Not as boring as expected: triploids, pentaploids and aneuploids of invasive Solidago species revealed by detailed karyological examination in central Europe

Three alien Solidago species of North American origin have become naturalized in Europe. While S. canadensis and S. gigantea are considered among the most aggressive plant invaders, S. altissima is rarely found. Here, the detailed karyological variation and cytogeography of alien Solidago species in central Europe and the genetic relationships among the cytotypes is elucidated in their native and other invaded areas. Almost 4,800 plants from 800 sites across central Europe were studied using flow cytometry and complementary chromosome counting. A representative subset was sequenced (ITS, cpDNA) and compared with available data. The findings are in accord with previous reports, with the diploids of S. canadensis and tetraploids of S. gigantea dominating the invaded range. There was up to 10% variation in the genome size among the studied populations of particular species, the relationship between genome size and the geographical location and altitude of the population was statistically confirmed. In addition, for the first time in the invaded range, rare cytotypes, i.e. triploids of S. canadensis and pentaploids and aneuploids of S. gigantea, were recorded. The record of S. altissima in the study area was not confirmed and its previous hexaploid record was refuted based on re-examination. The triploids of S. canadensis were represented by scattered plants within diploid populations, and they most likely resulted from an occasional fusion of reduced and unreduced gametes. The origin of the S. gigantea aneuploids and especially pentaploids is more obscure as almost three hundreds were recorded, although so far, only one pentaploid has been reported from North America. Samples from the invaded range shared ITS ribotypes with those from the native range; however, ITS data indicate slight genetic differentiation of pentaploids and aneuploids of S. gigantea. It was also revealed that two widespread haplotypes of cpDNA in central Europe were shared by all cytotypes of S. gigantea and S. canadensis. The cytogeographic pattern of invasive Solidago species in Europe is more diverse than expected. Because genome duplication can promote invasiveness, the origin, distribution and invasive potential of newly discovered polyploids need to be investigated in order to prevent their negative effect on the native flora.