The genomic content of the subtelomeric repeated sequences Spelt1 and Spelt52 was studied by dot, Southern, and in situ hybridization in 11 newly synthesized amphiploids of Aegilops and Triticum, and data were compared with the parental plants. Spelt1 had reduced copy numbers in the first generation of three synthetic amphiploids, but two others did not change; Spelt52 was amplified in nine amphiploids and did not change in two. In the second allopolyploid generation, Spelt1 copy number did not change, whereas there was amplification of Spelt52 in some allopolyploids and decreases in others. Neither allopolyploidy level nor the direction of the cross affected the patterns of change in the newly synthesized amphiploids. Changes did not result from intergenomic recombination because similar alterations were noticed in allopolyploids with and without Ph1, a gene that suppresses homoeologous pairing. No differences in Spelt1 and Spelt52 tandem organization were found by Southern hybridization. The significance of these data are discussed in relation to the establishment of newly formed allopolyploids.
"These sequences were classified by us (Feldman et al. 1997) into the following four classes (Figure 1): (i) nonspecific sequences (mainly repetitious sequences) that are found in all or many of the wheat chromosomes; (ii) groupspecific sequences (mainly coding sequences) that occur in the chromosomes of one homeologous group, e.g., 1A, 1B, and 1D; (iii) genome-specific sequences that occur in several chromosomes of one genome, e.g., 1A, 2A, 3A, etc.; and (iv) chromosome-specific sequences (CSSs) that occur in only one homologous chromosome pair, e.g., 1A and 1A. Most of the CSSs are noncoding sequences (Feldman et al. 1997) and are present in all the diploid species of Aegilops and Triticum, but occur in only one pair of chromosomes of allopolyploid wheat, suggesting that they were lost during or after allopolyploidization (Feldman et al. 1997) A most surprising discovery is that allopolyploidization in the wheat group causes immediate nonrandom elimination of specific noncoding, low-copy, and high-copy DNA sequences (Feldman et al. 1997; Liu et al. 1998a,b; Ozkan et al. 2001; Shaked et al. 2001; Han et al. 2003, 2005; Salina et al. 2004). The extent of DNA elimination was estimated by determining the amounts of nuclear DNA in natural allopolyploids and in their diploid progenitors, as well as in newly synthesized allopolyploids and their parental plants (Ozkan et al. 2003; Eilam et al. 2008, 2010). "
[Show abstract][Hide abstract] ABSTRACT: The wheat group has evolved through allopolyploidization, namely, through hybridization among species from the plant genera Aegilops and Triticum followed by genome doubling. This speciation process has been associated with ecogeographical expansion and with domestication. In the past few decades, we have searched for explanations for this impressive success. Our studies attempted to probe the bases for the wide genetic variation characterizing these species, which accounts for their great adaptability and colonizing ability. Central to our work was the investigation of how allopolyploidization alters genome structure and expression. We found in wheat that allopolyploidy accelerated genome evolution in two ways: (1) it triggered rapid genome alterations through the instantaneous generation of a variety of cardinal genetic and epigenetic changes (which we termed "revolutionary" changes), and (2) it facilitated sporadic genomic changes throughout the species' evolution (i.e., evolutionary changes), which are not attainable at the diploid level. Our major findings in natural and synthetic allopolyploid wheat indicate that these alterations have led to the cytological and genetic diploidization of the allopolyploids. These genetic and epigenetic changes reflect the dynamic structural and functional plasticity of the allopolyploid wheat genome. The significance of this plasticity for the successful establishment of wheat allopolyploids, in nature and under domestication, is discussed.
"For instance, loss of ribosomal RNA genes (rDNA) was reported in several different allopolyploid species (Franzke and Mummenhoff 1999; Volkov et al. 1999; Lim et al. 2000; Fulneček et al. 2002; Matyasek et al. 2003; Skalická et al. 2003, 2005; Kovarik et al. 2004, 2005; Shcherban et al. 2008a, 2008b). In the wheat group it was found that elimination of various low-copy and high-copy DNA sequences occurs due to allopolyploidization and happens during or soon after this process (Feldman et al. 1997; Liu et al. 1998a, 1998b; Ozkan et al. 2001, 2002; Shaked et al. 2001; Kashkush et al. 2002, 2003; Levy and Feldman 2002, 2004; Ma et al. 2002, 2004; Han et al. 2003, 2005; Salina et al. 2004; Feldman and Levy 2005; Ma and Gustafson 2005, 2006). "
[Show abstract][Hide abstract] ABSTRACT: Two classes of 5S DNA units, namely the short (containing units of 410 bp) and the long (containing units of 500 bp), are recognized in species of the wheat (the genera Aegilops and Triticum) group. While every diploid species of this group contains 2 unit classes, the short and the long, every allopolyploid species contains a smaller number of unit classes than the sum of the unit classes of its parental species. The aim of this study was to determine whether the reduction in these unit classes is due to the process of allopolyploidization, that is, interspecific or intergeneric hybridization followed by chromosome doubling, and whether it occurs during or soon after the formation of the allopolyploids. To study this, the number and types of unit classes were determined in several newly formed allotetraploids, allohexaploids, and an allooctoploid of Aegilops and Triticum. It was found that elimination of unit classes of 5S DNA occurred soon (in the first 3 generations) after the formation of the allopolyploids. This elimination was reproducible, that is, the same unit classes were eliminated in natural and synthetic allopolyploids having the same genomic combinations. No further elimination occurred in the unit classes of the 5S DNA during the life of the allopolyploid. The genetic and evolutionary significance of this elimination as well as the difference in response to allopolyploidization of 5S DNA and rDNA are discussed.
"Studies of newly formed allopolyploids such as Brassica, wheat, and triticale have shown that allopolyploidization can lead to rapid and extensive genomic changes (Song et al. 1995; Feldman et al. 1997; Liu et al. 1998a, 1998b; Ozkan et al. 2001; Shaked et al. 2001; Kashkush et al. 2002, 2003; Ma and Gustafson 2008). In particular, newly formed allopolyploids of the wheat (Aegilops-Triticum) group have shown that allopolyploid formation is accompanied by extensive genomic changes at the molecular level, including rapid and nonrandom elimination of low-and high-copy coding and noncoding DNA sequences, as well as other types of genomic modifications (Ozkan et al. 2001; Shaked et al. 2001; Madlung et al. 2002; Ma and Gustafson 2004; Salina et al. 2004; Han et al. 2003, 2005). Recently, Gaeta et al. (2007) reported that exchanges among homoeologous chromosomes are a major mechanism creating novel allele combinations and phenotypic variation in newly formed Brassica napus allopolyploids. "
[Show abstract][Hide abstract] ABSTRACT: Recent studies in the genera Aegilops and Triticum showed that allopolyploid formation triggers rapid genetic and epigenetic changes that lead to cytological and genetic diploidization. To better understand the consequences of cytological diploidization, chromosome pairing and seed fertility were studied in S1, S2, and S3 generations of 18 newly formed allopolyploids at different ploidy levels. Results showed that bivalent pairing at first meiotic metaphase was enhanced and seed fertility was improved during each successive generation. A positive linear relationship was found between increased bivalent pairing, improved fertility, and elimination of low-copy noncoding DNA sequences. These findings support the conclusion that rapid elimination of low-copy noncoding DNA sequences from one genome of a newly formed allopolyploid, different sequences from different genomes, is an efficient way to quickly augment the divergence between homoeologous chromosomes and thus bring about cytological diploidization. This facilitates the rapid establishment of the raw allopolyploids as successful, competitive species in nature.
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