweed in North America is tetraploid while the diffuse knapweed is likely all diploid. Genetic incompatibilities associated with these two cytotypes likely prevents on-going hybridization. Instead, the diffuse knapweed in North America was probably introduced with individuals of hybrid origin.-I used amplified fragment length polymorphisms (AFLPs) to examine hybridization at the molecular level. The Bayesian clustering program, STRUCTURE, found that a majority of the assayed North American diffuse knapweed sites contained individuals with significant introgression from spotted knapweed. Counter to expectation, the hybrid-like plants did not contain more admixture than the plants that looked diffuse-like. A century of back-crossing with diffuse knapweed has likely decoupled the relationship between floral morphology and introgression at the molecular level.-Biological control agents were not strongly influenced by the presence of plants with hybrid morphology. Within North America diffuse-like and hybrid-like plants carried similar herbivore loads. In paired preference tests the seedhead weevil Larinus minutus demonstrated a preference for newly created artificial hybrids over North American diffuse knapweed- this preference would likely be short-lived in the field as introgression occurred.