Cytogeography of invasive knotweeds (Fallopia sect. Reynoutria) in central Europe

Fallopia sect. Reynoutria members (knotweeds) represent one of the most invasive alien plants in Europe. However, several aspects of their biology, including cytological variation are poorly understood. Specifically, some taxa have multiple ploidy levels and the frequency of cytotypes in seeds and seedlings does not correspond to that recorded in adult plants. In this study, flow cytometry was used to determine the relative genome size of 1,106 established plants of knotweeds (960 populations) collected in the Western Carpathians and adjacent area in Pannonia, and the results were combined with previous data (53 individuals from 43 populations) from the Krivánska Malá Fatra Mts. In total, 1,159 individuals from 1,003 populations (825 individuals from Slovakia, 173 from Hungary, 70 from Poland, 63 from Czechia and 28 from Austria) were evaluated. Fallopia sect. Reynoutria is represented mainly by the hexaploid (2n = 6x ~ 66) cytotype of F. ×bohemica (809 individuals) and octoploid (2n = 8x ~ 88) cytotype of F. japonica var. japonica (297 individuals) in the area studied. To a limited extent, the tetraploid (2n = 4x ~ 44) and hexaploid (2n = 6x ~ 66) cytotypes of F. sachalinensis (43 and 9 individuals, respectively) were also recorded. In addition, for the first time, adult aneuploid knotweed plants with 2n = 65 and 2n = 107 chromosomes (both in F. ×bohemica) were recorded in continental Europe and the world, respectively. In contrast, the occurrence of the tetraploid (2n = 4x ~ 44) cytotype of F. japonica previously reported in the area studied was not confirmed and it is likely this information is incorrect. This study revealed evidence of a climatically determined distribution of invasive knotweeds in the area studied. The occurrence of F. japonica var. japonica and F. sachalinensis decreased markedly along a north-west–south-east gradient, whereas the frequency of F. ×bohemica occurrence increased (from 15.7% in the Polish part of the Western Carpathians up to 98.5% in the Hungarian part of Pannonia).