Population structure, ploidy levels and allelopathy of Centaurea maculosa (spotted knapweed) and C. diffusa (diffuse knapweed) in North America and Eurasia.

Understanding the origins and basic biology of non-indigenous invasive plants can help lay a strong foundation for successful control of such invaders. Centaurea maculosa (spotted knapweed) and its congener C. diffusa (diffuse knapweed) were introduced into North America from Eurasia. These species have diploid and tetraploid forms, and they are thought to hybridize. We are investigating: 1) the Eurasian origins of these plants and their population structure in North America using cpDNA sequence data; 2) ploidy levels of introduced populations; and 3) production of a potentially allelopathic root exudate ([–]-catechin) by C. maculosa and putative hybrids. We sequenced four noncoding regions of the chloroplast genome (4,050 bp) of 14 individuals. For two of the regions (2,161 bp) we sequenced an additional 12 individuals. The sequence data show complex patterns. Haplotypes do not segregate neatly between C. maculosa and C. diffusa. The data suggest that at least two distinct introductions of C. maculosa into North America have occurred – one of individuals related to those in southern France and one of individuals with haplotypes found in western Europe and Ukraine. The cytology shows that C. maculosa populations in North America comprise both diploid and tetraploid individuals, while C. diffusa populations are predominantly diploid. We examined root exudates from individuals collected from what appeared to be a hybrid swarm. Offspring from C. maculosa phenotypes produced the most (–)-catechin, while offspring from putative hybrids produced almost no (–)catechin. This suggests that the ability to produce (–)-catechin is lost through hybridization. This research will aid in focusing the search for new biological control agents. In addition, it lays the foundation for testing two important hypotheses: that introduced populations have evolved to be more aggressive than their native counterparts, and that herbivores and pathogens from the area of origins of introduced plants make more effective biological control agents.