Uncovering the genomic basis of an extraordinary plant invasion.

Invasive species are a key driver of the global biodiversity crisis, but the drivers of invasiveness, including the role of pathogens, remain debated. We investigated the genomic basis of invasiveness in Ambrosia artemisiifolia (common ragweed), introduced to Europe in the late 19th century, by resequencing 655 ragweed genomes, including 308 herbarium specimens collected up to 190 years ago. In invasive European populations, we found selection signatures in defense genes and lower prevalence of disease-inducing plant pathogens. Together with temporal changes in population structure associated with introgression from closely related Ambrosia species, escape from specific microbial enemies likely favored the plant's remarkable success as an invasive species.

Competition between cytotypes changes across a longitudinal gradient in Centaurea stoebe (Asteraceae).

PREMISE OF THE STUDY: Polyploidy resulting from whole genome duplication has contributed to the adaptive evolution of many plant species. However, the conditions necessary for successful polyploid evolution and subsequent establishment and persistence in sympatry with diploid progenitors are often quite limited. One condition thought to be necessary for establishment is a substantial competitive superiority of the polyploid. METHODS: We conducted a pairwise competition experiment using diploid and tetraploid cytotypes of Centaurea stoebe L. to determine whether (1) tetraploids have greater competitive ability than diploids, (2) cytotypes from mixed-cytotype populations have more balanced competitive abilities than single-cytotype populations, and (3) competitive abilities change along a longitudinal gradient. KEY RESULTS: Across sampling localities, tetraploids did not produce greater aboveground biomass than diploids but suffered from greater intracytotypic competition. Tetraploids allocated greater biomass belowground than diploids, regardless of competition treatment, and had greater performance for traits associated with long-term persistence (bolted more frequently and produced more accessory rosettes). Competitive ability of tetraploids did not differ between single- and mixed-cytotype populations but varied along a longitudinal gradient. Tetraploids were stronger intercytotypic competitors in Western Europe (Switzerland and Germany) than in Eastern Europe (Hungary and Slovakia), which indicates that cytotype coexistence may be more likely in Eastern Europe, the proposed origin of tetraploids, than in Western Europe. CONCLUSIONS: Our study addresses the importance of examining competitive interactions between cytotypes across their distributional range, as competitive interactions were not consistent across sampling localities.

Escape from competition: neighbors reduce Centaurea stoebe performance at home but not away.

The greater abundance of some exotic plants in their nonnative ranges might be explained in part by biogeographic differences in the strength of competition, but these competitive effects have not been experimentally examined in the field. We compared the effects of neighbors on the growth and reproduction of spotted knapweed (Centaurea stoebe) in Europe, where it is native, and in Montana, where it is invasive. There were strong negative competitive effects of neighboring vegetation on C. stoebe growth and reproduction in Europe. In contrast, identical experiments in Montana resulted in insignificant impacts on C. stoebe. Although the mechanisms that produce this dramatic biogeographic difference in competitive outcome remain unknown, our results indicate that differences in net competitive interactions between ranges may contribute to the striking dominance of C. stoebe in parts of North America.