Natural hybrids between Tragopogon mirus and T. miscellus (Asteraceae): a new perspective on karyotypic changes following hybridization at the polyploid level.

UNLABELLED PREMISE OF THE STUDY Natural hybrids have formed in Pullman, Washington, United States between the recently formed allotetraploids Tragopogon miscellus and T. mirus. In addition to forming spontaneously, these hybrids are semifertile, propagating via achenes. Previous work indicated that the tetraploid hybrids have genetic contributions from three progenitor diploids: T. dubius, T. pratensis, and T. porrifolius. Because the hybrids contain genomes from three species, they should be karyotypically variable and have very low fertility. To better understand how these hybrids are semifertile, we applied fluorescent probes to determine chromosome composition. • METHODS We sequentially conducted fluorescence and genomic in situ hybridization to generate karyotypes for five hybrid individuals grown from field-collected achenes. • KEY RESULTS All plants had the expected somatic chromosome number (2n = 24), but none showed an additive F1 chromosome complement, i.e., two sets of chromosomes from T. dubius and one set of chromosomes each from T. porrifolius and T. pratensis. No individuals shared an identical karyotype, but chromosomal variation followed a compensatory pattern of substitutions, with all groups of putatively homeologous chromosomes consistently totaling four. • CONCLUSIONS The hybrids appear to be shifting away from a parentally additive F1 karyotype to chromosomal compositions that are mostly, or entirely, disomic. We hypothesize that this process may eventually lead to the elimination of chromosomes from a population and produce a stabilized karyotype distinct from both allotetraploid parents. This work has implications for other hybrids formed between polyploids, in that they may be hard to detect using sequence data alone due to multilateral patterns of chromosome elimination.