Although polyploidy is a significant force in the diversification of plants, the evolutionary consequences of polyploidization are not thoroughly understood. One possible consequence of polyploidy predicted by most population genetic theories is that the newly synthesized polyploid will self-fertilize at a greater rate than its diploid progenitors. To test for increased selfing rates in a polyploid, the mating systems of the allotetraploid Tragopogon mirus and one of its diploid progenitors, T. dubius, were compared. Tragopogon mirus is a recently derived species that arose sometime in the last 80 years and thus provides an opportunity to probe how quickly a shift in outcrossing rates might occur. Based on analyses of variation in maternal plants and their progeny arrays, the two tetraploid populations surveyed have higher outcrossing rates than the two diploid populations. This result is the opposite of that predicted by population genetic theory. This discrepancy between theoretical and empirical results may result from bias in the genetic sample, traits in the natural histories of the taxa involved or a lack of sufficient time since the formation of the polyploid (80 years or 40-80 generations) for a shift towards increased selfing to have occurred. Alternatively, the partial dominance model of inbreeding depression typically applied to polyploids may not be appropriate; the overdominance model predicts outcrossing rates in diploids and their tetraploid derivatives that are consistent with those observed in T. dubius and T. mirus.
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"Most studies on polyploidization have been focused on establishing the frequencies or patterns of homeologous gene expression in the context of polyploidization (Tate et al. 2004; Wendel and Doyle 2005; Adams 2007). Moreover, most of them employed the synthesized haploids and polyploids, while the natural polyploids were seldom used (Peng et al. 2008), except some recent polyploids such as Spartina (Baumel et al. 2001), Tragopogon (Cook and Soltis 1999) and Senecio (Abbott and Lowe 2004; Hegarty et al. 2006). As we know, both inducing treatment and tissue culture conditions have the potential to modify the DNA structure, influence gene expression, and eventually interference the accuracy of study on ploidy effects. "
[Show abstract][Hide abstract] ABSTRACT: Interspecific hybridization has a much greater effect than chromosome doubling on gene expression; however, the associations between homeologous gene expression changes and polyhaploidization had rarely been addressed. In this study, cDNA-single strand conformation polymorphism analysis was applied to measure the expression of 30 homeologous transcripts in naturally occurring haploid (ABD, 2n = 21) and its polyploid maternal parent Yumai 21A (AABBDD, 2n = 42) in wheat. Only one gene (TC251989) showed preferentially silenced homoeoalleles in haploids. Further analyses of 24 single-copy genes known to be silenced in the root and/or leaf also found no evidence of homeologous silencing in 1-month-old haploids and two ESTs (BF484100 and BF473379) exhibit different expression patterns between 4-month-old haploids and hexaploids. Global analysis of the gene expression patterns using the Affymetrix GeneChip showed that of the 55,052 genes probed, only about 0.11% in the shoots and 0.25% in the roots were activated by polyhaploidization. The results demonstrate that activation and silencing of homoeoalleles were not widespread in haploid seedlings.
Full-text · Article · Jun 2011 · G3-Genes Genomes Genetics
"This sort of heterozygosity would be evident if there were a very low-copy array and a very high-copy array in parents. However, FISH analysis revealed no indication of rDNA heterozygosity in the diploid Tragopogon individuals investigated, a situation which might be expected for species that are largely selfing [58,59], and in plants that were derived from inbred lines propagated in a greenhouse (at least one generation). In estimating rDNA locus sizes, and hence relative copy numbers at individual loci, FISH may be influenced by the condensation state of the chromatin. "
[Show abstract][Hide abstract] ABSTRACT: Tragopogon mirus and T. miscellus are allotetraploids (2n = 24) that formed repeatedly during the past 80 years in eastern Washington and adjacent Idaho (USA) following the introduction of the diploids T. dubius, T. porrifolius, and T. pratensis (2n = 12) from Europe. In most natural populations of T. mirus and T. miscellus, there are far fewer 35S rRNA genes (rDNA) of T. dubius than there are of the other diploid parent (T. porrifolius or T. pratensis). We studied the inheritance of parental rDNA loci in allotetraploids resynthesized from diploid accessions. We investigate the dynamics and directionality of these rDNA losses, as well as the contribution of gene copy number variation in the parental diploids to rDNA variation in the derived tetraploids.
Using Southern blot hybridization and fluorescent in situ hybridization (FISH), we analyzed copy numbers and distribution of these highly reiterated genes in seven lines of synthetic T. mirus (110 individuals) and four lines of synthetic T. miscellus (71 individuals). Variation among diploid parents accounted for most of the observed gene imbalances detected in F1 hybrids but cannot explain frequent deviations from repeat additivity seen in the allotetraploid lines. Polyploid lineages involving the same diploid parents differed in rDNA genotype, indicating that conditions immediately following genome doubling are crucial for rDNA changes. About 19% of the resynthesized allotetraploid individuals had equal rDNA contributions from the diploid parents, 74% were skewed towards either T. porrifolius or T. pratensis-type units, and only 7% had more rDNA copies of T. dubius-origin compared to the other two parents. Similar genotype frequencies were observed among natural populations. Despite directional reduction of units, the additivity of 35S rDNA locus number is maintained in 82% of the synthetic lines and in all natural allotetraploids.
Uniparental reductions of homeologous rRNA gene copies occurred in both synthetic and natural populations of Tragopogon allopolyploids. The extent of these rDNA changes was generally higher in natural populations than in the synthetic lines. We hypothesize that locus-specific and chromosomal changes in early generations of allopolyploids may influence patterns of rDNA evolution in later generations.
"An estimated 45–70% of angiosperm species are polyploid descendents of taxa with lower ploidy levels (Grant 1981, Masterson 1994), and the process of polyploidization can alter morphology, physiology, phenology, rates of genetic drift, ecological tolerances, and population dynamics (Stebbins 1971, Eagles and Othman 1978, Burdon and Marshall 1981, Grant 1981, Levin 1983, Warner and Edwards 1989, Moody et al. 1993, Dhawan and Lavania 1996, Segraves and Thompson 1999, Otto and Whitton 2000, Bretagnolle and Thompson 2001, Mu¨nzbergova´2007). Polyploids also often differ from diploids in mating systems, thereby altering patterns of gene flow within and among populations that differ in cytotype (Brochmann et al. 1993, Jacquemart and Thompson 1996, Cook and Soltis 1999, Lichtenzveig et al. 2000, Quarin et al. 2001). "
[Show abstract][Hide abstract] ABSTRACT: One of the major mechanisms of plant diversification has been the evolution of polyploid populations that differ from their diploid progenitors in morphology, physiology, and environmental tolerances. Recent studies have indicated that polyploidy may also have major effects on ecological interactions with herbivores and pollinators. We evaluated pollination of sympatric diploid and tetraploid plants of the rhizomatous herb Heuchera grossulariifolia (Saxifragaceae) along the Selway and Salmon Rivers of northern Idaho, USA, during four consecutive years. Previous molecular and ecological analyses had indicated that the tetraploid populations along these two river systems are independently derived and differ from each other in multiple traits. In each region, we evaluated floral visitation rate by all insect visitors, pollination efficacy of all major visitors, and relative contribution of all major pollinators to seed set. In both regions, diploid and tetraploid plants attracted different suites of floral visitors. Most pollination was attributable to several bee species and the moth Greya politella. Lasioglossum bees preferentially visited diploid plants at Lower Salmon but not at Upper Selway, queen Bombus centralis preferentially visited tetraploids at both sites, and worker B. centralis differed between sites in their cytotype preference. Hence, diploid and autotetraploid H. grossulariifolia plants act essentially as separate ecological species and may experience partial reproductive isolation through differential visitation and pollination by their major floral visitors. Overall the results, together with recent results from other studies, suggest that the repeated evolution of polyploidy in plants may contribute importantly to the structure and diversification of ecological interactions in terrestrial communities.