The Arctic Daisy, Chrysanthemum arcticum, Trifecta Is a Genetically Variable Polyploid Series

Background/Objectives: Accurate ploidy determination is essential for understanding population structure and evolutionary history for breeding and domestication. The Chrysanthemum arcticum (Arctic daisy) complex, comprising C. arcticum and subspecies arcticum and polaré, exhibits variability in ploidy variation with reports ranging from diploid (2n = 2x = 18) to octoploid (2n = 8x = 72). Methods: The objective of this study was to assess ploidy levels in n = 225 genotypes from n = 46 wild, native populations of the three taxa collected across mainland Alaska, Attu Island (Alaska), and Hudson Bay (Canada) using flow cytometry, and using C. nankingense (2n = 2x = 18) as the diploid reference. Results: Genome sizes were from 5.27 to 20.69 pg (2C), corresponding to diploid through hexaploid. Triploids were the most frequent (64%) before, as well as after, applying the reference standard bias (10% correction). All ploidy levels were present in multiple geographic regions, with no clear spatial, taxonomic, or latitudinal segregation. The high incidence of triploids, most of which were self-compatible and highly fertile, may reflect genetic instability, underlying aneuploidy (which is common in several Chrysanthemum polyploids), or systematic bias from reference standard differences. Inconsistencies between flow cytometry estimates and observed reproductive compatibility, such as successful crossings with known diploids, suggest additional genomic complexity. Potential historical influences creating genomic instability, including environmental disturbances (chemical, radiation warfare remnants) from World War II military activities at Attu Island and Hudson Bay, are discussed. Conclusions: This study shows the challenges of accurately determining ploidy levels in the C. arcticum trifecta complex and highlights the need for other approaches, including high-density SNP genotyping and chromosome imaging, to resolve ploidy questions and guide future breeding strategies.