Recent and recurrent polyploidy in Tragopogon (Asteraceae): cytogenetic, genomic and genetic comparisons

Abstract

Tragopogon mirus Ownbey and T. miscellus Ownbey are allopolyploids that formed repeatedly during the past 80 years following the introduction of three diploids ( T. dubius Scop . , T. pratensis L . and T. porrifolius L.) from Europe to western North America. These polyploid species of known parentage are useful for studying the consequences of recent and recurrent polyploidization. We summarize recent analyses of the cytogenetic, genomic and genetic consequences of polyploidy in Tragopogon . Analyses of rDNA ITS (internal transcribed spacer) + ETS (external transcribed spacer) sequence data indicate that the parental diploids are phylogenetically well separated within Tragopogon (a genus of perhaps 150 species), in agreement with isozymic and cpDNA data. Using Southern blot and cloning experiments on tissue from early herbarium collections of T. mirus and T. miscellus (from 1949) to represent the rDNA repeat condition closer to the time of polyploidization than samples collected today, we have demonstrated concerted evolution of rDNA. Concerted evolution is ongoing, but has not proceeded to completion in any polyploid population examined; rDNA repeats of the diploid T. dubius are typically lost or converted in both allopolyploids, including populations of independent origin. Molecular cytogenetic studies employing rDNA probes, as well as centromeric and subtelomeric repeats isolated from Tragopogon , distinguished all chromosomes among the diploid progenitors (2 n = 12). The diploid chromosome complements are additive in both allopolyploids (2 n = 24); there is no evidence of major chromosomal rearrangements in populations of either T. mirus or T. miscellus . cDNA-AFLP display revealed differences in gene expression between T. miscellus and its diploid parents, as well as between populations of T. miscellus of reciprocal origin. Approximately 5% of the genes examined in the allopolyploid populations have been silenced, and an additional 4% exhibit novel gene expression relative to their diploid parents. Some of the differences in gene expression represent maternal or paternal effects. Multiple origins of a polyploid species not only affect patterns of genetic variation in natural populations, but also contribute to differential patterns of gene expression and may therefore play a major role in the long-term evolution of polyploids.

Arabidopsis hybrid speciation processes

“…Although diploids of each species are well separated according to morphological and neutral molecular marker data, hybridization frequently occurs, especially in plant families that underwent radiation events, e.g., Asteraceae [Helianthus (22), Senecio (23, 24), Tragopogon (25, 26)] and Brassicaceae [Boechera (27), Cardamine (28)]. In numerous of these study systems, hybridization is not one single event, but occurs multiple times-independently and sometimes even polytopically (24,(25)(26)(27)(28). For the A. lyrata × A. arenosa hybrid/introgression system, microsatellite marker data indicate that populations from the eastern Austrian Forealps with their strong hybrid index mark the initial hybridization event, which could have happened during Pleistocene glaciation and deglaciation cycles.…”

Silenced rRNA genes are activated and substitute for partially eliminated active homeologs in the recently formed allotetraploid, Tragopogon mirus (Asteraceae)

“…(2n = 2x = 12) and T. porrifolius L. (2n = 2x = 12) from Europe to North America (Soltis et al, 2004). In plants from most natural populations studied in the Palouse area of the northwestern USA, there are fewer rDNA units of T. dubius origin than of T. porrifolius origin .…”

Gene loss and silencing in Tragopogon miscellus (Asteraceae): comparison of natural and synthetic allotetraploids

“…It arose naturally o80 years ago through allopolyploidisation, with the diploid species T. dubius and T. pratensis as parents (Ownbey, 1950;Soltis et al, 2004a). These parents are phylogenetically divergent (Mavrodiev et al, 2005;Buggs et al, 2008); therefore, homoeologues in T. miscellus can be distinguished by sequence differences (for example, Tate et al, 2006).…”

“…These parents are phylogenetically divergent (Mavrodiev et al, 2005;Buggs et al, 2008); therefore, homoeologues in T. miscellus can be distinguished by sequence differences (for example, Tate et al, 2006). The species formed repeatedly in different localities from separate populations of the diploid progenitors (reviewed in Soltis et al, 1995Soltis et al, , 2004a, giving replicated independent natural allopolyploid lines. In addition, we now have multiple, reciprocal synthetic allopolyploid lines of the species (Tate et al, in press).…”