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

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.

An assessment of karyotype restructuring in the neoallotetraploid Tragopogon miscellus (Asteraceae).

Tragopogon miscellus and Tragopogon mirus are two rare examples of allopolyploids that have formed recently in nature. Molecular cytogenetic studies have revealed chromosome copy number variation and intergenomic translocations in both allotetraploids. Due to a lack of interstitial chromosome markers, there remained the possibility of additional karyotype restructuring in these neopolyploids, via intrachromosomal and intragenomic rearrangements. To address this issue, we searched for additional high-copy tandem repeats in genomic sequences of the diploid progenitor species-Tragopogon dubius, Tragopogon pratensis and Tragopogon porrifolius-for application to the chromosomes of the allotetraploids. Eight novel repeats were localised by fluorescence in situ hybridisation (FISH) in the diploids; one of these repeats, TTR3, provided interstitial coverage. TTR3 was included in a cocktail with other previously characterised probes, producing better-resolved karyotypes for the three diploids. The cocktail was then used to test a hypothesis of karyotype restructuring in the recent allotetraploid T. miscellus by comparing repeat distributions to its diploid progenitors, T. dubius and T. pratensis. Five individuals of T. miscellus were selected from across the range of karyotypic variation previously observed in natural populations. FISH signal distributions mostly matched those observed in the diploid progenitors, with the exception of several losses or gains of signal at chromosomal subtermini and previously noted intergenomic translocations. Thus, in T. miscellus, we find most changes restricted to the subterminal regions, and although some were recurrent, none of the changes were common to all individuals analysed. We consider these findings in relation to the gene loss reported previously for T. miscellus.