Chromosomal Composition and Fertility in the Early Generations of Two Resynthesized Allopolyploids in Tragopogon (Asteraceae)

The nascent allopolyploid species Tragopogon mirus and T. miscellus are both characterized by widespread karyotypic variation, including frequent aneuploidy and intergenomic translocations. Our study illuminates the origins and early impacts of this variation by addressing two questions: how quickly does karyotypic variation accumulate in Tragopogon allopolyploids following whole-genome duplication (WGD), and how does the fertility of neopolyploid Tragopogon evolve shortly after WGD? We used genomic in situ hybridization (GISH) and lactophenol-cotton blue staining to estimate the karyotypic variation and pollen stainability, respectively, of resynthesized neopolyploid T. mirus and T. miscellus during the first five generations after WGD. Widespread karyotypic variation developed quickly and resembled that of nascent T. mirus and T. miscellus by generation S₄. Pollen stainability in resynthesized allopolyploids was consistently lower than that of nascent T. mirus and T. miscellus, as well as their respective diploid progenitor species. Logistic regression showed that mean pollen stainability increased slightly over four generations in T. mirus, but remained at a constant level in T. miscellus. Our results identify some of the changes that occur in T. mirus and T. miscellus between the neopolyploid and nascent polyploid stages, which have critical consequences for allopolyploid establishment and evolution.