Molecular cytogenetic analysis of recently evolved Tragopogon (Asteraceae) allopolyploids reveal a karyotype that is additive of the diploid progenitors
Tragopogon (Asteraceae) has approximately 150 species native to Eurasia. Three diploid (2n = 2x =12) species (T. dubius, T. pratensis, and T. porrifolius) that were introduced into eastern Washington State, USA, and adjacent Idaho in the early 1900s have recently and recurrently formed two allopolyploid (2n = 4x = 24) species, T. mirus (T. dubius × T. porrifolius) and T. miscellus (T. dubius × T. pratensis) in western North America (Ownbey, 1950⇓; Ownbey and McCollum, 1954⇓; Brown and Schaak, 1972⇓; Soltis and Soltis, 1989⇓, 1991⇓, 1999⇓; Soltis et al., 1995⇓; Cook et al., 1998⇓). Ownbey and McCollum (1954)⇓ used traditional cytogenetic methods to karyotype the six pairs of chromosomes in the introduced diploid species of Tragopogon. Along with morphological characters, Ownbey and McCollum (1954)⇓ observed sufficient chromosomal variation (e.g., terminal knobs and secondary constrictions) among the different populations of diploid species to infer the multiple origins of the two recently formed allopolyploids, T. mirus and T. miscellus. These recurrent formations were later confirmed with molecular methods (Soltis et al., 1995⇓)....
...Our goal is to determine whether chromosomal rearrangements or changes in genome size have occurred in the Tragopogon allotetraploids since their recent formation. One way to survey for genomic rearrangements in polyploids is to locate repetitive DNA physically on chromosomes using the tools of molecular cytogenetics such as fluorescent in situ hybridization (FISH), a technique that has provided insights into genome and chromosome evolution (reviewed in Heslop-Harrison, 1991⇓, 2000⇓; Jiang and Gill, 1994⇓, 1996⇓; Leitch and Bennett, 1997⇓; Schwarzacher and Heslop-Harrison, 2000⇓; Singh, 2003⇓)....
...We used molecular cytogenetics (FISH) to determine the number and distribution of these four tandem repeats to investigate the genetic consequences of allopolyploidy in Tragopogon. The FISH technique was carried out on multiple populations of the three diploid species (T. dubius, T. pratensis, and T. porrifolius), and karyotypes were constructed. The same probes were then hybridized to multiple populations of the recently formed allotetraploid species (T. mirus and T. miscellus) to determine if chromosomal rearrangements had occurred subsequent to polyploidization. We also measured DNA C values for the same diploid and tetraploid Tragopogon species to determine the dynamics of genome size evolution in this polyploid complex...
...Table 1 shows the DNA C values calculated from plants of different populations of diploids (T. dubius, T. pratensis, and T. porrifolius) and allotetraploids (T. miscellus and T. mirus). Genome size varied among different populations of the diploid species. Among the diploid species, T. dubius appears to have the smallest genome size and T. porrifolius the largest.
The DNA C values of T. mirus are not substantially different from the sum of the diploid parents. The genome size of T. mirus (population 2601 from Pullman, Washington, USA; mean 4C DNA value of 24.33) is additive of the genome sizes of its putative diploid progenitor populations, T. dubius (population 2613 from Pullman, Washington USA; mean 4C DNA value of 11.76) and T. porrifolius (population 2611 Pullman, Washington, USA; mean 4C DNA value of 12.5). Although we measured 4C DNA values for another population of T. mirus (2603 from Rosalia, Washington, USA), we cannot comment unequivocally on the additivity of its genome size because we were able to measure only the genome size of the T. dubius parent; plants of the second parent, T. porrifolius, are no longer present in Rosalia and are presumed extinct. Nonetheless, the two 4C DNA measurements for plants of T. porrifolius from other localities are very similar.
In contrast, the genome size of at least some populations of T. miscellus appears to have undergone downsizing when compared to its diploid progenitors. Populations 2604 (Moscow, Idaho, USA) and 2605 (Pullman, Washington, USA) have the same diploid parental genotypes (T. pratensis [2608 Moscow, Idaho, USA] and T. dubius [2613 Pullman, Washington, USA]), but are the result of reciprocal parentage (Soltis and Soltis, 1989⇓). The genome size values for these two populations are 20.30 pg and 20.99 pg, respectively, which are lower than the value predicted (24.20 or 23.27 pg) by adding the values of the diploid parents (t = 18.79958, P < 0.05; t = 13.88154, P < 0.05, for predicted values of 24.20 and 23.27, respectively). This level of downsizing—approximately 15%—is similar to that reported in the allopolyploids Brassica napus, B. juncea, and B. carinata (Naryan, 1998⇓). The genome size of a third population of T. miscellus (2606 from Spangle, Washington, USA) is 21.76 pg; the value for the parental T. pratensis from Spangle is 11.09 pg, but we lack genome size data for T. dubius from Spangle...
...Collectively, the distribution of the four tandem repetitive DNA loci (TPRMBO, TGP7, 18S-5.8S-26S rDNA, and 5SrDNA) among the chromosome pairs allowed the construction of molecular cytogenetic karyotypes for the three diploid species of Tragopogon (Fig. 5g). Thus, the number, location, and intensity of the FISH signals for all the mapped loci allowed for the identification of several of the diploid parental chromosomes in the polyploids. If rearrangements had taken place upon or immediately following polyploidization, we would expect to observe nonadditive patterns in the polyploids. For example, the number of rDNA loci in a polyploid could be greater or fewer than that found separately in the two diploid progenitors. Alternatively, rearrangements could move subtelomeric repeats found in the diploids to interstitial locations in the polyploids. Some synthetic allopolyploid species, including members of Triticeae (Shaked et al., 2001⇓) and Brassica (Song et al., 1995⇓), display rapid (within a few generations) evolution of polymorphic markers. However, other polyploid species show additive patterns, as we observed for Tragopogon; these include synthetic polyploids in Gossypium (Liu et al., 2001⇓) and the natural Nicotiana polyploids (N. rustica and N. arentsii) compared with their putative diploid parents (Lim et al., 2004⇓)....
... We found no evidence for major genomic rearrangements in the allopolyploids T. mirus and T. miscellus. The number and location of the tandem repetitive sequences TGP7 and TPRMBO appear to be directly inherited in the allopolyploids from their corresponding diploid ancestors without organizational or distributional changes. Similarly, the 18S-5.8S-26S and 5S rDNA loci in T. miscellus and T. mirus were exactly as predicted from the number and location of these loci in their diploid progenitors.