Overlooked giants: post-invasion interploidy dynamics of a cryptic cytotype of alien Solidago gigantea (Asteraceae) in Europe

Background: Novel invasive genotypes can arise through polyploidisation, hybridisation, or gene flow between populations of distinct origins or related species. Solidago gigantea, a notorious European invader, has long been reported exclusively as tetraploid in its invasive range. Recently, mixed-ploidy populations, including tetraploid and pentaploid plants, were discovered; yet the potential role of novel pentaploid cytotype (and its progeny) in S. gigantea invasions remains poorly understood. This study aims to elucidate the origin of pentaploids, the cytotype and genetic structure of mixed-ploidy populations, characterise reproductive mode and mating interactions of pentaploid plants, and assess their fitness and potential contribution to invasiveness using relative DNA content screening, ddRADseq population genetics, and reproductive potential and fitness assessments.Results: Molecular analyses revealed that pentaploids constitute a genetically distinct lineage within S. gigantea. Our results rule out both an autopolyploid origin from the common tetraploid cytotype and an allopolyploid origin via hybridisation with co-occurring native or invasive Solidago species. The pentaploid cytotype reproduces exclusively through clonal propagation; its low genetic variability suggests that both studied populations may belong to a single extensive clonal genet. Pentaploids produce viable gametes but appear to exhibit strict self-incompatibility, preventing the formation of offspring withing the same genotype. However, pentaploid S. gigantea engages in bidirectional mating with co-occurring tetraploid plants, yielding well-developed seeds with offspring ploidy ranging from 4x to 5x (predominantly aneuploid). Despite this cytological variability, progeny from mixed-ploidy populations displayed germination rates and early growth comparable to those from pure tetraploid populations. Notably, at least some tetraploid offspring from 4x-5x crosses successfully established, flowered, and backcrossed with pentaploid plants to produce viable seeds of subsequent introgressed generations.Conclusions: The pentaploid cytotype of S. gigantea introduces a new post-invasion dynamic to its invasive populations. Rather than being an evolutionary dead-end, this cytotype may potentially enhance the species' invasiveness through three evolutionary pathways: (1) a highly successful clonal life strategy enabling both local and long-distance spread; (2) genetic enrichment of tetraploid populations via ongoing interploidy crosses; and (3) establishment of novel aneuploid genotypes due to the remarkable tolerance of chromosomal instability observed in S. gigantea.