水稻和其他禾本科植物基因组多倍体起源的证据

基因加倍(Gene duplication)被认为是进化的加速器。古老的基因组加倍事件已经在多个物种中被确定,包括酵母、脊椎动物以及拟南芥等。本研究发现水稻基因组同样存在全基因组加倍事件,大概发生在禾谷类作物分化之前,距今约7000万年。在水稻基因组中,共找到117个加倍区段(Duplicated block),分布在水稻的全部12条染色体,覆盖约60％的水稻基因组。在加倍区段,大约有20％的基因保留了加倍后的姊妹基因对(Duplicated pairs)。与此形成鲜明对照的是加倍区段的转录因子保留了60％的姊妹基因。禾本科植物全基因组加倍事件的确定对研究禾本科植物基因组的进化具有重要影响,暗示了多倍体化及随后的基因丢失、染色体重排等在禾谷类物种分化中扮演了重要角色。     

High rate of recent intron gain and loss in simultaneously duplicated Arabidopsis genes

We examined the gene structure of a set of 2563 Arabidopsis thaliana paralogous pairs that were duplicated simultaneously 20-60 MYA by tetraploidy. Out of a total of 23,164 introns in these genes, we found that 10,004 pairs have been conserved and 578 introns have been inserted or deleted in the time since the duplication event. This intron insertion/deletion rate of 2.7 x 10(-3) to 9.1 x 10(-4) per site per million years is high in comparison to previous studies. At least 56 introns were gained and 39 lost based on parsimony analysis of the phylogenetic distribution of these introns. We found weak evidence that genes undergoing intron gain and loss are biased with respect to gene ontology terms. Gene pairs that experienced at least 2 intron insertions or deletions show evidence of enrichment for membrane location and transport and transporter activity function. We do not find any relationship of intron flux to expression level or G + C content of the gene. Detection of a bias in the location of intron gains and losses within a gene depends on the method of measurement: an intragene method indicates that events (specifically intron losses) are biased toward the 3' end of the gene. Despite the relatively recent acquisition of these introns, we found only one case where we could identify the mechanism of intron origin--the TOUCH3 gene has experienced 2 tandem, partial, internal gene duplications that duplicated a preexisting intron and also created a novel, alternatively spliced intron that makes use of a duplicated pair of cryptic splice sites.     

In silico evidence for functional specialization after genome duplication in yeast

A fairly recent whole-genome duplication (WGD) event in yeast enables the effects of gene duplication and subsequent functional divergence to be characterized. We examined 15 ohnolog pairs (i.e. paralogs from a WGD) out of c . 500 Saccharomyces cerevisiae ohnolog pairs that have persisted over an estimated 100 million years of evolution. These 15 pairs were chosen for their high levels of asymmetry, i.e. within the pair, one ohnolog had evolved much faster than the other. Sequence comparisons of the 15 pairs revealed that the faster evolving duplicated genes typically appear to have experienced partially – but not fully – relaxed negative selection as evidenced by an average nonsynonymous/synonymous substitution ratio (d N /d S _avg=0.44) that is higher than the slow-evolving genes' ratio (d N /d S _avg=0.14) but still <1. Increased number of insertions and deletions in the fast-evolving genes also indicated loosened structural constraints. Sequence and structural comparisons indicated that a subset of these pairs had significant differences in their catalytically important residues and active or cofactor-binding sites. A literature survey revealed that several of the fast-evolving genes have gained a specialized function. Our results indicate that subfunctionalization and even neofunctionalization has occurred along with degenerative evolution, in which unneeded functions were destroyed by mutations.     

Rate asymmetry after genome duplication causes substantial long-branch attraction artifacts in the phylogeny of Saccharomyces species

Whole-genome duplication (WGD) produces sets of gene pairs that are all of the same age. We therefore expect that phylogenetic trees that relate these pairs to their orthologs in other species should show a single consistent topology. However, a previous study of gene pairs formed by WGD in the yeast Saccharomyces cerevisiae found conflicting topologies among neighbor-joining (NJ) trees drawn from different loci and suggested that this conflict was the result of "asynchronous functional divergence" of duplicated genes (Langkjaer, R. B., P. F. Cliften, M. Johnston, and J. Piskur. 2003. Yeast genome duplication was followed by asynchronous differentiation of duplicated genes. Nature 421:848-852). Here, we test whether the conflicting topologies might instead be due to asymmetrical rates of evolution leading to long-branch attraction (LBA) artifacts in phylogenetic trees. We constructed trees for 433 pairs of WGD paralogs in S. cerevisiae with their single orthologs in Saccharomyces kluyveri and Candida albicans. We find a strong correlation between the asymmetry of evolutionary rates of a pair of S. cerevisiae paralogs and the topology of the tree inferred for that pair. Saccharomyces cerevisiae gene pairs with approximately equal rates of evolution tend to give phylogenies in which the WGD postdates the speciation between S. cerevisiae and S. kluyveri (B-trees), whereas trees drawn from gene pairs with asymmetrical rates tend to show WGD pre-dating this speciation (A-trees). Gene order data from throughout the genome indicate that the "A-trees" are artifacts, even though more than 50% of gene pairs are inferred to have this topology when the NJ method as implemented in ClustalW (i.e., with Poisson correction of distances) is used to construct the trees. This LBA artifact can be ameliorated, but not eliminated, by using gamma-corrected distances or by using maximum likelihood trees with robustness estimated by the Shimodaira-Hasegawa test. Tests for adaptive evolution indicated that positive selection might be the cause of rate asymmetry in a substantial fraction (19%) of the paralog pairs.     

Functional Divergence in a Multi-gene Family Is a Key Evolutionary Innovation for Anaerobic Growth in Saccharomyces cerevisiae

The amplification and diversification of genes into large multi-gene families often mark key evolutionary innovations, but this process often creates genetic redundancy that hinders functional investigations. When the model budding yeast Saccharomyces cerevisiae transitions to anaerobic growth conditions, the cell massively induces the expression of seven serine/threonine-rich anaerobically-induced cell wall mannoproteins (anCWMPs): TIP1, TIR1, TIR2, TIR3, TIR4, DAN1, and DAN4. Here, we show that these genes likely derive evolutionarily from a single ancestral anCWMP locus, which was duplicated and translocated to new genomic contexts several times both prior to and following the budding yeast whole genome duplication (WGD) event. Based on synteny and their phylogeny, we separate the anCWMPs into four gene subfamilies. To resolve prior inconclusive genetic investigations of these genes, we constructed a set of combinatorial deletion mutants to determine their contributions toward anaerobic growth in S. cerevisiae. We found that two genes, TIR1 and TIR3, were together necessary and sufficient for the anCWMP contribution to anaerobic growth. Overexpressing either gene alone was insufficient for anaerobic growth, implying that they encode non-overlapping functional roles in the cell during anaerobic growth. We infer from the phylogeny of the anCWMP genes that these two important genes derive from an ancient duplication that predates the WGD event, whereas the TIR1 subfamily experienced gene family amplification after the WGD event. Taken together, the genetic and molecular evidence suggests that one key anCWMP gene duplication event, several auxiliary gene duplication events, and functional divergence underpin the evolution of anaerobic growth in budding yeasts.     

Chromosomal-level genome assembly of the orchid tree Bauhinia variegata (Leguminosae; Cercidoideae) supports the allotetraploid origin hypothesis of Bauhinia

Cercidoideae, one of the six subfamilies of Leguminosae, contains one genus Cercis with its chromosome number 2n = 14 and all other genera with 2n = 28. An allotetraploid origin hypothesis for the common ancestor of non-Cercis genera in this subfamily has been proposed; however, no chromosome-level genomes from Cercidoideae have been available to test this hypothesis. Here, we conducted a chromosome-level genome assembly of Bauhinia variegata to test this hypothesis. The assembled genome is 326.4 Mb with the scaffold N50 of 22.1 Mb and contains 37,996 protein-coding genes. The Ks distribution between gene pairs in the syntenic regions indicates two whole-genome duplications (WGDs): one is B. variegata-specific, and the other is shared among core eudicots. Although Ks between gene pairs generated by the recent WGD in Bauhinia is greater than that between Bauhinia and Cercis, the WGD was not detected in Cercis, which can be explained by an accelerated evolutionary rate in Bauhinia after divergence from Cercis. Ks distribution and phylogenetic analysis for gene pairs generated by the recent WGD in Bauhinia and their corresponding orthologs in Cercis support the allopolyploidy origin hypothesis of Bauhinia. The genome of B. variegata also provides a genomic resource for dissecting genetic basis of its ornamental traits.     

Whole genome duplication of intra- and inter-chromosomes in the tomato genome

Whole genome duplication(WGD)events have been proven to occur in the evolutionary history of most angiosperms.Tomato is considered a model species of the Solanaceae family.In this study,we describe the details of the evolutionary process of the tomato genome by detecting collinearity blocks and dating the WGD events on the tree of life by combining two different methods:synonymous substitution rates(Ks)and phylogenetic trees.In total,593 collinearity blocks were discovered out of 12 pseudo-chromosomes constructed. It was evident that chromosome 2 had experienced an intra-chromosomal duplication event.Major inter-chromosomal duplication occurred among all the pseudo-chromosome.We calculated the Ks value of these collinearity blocks.Two peaks of Ks distribution were found,corresponding to two WGD events occurring approximately 36-82 million years ago(MYA)and 148-205 MYA.Additionally, the results of phylogenetic trees suggested that the more recent WGD event may have occurred after the divergence of the rosidasterid clade,but before the major diversification in Solanaceae.The older WGD event was shown to have occurred before the divergence of the rosid-asterid clade and after the divergence of rice-Arabidopsis(monocot-dicot).     

The rates and patterns of deletions in the human factor IX gene.

Deletions are commonly observed in genes with either segments of highly homologous sequences or excessive gene length. However, in the factor IX gene and in most genes, deletions (of > or = 21 bp) are uncommon. We have analyzed DNA from 290 families with hemophilia B (203 independent mutations) and have found 12 deletions > 20 bp. Eleven of these are > 2 kb (range > 3-163 kb), and one is 1.1 kb. The junctions of the four deletions that are completely contained within the factor IX gene have been determined. A novel mutation occurred in patient HB128: the data suggest that a 26.8-kb deletion occurred between two segments of alternating purines and pyrimidines and that a 2.3-kb sense strand segment derived from the deleted region was inserted. For our sample of 203 independent mutations, we estimate the "baseline" rates of deletional mutation per base pair per generation as a function of size. The rate for large (> 2 kb) deletions is exceedingly low. For every mutational event in which a given base is at the junction of a large deletion, there are an estimated 58 microdeletions (< 20 bp) and 985 single-base substitutions at that base. Analysis of the nine reported deletion junctions in the factor IX gene literature reveals that (i) five are associated with inversions, orphan sequences, or sense strand insertions; (ii) four are simple deletions that display an excess of short direct repeats at their junctions; (iii) there is no dramatic clustering of junctions within the gene; and (iv) with the exception of alternating purines and pyrimidines, deletion junctions are not preferentially associated with repetitive DNA.     

High‐resolution array‐CGH in patients with oculocutaneous albinism identifies new deletions of the TYR,OCA2, and SLC45A2 genes and a complex rearrangement of the OCA2 gene

Oculocutaneous albinism (OCA) is caused by mutations in six different genes, and their molecular diagnosis encompasses the search for point mutations and intragenic rearrangements. Here, we used high‐resolution array‐comparative genome hybridization (CGH) to search for rearrangements across exons, introns and regulatory sequences of four OCA genes: TYR, OCA2, TYRP1, and SLC45A2. We identified a total of ten new deletions in TYR, OCA2, and SLC45A2. A complex rearrangement of OCA2 was found in two unrelated patients. Whole‐genome sequencing showed deletion of a 184‐kb fragment (identical to a deletion previously found in Polish patients), whereby a large portion of the deleted sequence was re‐inserted after severe reshuffling into intron 1 of OCA2. The high‐resolution array‐CGH presented here is a powerful tool to detect gene rearrangements. Finally, we review all known deletions of the OCA1–4 genes reported so far in the literature and show that deletions or duplications account for 5.6% of all mutations identified in the OCA1–4 genes.     

Consistent patterns of rate asymmetry and gene loss indicate widespread neofunctionalization of yeast genes after whole-genome duplication

We investigated patterns of rate asymmetry in sequence evolution among the gene pairs (ohnologs) formed by whole-genome duplication (WGD) in yeast species. By comparing three species (Saccharomyces cerevisiae, Candida glabrata, and S. castellii) that underwent WGD to a nonduplicated outgroup (Kluyveromyces lactis), and by using a synteny framework to establish orthology and paralogy relationships at each duplicated locus, we show that 56% of ohnolog pairs show significantly asymmetric protein sequence evolution. For ohnolog pairs that remain duplicated in two species there is a strong tendency for the faster-evolving copy in one species to be orthologous to the faster copy in the other species, which indicates that the evolutionary rate differences were established before speciation and hence soon after the WGD. We also present evidence that in cases where one ohnolog has been lost from the genome of a post-WGD species, the lost copy was likely to have been the faster-evolving member of the pair prior to its loss. These results suggest that a significant fraction of the retained ohnologs in yeast species underwent neofunctionalization soon after duplication.     

Genome-Wide Reconstruction of Rediploidization Following Autopolyploidization across One Hundred Million Years of Salmonid Evolution

The long-term evolutionary impacts of whole-genome duplication (WGD) are strongly influenced by the ensuing rediploidization process. Following autopolyploidization, rediploidization involves a transition from tetraploid to diploid meiotic pairing, allowing duplicated genes (ohnologs) to diverge genetically and functionally. Our understanding of autopolyploid rediploidization has been informed by a WGD event ancestral to salmonid fishes, where large genomic regions are characterized by temporally delayed rediploidization, allowing lineage-specific ohnolog sequence divergence in the major salmonid clades. Here, we investigate the long-term outcomes of autopolyploid rediploidization at genome-wide resolution, exploiting a recent “explosion” of salmonid genome assemblies, including a new genome sequence for the huchen (Hucho hucho). We developed a genome alignment approach to capture duplicated regions across multiple species, allowing us to create 121,864 phylogenetic trees describing genome-wide ohnolog divergence across salmonid evolution. Using molecular clock analysis, we show that 61% of the ancestral salmonid genome experienced an initial “wave” of rediploidization in the late Cretaceous (85–106 Ma). This was followed by a period of relative genomic stasis lasting 17–39 My, where much of the genome remained tetraploid. A second rediploidization wave began in the early Eocene and proceeded alongside species diversification, generating predictable patterns of lineage-specific ohnolog divergence, scaling in complexity with the number of speciation events. Using gene set enrichment, gene expression, and codon-based selection analyses, we provide insights into potential functional outcomes of delayed rediploidization. This study enhances our understanding of delayed autopolyploid rediploidization and has broad implications for future studies of WGD events.     

PCR-mediated one-step deletion of targeted chromosomal regions in haploid <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis>

Chromosome rearrangements, especially chromosomal deletions, have been exploited as important resources for functional analysis of genomes. To facilitate this analysis, we applied a previously developed method for chromosome splitting for the direct deletion of a designed internal or terminal chromosomal region carrying many nonessential genes in haploid Saccharomyces cerevisiae. The method, polymerase chain reaction (PCR)-mediated chromosomal deletion (PCD), consists of a two-step PCR and one transformation per deletion event. In this paper, we show that the PCD method efficiently deletes internal regions in a single transformation. Of the six chromosomal regions targeted for deletion by this method, five regions (16 to 38 kb in length) containing 10 to 19 nonessential genes were successfully eliminated at high efficiency. The one targeted region on chromosome XIII that was not deleted was subsequently found to contain sequences essential for yeast growth. While 14 individual genes in this region have been reported to be nonessential, synthetic lethal interactions may occur among these nonessential genes. Phenotypic analysis showed that four deletion strains still exhibited normal growth while possible synthetic growth defects were observed in another strain harboring a 19-gene deletion on chromosome XV. These results demonstrate that the PCD method is a useful tool for deleting genes and for analyzing their functions in defined chromosomal regions.     

Random genome deletion methods applicable to prokaryotes

Through their enabling of simultaneous identification of multiple non-essential genes in a genome, large-segment genome deletion methods are an increasingly popular approach to minimize and tailor microbial genomes for specific functions. At present, difficulties in identifying target regions for deletion are a result of inadequate knowledge to define gene essentiality. Furthermore, with the majority of predicted open reading frames of completely sequenced genomes still annotated as putative genes, essential or important genes are found scattered throughout the genomes, limiting the size of non-essential segments that can be safely deleted in a single sweep. Recently described large-segment random genome deletion methods that utilize transposons enable the generation of random deletion strains, analysis of which makes identification of non-essential genes less tedious. Such and other efforts to determine the minimum genome content necessary for cell survival continue to accumulate important information that should help improve our understanding of genome function and evolution. This review presents an assessment of technological advancements of random genome deletion methods in prokaryotes to date.     

Protein complexity, gene duplicability and gene dispensability in the yeast genome

Using functional genomic and protein structural data we studied the effects of protein complexity (here defined as the number of subunit types in a protein) on gene dispensability and gene duplicability. We found that in terms of gene duplicability the major distinction in protein complexity is between hetero-complexes, each of which includes at least two different types of subunits (polypeptides), and homo-complexes, which include monomers and complexes that consist of only subunits of one polypeptide type. However, gene dispensability decreases only gradually as the number of subunit types in a protein complex increases. These observations suggest that the dosage balance hypothesis can explain well gene duplicability of complex proteins, but cannot completely explain the difference in dispensabilities between hetero-complex subunits. It is likely that knocking out a gene coding for a hetero-complex subunit would disrupt the function of the whole complex, so that the deletion effect on fitness would increase with protein complexity. We also found that multi-domain polypeptide genes are less dispensable but more duplicable than single-domain polypeptide genes. Duplicate genes derived from the whole genome duplication event in yeast are more dispensable (except for ribosomal protein genes) than other duplicate genes. Further, we found that subunits of the same protein complex tend to have similar expression levels and similar effects of gene deletion on fitness. Finally, we estimated that in yeast the contribution of duplicate genes to genetic robustness against null mutation is approximately 9%, smaller than previously estimated. In yeast, protein complexity may serve as a better indicator of gene dispensability than do duplicate genes.     

Chromosomal stasis versus plasmid plasticity in aphid endosymbiont Buchnera aphidicola

The study of three genomes of the aphid endosymbiont Buchnera aphidicola has revealed an extraordinary stasis: conservation of gene order and genetic composition of the chromosome, while the chromosome size and number of genes has reduced. The reduction in genome size appears to be ongoing since some lineages we now know to have even smaller chromosomes than the first B. aphidicola analysed. The current sequencing by our group of one of these smaller genomes with an estimated size of 450 kb, and its comparison with the other three available genomes provide insights into the nature of processes involved in shrinkage. We discuss whether B. aphidicola might be driven to extinction and be replaced by secondary aphid endosymbionts. In some lineages, genes encoding key enzymes in the pathways leading to tryptophan and leucine biosynthesis (trpEG and leuABCD, respectively) are located on plasmids, rather than the main chromosome. In contrast to the stasis of the main chromosome, plasmid genes have frequently been transferred to the main chromosome and undergone other gene rearrangements. We propose a two-step scenario to explain these contrasting modes of evolution. Essential genes may have escaped regulation by moving to plasmids in a moving B. aphidicola ancestor. B. aphidicola became polyploidy at a given stage of its evolution and plasmid genes have been transferred to the main chromosome through several independent events.