Centaurea solstitialis L. (yellow starthistle, Asteraceae) is a Eurasian native plant introduced as an exotic into North and South America, and Australia, where it is regarded as a noxious invasive. Changes in ploidy level have been found to be responsible for numerous plant biological invasions, as they are involved in trait shifts critical to invasive success, like increased growth rate and biomass, longer life-span, or polycarpy. C. solstitialis had been reported to be diploid (2n = 2x = 16 chromosomes), however, actual data are scarce and sometimes contradictory. We determined for the first time the absolute nuclear DNA content by flow cytometry and estimated ploidy level in 52 natural populations of C. solstitialis across its native and non-native ranges, around the world. All the C. solstitialis populations screened were found to be homogeneously diploid (average 2C value of 1.72 pg, SD = ±0.06 pg), with no significant variation in DNA content between invasive and non-invasive genotypes. We did not find any meaningful difference among the extensive number of native and non-native C. solstitialis populations sampled around the globe, indicating that the species invasive success is not due to changes in genome size or ploidy level.

Dioecious and thorny Rochefortia Sw. is a poorly known though distinct element of the Ehretiaceae comprising woody plants restricted to the Caribbean and the adjacent American mainland. The approximately ten species display a great morphological variability and overlapping taxonomic boundaries, which makes it difficult to differentiate them (particularly in the Caribbean region). We investigated the phylogenetic relationships of Rochefortia using DNA sequence data from one nuclear locus (Internal Transcribed Spacer) and three chloroplast DNA loci (rps16, trnL-trnF, trnS-trnG). The monophyly of Rochefortia was confirmed, with a sister group relationship between an American mainland clade and a Caribbean clade. The latter segregates into three, morphologically rather variable lineages, distributed either in the Lesser Antilles or in the eastern Greater Antilles or in the western Greater Antilles. Thus, geographic occurrence rather than morphology is indicative of taxonomic delimitation in Rochefortia.

Allelopathy, the release of chemicals by plants that inhibit the germination and growth of competing species, can be an important trait for invasive success. However, little is known about potential biogeographical differences in allelopathy due to divergent regional eco-evolutionary histories. To test this, we examined the allelochemical potential of the highly invasive species Centaurea solstitialis from six world regions including native (Spain, Turkey) and non-native ranges (Argentina, Chile, California and Australia). Seeds from several populations in each region were collected and grown under common garden conditions. Allelopathic potential and chemical composition of three leaf extract concentrations of C. solstitialis from each region: 0.25%, 0.5% 0.75% (w/v−1) were assessed on the phytometer Lactuca sativa. The main allelochemicals present in the leaf-surface extract were sesquiterpene lactones that varied in major constitutive compounds across regions. These leaf extracts had strong inhibitory effects on L. sativa seed germination and net growth. Summed across regions, the 0.25% concentration suppressed germination by 72% and radicle elongation by 66%, relative to the controls. At the 0.5% concentration, no seeds germinated when exposed to extracts from the non-native ranges of Argentina and Chile, whereas germination and radicle growth were reduced by 98% and 89%, respectively, in the remaining regions, relative to controls. Germination and seedling growth were completely inhibited at the 0.75% concentration extract for all regions. Some non-native regions were characterized by relatively lower concentrations of allelochemicals, suggesting that there is biogeographical variation in allelopathic expression. These findings imply that rapid selection on the biochemical signatures of an exotic invasive plant species can be highly region-specific across the world.