Allopolyploidy-Induced Rapid Genome Evolution in the Wheat (Aegilops-Triticum) Group

Abstract

To better understand genetic events that accompany allopolyploid formation, we studied the rate and time of elimination of eight DNA sequences in F1 hybrids and newly formed allopolyploids of Aegilops and TRITICUM: In total, 35 interspecific and intergeneric F1 hybrids and 22 derived allopolyploids were analyzed and compared with their direct parental plants. The studied sequences exist in all the diploid species of the Triticeae but occur in only one genome, either in one homologous pair (chromosome-specific sequences [CSSs]) or in several pairs of the same genome (genome-specific sequences [GSSs]), in the polyploid wheats. It was found that rapid elimination of CSSs and GSSs is a general phenomenon in newly synthesized allopolyploids. Elimination of GSSs was already initiated in F1 plants and was completed in the second or third allopolyploid generation, whereas elimination of CSSs started in the first allopolyploid generation and was completed in the second or third generation. Sequence elimination started earlier in allopolyploids whose genome constitution was analogous to natural polyploids compared with allopolyploids that do not occur in nature. Elimination is a nonrandom and reproducible event whose direction was determined by the genomic combination of the hybrid or the allopolyploid. It was not affected by the genotype of the parental plants, by their cytoplasm, or by the ploidy level, and it did not result from intergenomic recombination. Allopolyploidy-induced sequence elimination occurred in a sizable fraction of the genome and in sequences that were apparently noncoding. This finding suggests a role in augmenting the differentiation of homoeologous chromosomes at the polyploid level, thereby providing the physical basis for the diploid-like meiotic behavior of newly formed allopolyploids. In our view, this rapid genome adjustment may have contributed to the successful establishment of newly formed allopolyploids as new species.

Buffering of crucial functions by paleologous duplicated genes may contribute cyclicality to angiosperm genome duplication

“…Such genetic buffering may be especially important in survival of lineages experiencing the early ''genomic turmoil'' that immediately follows polyploid formation (20)(21)(22)(23)(24)(25). However, our data show that such buffering still markedly affects diversity among ecotypes of Arabidopsis thaliana and subspecies of Oryza sativa, species that each trace to genome duplications that occurred 60 million years or more before speciation (1,3).…”

“…Most higher organisms are thought to continuously produce aberrant unreduced gametes at low rates, but the rarity of genome duplication shows that the overwhelming majority do not survive. Study of both natural polyploids and synthetic polyploids formed by colchicine-based manipulation of interspecific hybrids reveals immediate consequences of polyploidization that ostensibly seem maladaptive, including loss and restructuring of low-copy DNA sequences (20)(21)(22)(23)(24)(25), activation of genes and retrotransposons (26,27), and gene silencing (28-31).…”

Transcripts of Vp - 1 homeologues are misspliced in modern wheat and ancestral species

“…Diploid species showed in each case a single Vp-1-related transcript, whereas the tetraploid T. turgidum showed multiple missplicing examples in addition to those shown by the diploid species; it therefore may be possible that increasing ploidy levels (and hence homeologue gene number) also may increase the level of missplicing of the Vp-1 genes. This hypothesis can be tested directly by analyzing missplicing events by using synthetic wheat (''AABBDD'') hybrids that have been constructed recently from T. turgidum and various ancestral diploids (31). The occurrence of multiple misspliced Vp-1 transcripts in the cytoplasm of embryos suggests that Vp-1 gene function may be compromised in modern wheat.…”

Three genomes differentially contribute to the biosynthesis of benzoxazinones in hexaploid wheat

“…Polyploidization is accompanied by changes in genome structure and gene expression, including the loss of gene sequences, silencing, and down-͞up-regulation of some homoeologs (16)(17)(18)(19)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39). We found that the loci of the five Bx biosynthetic genes are retained in all three genomes of hexaploid wheat (10) and isolated three homoeologous cDNAs each for TaBx1-TaBx5 genes in this study, showing that neither the loss nor the epigenetic silencing of the TaBx genes had occurred during the polyploidization.…”