Hybridization is hypothesized to enhance the ability of non-native species to invade. Although a number of invasive taxa are of hybrid origin, relatively few studies have compared the frequency of hybridization between the introduced and native ranges, or examined the performance of the hybrids relative to the parental species. Diffuse and spotted knapweed, two ecologically and economically devastating weeds in western North America, are known to hybridize in their native range in Europe. Based on floral morphology, hybrids were reported in North America. However, there has been confusion in the literature and field about whether or not plants with such intermediate morphology are indeed hybrids or are simply morphological variants of diffuse knapweed. To resolve this debate, extensive field surveys in the native (Eurasian) and introduced (North America) ranges were conducted in 2005 and 2006. Additionally, molecular data (Amplified Fragment Length Polymorphisms) were collected to determine if intermediate morphology reflects hybridization at the genetic level.  

Results/Conclusions

In the introduced range plants with hybrid morphology are present frequently in diffuse knapweed sites (38 out of 39 in western North America), but not in spotted knapweed sites. Based on molecular data, the plants with hybrid morphology are indicative of interspecific hybridization at the population level. It seems unlikely, however, that hybridization took place in North America; the spotted knapweed here is likely tetraploid and the diffuse knapweed is diploid and multiple attempts to make F1 hybrids between them via hand-pollination failed. Rather, the data suggest that individuals of hybrid origin were introduced with diffuse knapweed. Plants with hybrid morphology within diffuse knapweed sites were taller and more frequently exhibited polycarpy than plants that appeared as typical diffuse knapweed. Biological control agents, both seedhead feeders and root miners, do not discriminate between hybrid-type and diffuse-type plants within diffuse knapweed sites. Because hybridization between spotted and diffuse knapweed occurred prior to introduction approximately 100 years ago, and because biological control agents do not avoid hybrids, it seems unlikely that hybridization is currently altering diffuse knapweed’s invasion. It is, however, plausible that the inclusion of hybrids with the introduction of diffuse knapweed contributed to the success of this species in North America. A large greenhouse common garden is examining this possibility.