Intergeneric hybrid origin of the invasive tetraploid Cirsium vulgare

The invasive tetraploid Cirsium vulgare hybridizes with both Cirsium and Lophiolepis. Its conflicted position in molecular phylogenies, and its peculiar combination of morphological, anatomical, and genomic features that are alternatively shared with representatives of Cirsium or Lophiolepis, strongly suggest its intergeneric hybrid origin. Genetic relationships of C. vulgare (8 samples) with genus Lophiolepis (11 species) and other representatives of genus Cirsium (12 species) were evaluated using restriction site-associated DNA sequencing (RADseq) and examined using analytical and imaging approaches, such as NeighborNet, Heatmap, and STRUCTURE, to identify nuclear genomes admixture. Estimation of the intensity of spontaneous hybridization within and between Cirsium and Lophiolepis was based on herbarium revisions and published data for all reported hybrids pertinent to taxa currently included in Cirsium or Lophiolepis. The genome of any examined Cirsium species is more similar to C. vulgare than to any Lophiolepis species, and vice versa. The nuclear genome of the tetraploid C. vulgare is composed of two equivalent parts, each attributable either to Lophiolepis or to Cirsium; the organellar RADseq data clustered C. vulgare with the genus Cirsium. Spontaneous hybridization between Cirsium and Lophiolepis is significantly less intensive than within these genera. Our analyses provide compelling evidence that the invasive species C. vulgare has an allotetraploid intergeneric origin, with the maternal parent from Cirsium and the paternal from Lophiolepis. For the purpose of delimiting monophyletic genera, we propose keeping Lophiolepis separate from Cirsium and segregating C. vulgare into the hybridogenous genus Ascalea.