Rapid Concerted Evolution of Nuclear Ribosomal DNA in Two Tragopogon Allopolyploids of Recent and Recurrent OriginSequence data from this article have been deposited with the EMBL/GenBank Data Libraries under accession nos. AY458586, AY458588, AY458589, and AY458587

Abstract

We investigated concerted evolution of rRNA genes in multiple populations of Tragopogon mirus and T. miscellus, two allotetraploids that formed recurrently within the last 80 years following the introduction of three diploids (T. dubius, T. pratensis, and T. porrifolius) from Europe to North America. Using the earliest herbarium specimens of the allotetraploids (1949 and 1953) to represent the genomic condition near the time of polyploidization, we found that the parental rDNA repeats were inherited in roughly equal numbers. In contrast, in most present-day populations of both tetraploids, the rDNA of T. dubius origin is reduced and may occupy as little as 5% of total rDNA in some individuals. However, in two populations of T. mirus the repeats of T. dubius origin outnumber the repeats of the second diploid parent (T. porrifolius), indicating bidirectional concerted evolution within a single species. In plants of T. miscellus having a low rDNA contribution from T. dubius, the rDNA of T. dubius was nonetheless expressed. We have apparently caught homogenization of rDNA repeats (concerted evolution) in the act, although it has not proceeded to completion in any allopolyploid population yet examined. (Ownbey 1950). The introduction of these dipsequences frequently show evidence of homogenizaloid species into the Palouse brought them into close tion, probably caused by processes such as unequal contact, something that rarely occurs in the Old World crossing over and gene conversion, mechanisms collecwhere the diploids are largely allopatric. Using mortively referred to as concerted evolution (Zimmer et al. phology and cytology, Ownbey (1950) demonstrated 1980;Dover 1982). The process of concerted evolution that T. mirus and T. miscellus are allotetraploids (2n ϭ can best be studied in synthetic polyploids or in natural 24) whose diploid (2n ϭ 12) parents are T. dubius and polyploids of clear and very recent ancestry. However, T. porrifolius and T. dubius and T. pratensis, respectively. only a few naturally occurring polyploid species areThe ancestries of both tetraploids were subsequently known to have arisen spontaneously within the past 150 confirmed through flavonoid, isozymic, and DNA studyears: Cardamine schulzii (Urbanska et al. 1997) clones from the polyploids suggested that the parental and P. S. Soltis, personal observation).rDNA types were not present in the allotetraploids in Variable numbers of rRNA genes coding for 18S-5.8S-equal frequency. In fact, T. dubius appeared to be under-26S RNA occur in different plant species (between 1000 represented in both allotetraploid species. To assess and Ͼ50,000 genes), forming multigene families in long whether concerted evolution has been operating in the tandem arrays (Hemleben and Zentgraf 1994). The recently formed allotetraploids, T. mirus and T. miscellus, intragenic (ITS) and intergenic spacers (IGS) associwe investigated the rDNA cistron using cloning, Southated with genic regions often vary among species and ern blots, slot blots, and single-str...

Highly efficient concerted evolution in the ribosomal DNA repeats: Total rDNA repeat variation revealed by whole-genome shotgun sequence data

“…A rapid homogenization model of concerted evolution is consistent with the results from numerous previous studies (Liao et al 1997;Scott 1998, 2002;Skalicka et al 2003;Averbeck and Eickbush 2005;Kovarik et al 2005). Furthermore, rDNA polymorphisms between individuals in a population (e.g., Carbone and Kohn 2001;James et al 2001;Ganley et al 2005) are also evidence for rapid homogenization.…”

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

“…¼ 19, t ¼ 2.027, P ¼ 0.057). A similar bias towards loss of T. dubius genetic material has been found for rDNA copy number (Kovarik et al, 2005). This could be because of cytoplasmic-nuclear interactions.…”

“…Perhaps polyploidy in T. miscellus sets the stage for noninstantaneous, yet rapid, mutational mechanisms that are ongoing through many generations. The most obvious of these is homoeologous recombination, in which fragments of chromosomes can be lost, though we cannot exclude the possibility of gene conversion, as has been found for rRNA genes in the species (Kovarik et al, 2005;Matyasek et al, 2007). Homoeologous recombination seems to have caused loss of chromosome fragments in re-synthesised Brassica allopolyploids (Song et al, 1995;Gaeta et al, 2007).…”

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

“…Given that the genome size of T. mirus is known (~21 pg/4C Pires et al, 2004), it was possible to calculate the number of rRNA gene copies from the strength of the hybridisation signals. We estimate that plant 33B had about 1700 ITS1 copies (~1300 copies in the P-genome and~400 copies in the D-genome) per haploid chromosome set, which is typical for this population of T. mirus (Rosalia; Kovarik et al, 2005). In contrast, plant 33A had less than 1400 ITS1 copies (~1300 from the P-genome and~70 copies from the D-genome).…”

“…Southern blot procedures followed the standard protocol described in Kovarik et al (2005). The membranes were exposed to a Storage Phosphor Screen, scanned (Typhoon 9410, GE Healthcare, Piscataway, NJ, USA) and the signal was quantified using Image Quant (GE Healthcare).…”