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The predictability of evolution is debatable, with recent evidence suggesting that outcomes may be constrained by gene interaction networks [1]. Whole-genome duplication (WGD; polyploidization-ubiquitous in plant evolution [2]) provides the opportunity to evaluate the predictability of genome reduction, a pervasive feature of evolution [3, 4]. Repeated patterns of genome reduction appear to have occurred via duplicated gene (homeolog) loss in divergent species following ancient WGD [5-9], with evidence for preferential retention of duplicates in certain gene classes [8-10]. The speed at which these patterns arise is unknown. We examined presence/absence of 70 homeologous loci in 59 Tragopogon miscellus plants from five natural populations of independent origin; this allotetraploid arose ~80 years ago via hybridization between diploid parents and WGD [11]. Genes were repeatedly retained or lost in clusters, and the gene ontology categories of the missing genes correspond to those lost after ancient WGD in the same family (Asteraceae; sunflower family) [6] and with gene dosage sensitivity [8]. These results provide evidence that the outcomes of WGD are predictable, even in 40 generations, perhaps due to the connectivity of gene products [8, 10, 12]. The high frequency of single-allele losses detected and low frequency of changes fixed within populations provide evidence for ongoing evolution., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Premise of the Study: Hybridization and polyploidization (allopolyploidy) are ubiquitous in the evolution of plants, but tracing the origins and subsequent evolution of the constituent genomes of allopolyploids has been challenging. Genome doubling greatly complicates genetic analyses, and this has long hindered investigation in that most allopolyploid species are "nonmodel" organisms. However, recent advances in sequencing and genomics technologies now provide unprecedented opportunities to analyze numerous genetic markers in multiple individuals in any organism., Methods: Here we review the application of next-generation sequencing technologies to the study of three aspects of allopolyploid genome evolution: duplicated gene loss and expression in two recently formed Tragopogon allopolyploids, intergenomic interactions and chromosomal evolution in Tragopogon miscellus, and repetitive DNA evolution in Nicotiana allopolyploids., Key Results: For the first time, we can explore on a genomic scale the evolutionary processes that are ongoing in natural allopolyploids and not be restricted to well-studied crops and genetic models., Conclusions: These approaches can be easily and inexpensively applied to many other plant species-making any evolutionarily provocative system a new "model" system.

Background: Recent phylogenetic analyses have identified Amborella trichopoda, an understory tree species endemic to the forests of New Caledonia, as sister to a clade including all other known flowering plant species. The Amborella genome is a unique reference for understanding the evolution of angiosperm genomes because it can serve as an outgroup to root comparative analyses. A physical map, BAC end sequences and sample shotgun sequences provide a first view of the 870 Mbp Amborella genome., Results: Analysis of Amborella BAC ends sequenced from each contig suggests that the density of long terminal repeat retrotransposons is negatively correlated with that of protein coding genes. Syntenic, presumably ancestral, gene blocks were identified in comparisons of the Amborella BAC contigs and the sequenced Arabidopsis thaliana, Populus trichocarpa, Vitis vinifera and Oryza sativa genomes. Parsimony mapping of the loss of synteny corroborates previous analyses suggesting that the rate of structural change has been more rapid on lineages leading to Arabidopsis and Oryza compared with lineages leading to Populus and Vitis. The gamma paleohexiploidy event identified in the Arabidopsis, Populus and Vitis genomes is shown to have occurred after the divergence of all other known angiosperms from the lineage leading to Amborella., Conclusions: When placed in the context of a physical map, BAC end sequences representing just 5.4% of the Amborella genome have facilitated reconstruction of gene blocks that existed in the last common ancestor of all flowering plants. The Amborella genome is an invaluable reference for inferences concerning the ancestral angiosperm and subsequent genome evolution.

Tragopogon miscellus (Asteraceae) is an evolutionary model for the study of natural allopolyploidy, but until now has been under-resourced as a genetic model. Using 454 and Illumina expressed sequence tag sequencing of the parental diploid species of T. miscellus, we identified 7782 single nucleotide polymorphisms that differ between the two progenitor genomes present in this allotetraploid. Validation of a sample of 98 of these SNPs in genomic DNA using Sequenom MassARRAY iPlex genotyping confirmed 92 SNP markers at the genomic level that were diagnostic for the two parental genomes. In a transcriptome profile of 2989 SNPs in a single T. miscellus leaf, using Illumina sequencing, 69% of SNPs showed approximately equal expression of both homeologs (duplicate homologous genes derived from different parents), 22% showed apparent differential expression and 8.5% showed apparent silencing of one homeolog in T. miscellus. The majority of cases of homeolog silencing involved the T. dubius SNP homeolog (164/254; 65%) rather than the T. pratensis homeolog (90/254). Sequenom analysis of genomic DNA showed that in a sample of 27 of the homeologs showing apparent silencing, 23 (85%) were because of genomic homeolog loss. These methods could be applied to any organism, allowing efficient and cost-effective generation of genetic markers.