利用叶绿体基因探讨稻属BBCC基因组物种的系统起源

基于9个叶绿体基因片段(atpA、atpB、matK、petA、psaA、psbA、psbB、psbC和rbcL), 深入探讨了稻属(Oryza)3个BBCC基因组异源四倍体和5个与之相关的BB或CC基因组二倍体物种间的谱系关系。进一步的系统发育分析表明: 3个具有相同BBCC基因组的四倍体物种并非同一次物种形成事件的产物, 而是在不同的分布区经历了至少3次分别的物种起源。其中, 四倍体Oryza punctata的母本可能来自同样分布在非洲并具有CC基因组的二倍体物种O. eichingeri; 而四倍体O. malampuzhaensis和O. minuta的母本则可能来自亚洲已经灭绝的具有BB基因组的不同二倍体。研究结果不但为追溯稻属异源四倍体的复杂网状进化提供了重要的分子证据, 而且拓展了我们对有花植物复杂物种形成的深入理解。     

Global Patterns of Subgenome Evolution in Organelle-Targeted Genes of Six Allotetraploid Angiosperms

Whole-genome duplications (WGDs) are a prominent process of diversification in eukaryotes. The genetic and evolutionary forces that WGD imposes on cytoplasmic genomes are not well understood, despite the central role that cytonuclear interactions play in eukaryotic function and fitness. Cellular respiration and photosynthesis depend on successful interaction between the 3,000+ nuclear-encoded proteins destined for the mitochondria or plastids and the gene products of cytoplasmic genomes in multi-subunit complexes such as OXPHOS, organellar ribosomes, Photosystems I and II, and Rubisco. Allopolyploids are thus faced with the critical task of coordinating interactions between the nuclear and cytoplasmic genes that were inherited from different species. Because the cytoplasmic genomes share a more recent history of common descent with the maternal nuclear subgenome than the paternal subgenome, evolutionary “mismatches” between the paternal subgenome and the cytoplasmic genomes in allopolyploids might lead to the accelerated rates of evolution in the paternal homoeologs of allopolyploids, either through relaxed purifying selection or strong directional selection to rectify these mismatches. We report evidence from six independently formed allotetraploids that the subgenomes exhibit unequal rates of protein-sequence evolution, but we found no evidence that cytonuclear incompatibilities result in altered evolutionary trajectories of the paternal homoeologs of organelle-targeted genes. The analyses of gene content revealed mixed evidence for whether the organelle-targeted genes are lost more rapidly than the non-organelle-targeted genes. Together, these global analyses provide insights into the complex evolutionary dynamics of allopolyploids, showing that the allopolyploid subgenomes have separate evolutionary trajectories despite sharing the same nucleus, generation time, and ecological context.     

利用多基因序列探讨稻属药稻复合体二倍体物种的系统发育关系

稻属药稻复合体Oryza officinalis complex中有5个二倍体物种,涉及B、C和E3个染色体组,分布在亚洲、非洲和大洋洲。其中,O.australiensis是稻属中惟一含E染色体组的物种;O.punctata则是惟一含B染色体组的物种。虽然O.officinalis、O.rhizomatis和O.eichingeri都含C染色体组,但由于它们间断分布在亚洲和非洲,可能在稻属异源多倍体成种中发挥了不同作用。染色体组B、C和E之间,以及上述二倍体物种之间的系统发育关系及其在稻属多倍体物种形成     

Homoeolog Expression Is Modulated Differently by Different Subgenomes in <Emphasis Type="Italic">Brassica napus</Emphasis> Hybrids and Allotetraploids

Synthetic and natural allotetraploid Brassica napus (2n?=?38, AACC) have been widely used as a model to study the genetic changes associated with allopolyploidization; however, there has been little research on the homoeolog expression patterns and the roles of cis and trans regulation. Herein, homoeolog expression patterns were assessed by using RNA-seq for two interspecific hybrids (A_nC_o with the extracted A subgenome from natural B. napus, and A_rC_o with the A subgenome from extant B. rapa), synthetic and natural allopolyploids (C_oC_oA_rA_r and A_nA_nC_nC_n), and the diploid parents. The ranges of homoeolog expression bias decreased after hybridization, whereas the extents of homoeolog expression bias and non-conserved expression, especially transgressive expression, increased over evolutionary time. Despite sharing the same C subgenome parent, these two hybrids showed different homolog expression patterns in many respects. In A_nC_o, the trans-regulatory factors from C_o subgenome tended to cause downregulation of A_n subgenome homoeologs, but trans-regulatory factors from the A_n subgenome acted as both activators and repressors, and such asymmetric effects of trans-regulatory factors might explain why the homoeolog expression was biased toward the C subgenome after genome merger. No significant asymmetric effects of trans-regulatory factors were found in A_rC_o, which was consistent with the overall balanced expression of homoeologs. These results suggested that A subgenomes with different regulatory systems might act differently in modulating homoeolog expression after merger with the C subgenome, resulting in either balanced or unbalanced homoeolog expression biases.     

Pericentromeric effects shape the patterns of divergence, retention, and expression of duplicated genes in the paleopolyploid soybean

The evolutionary forces that govern the divergence and retention of duplicated genes in polyploids are poorly understood. In this study, we first investigated the rates of nonsynonymous substitution (Ka) and the rates of synonymous substitution (Ks) for a nearly complete set of genes in the paleopolyploid soybean (Glycine max) by comparing the orthologs between soybean and its progenitor species Glycine soja and then compared the patterns of gene divergence and expression between pericentromeric regions and chromosomal arms in different gene categories. Our results reveal strong associations between duplication status and Ka and gene expression levels and overall low Ks and low levels of gene expression in pericentromeric regions. It is theorized that deleterious mutations can easily accumulate in recombination-suppressed regions, because of Hill-Robertson effects. Intriguingly, the genes in pericentromeric regions-the cold spots for meiotic recombination in soybean-showed significantly lower Ka and higher levels of expression than their homoeologs in chromosomal arms. This asymmetric evolution of two members of individual whole genome duplication (WGD)-derived gene pairs, echoing the biased accumulation of singletons in pericentromeric regions, suggests that distinct genomic features between the two distinct chromatin types are important determinants shaping the patterns of divergence and retention of WGD-derived genes.     

Genetic diversity patterns in Curcuma reflect differences in genome size

The relationships between genome size and the systematic and evolutionary patterns in vascular plants are equivocal, although a close relationship between genome size and evolutionary patterns has been previously reported. However, several studies have also revealed the dynamic nature of genome size evolution and its considerable ‘ups’ and ‘downs’. Thus, in this study, the phylogenetic relationships among three previously revealed genome size groups and among species of the highly polyploid genus Curcuma were evaluated using AFLP. Our results suggest two main lineages within Indian Curcuma reflecting evolution of genome size. The first one includes hexaploids and higher polyploids of the previously recognized genome size group I, and the second one includes mainly hexaploids of genome size groups II and III. Within genome size group I, relationships among species seem to be influenced by reticulate evolution and higher polyploids are likely to be of allopolyploid origin. Reproductive systems in Indian Curcuma vary considerably among ploidy levels and these differences considerably affect morphological and genetic variation. In general, clonally reproducing species are expected to exhibit low genotypic diversity, but, at the same time, species of allopolyploid origin are expected to maintain higher levels of heterozygosity compared with their progenitors. We investigated intra‐populational genetic variability in Curcuma spp. to evaluate whether mode of reproduction or ploidy represent the main factor influencing the degree of genetic diversity. We found that hexaploid species exhibited significantly higher genetic diversity than higher polyploids (9x, 15x). Our results suggest that this genetic diversity pattern is largely influenced by the mode of reproduction, as higher polyploids reproduce exclusively vegetatively, whereas hexaploids reproduce mainly sexually. © 2011 The Linnean Society of London, Botanical Journal of the Linnean Society, 2011, 165 , 388–401.     

Deciphering the origins of apomictic polyploids in the Cheilanthes yavapensis complex (Pteridaceae)

Deciphering species relationships and hybrid origins in polyploid agamic species complexes is notoriously difficult. In this study of cheilanthoid ferns, we demonstrate increased resolving power for clarifying the origins of polyploid lineages by integrating evidence from a diverse selection of biosystematic methods. The prevalence of polyploidy, hybridization, and apomixis in ferns suggests that these processes play a significant role in their evolution and diversification. Using a combination of systematic approaches, we investigated the origins of apomictic polyploids belonging to the Cheilanthes yavapensis complex. Spore studies allowed us to assess ploidy levels; plastid and nuclear DNA sequencing revealed evolutionary relationships and confirmed the putative progenitors (both maternal and paternal) of taxa of hybrid origin; enzyme electrophoretic evidence provided information on genome dosage in allopolyploids. We find here that the widespread apomictic triploid, Cheilanthes lindheimeri, is an autopolyploid derived from a rare, previously undetected sexual diploid. The apomictic triploid Cheilanthes wootonii is shown to be an interspecific hybrid between C. fendleri and C. lindheimeri, whereas the apomictic tetraploid C. yavapensis is comprised of two cryptic and geographically distinct lineages. We show that earlier morphology-based hypotheses of species relationships, while not altogether incorrect, only partially explain the complicated evolutionary history of these ferns.     

Genome physical mapping of polyploids: a BIBAC physical map of cultivated tetraploid cotton, Gossypium hirsutum L

Polyploids account for approximately 70% of flowering plants, including many field, horticulture and forage crops. Cottons are a world-leading fiber and important oilseed crop, and a model species for study of plant polyploidization, cellulose biosynthesis and cell wall biogenesis. This study has addressed the concerns of physical mapping of polyploids with BACs and/or BIBACs by constructing a physical map of the tetraploid cotton, Gossypium hirsutum L. The physical map consists of 3,450 BIBAC contigs with an N50 contig size of 863 kb, collectively spanning 2,244 Mb. We sorted the map contigs according to their origin of subgenome, showing that we assembled physical maps for the A- and D-subgenomes of the tetraploid cotton, separately. We also identified the BIBACs in the map minimal tilling path, which consists of 15,277 clones. Moreover, we have marked the physical map with nearly 10,000 BIBAC ends (BESs), making one BES in approximately 250 kb. This physical map provides a line of evidence and a strategy for physical mapping of polyploids, and a platform for advanced research of the tetraploid cotton genome, particularly fine mapping and cloning the cotton agronomic genes and QTLs, and sequencing and assembling the cotton genome using the modern next-generation sequencing technology.     

Comparative physical mapping between Oryza sativa (AA genome type) and O. punctata (BB genome type)

A comparative physical map of the AA genome (Oryza sativa) and the BB genome (O. punctata) was constructed by aligning a physical map of O. punctata, deduced from 63,942 BAC end sequences (BESs) and 34,224 fingerprints, onto the O. sativa genome sequence. The level of conservation of each chromosome between the two species was determined by calculating a ratio of BES alignments. The alignment result suggests more divergence of intergenic and repeat regions in comparison to gene-rich regions. Further, this characteristic enabled localization of heterochromatic and euchromatic regions for each chromosome of both species. The alignment identified 16 locations containing expansions, contractions, inversions, and transpositions. By aligning 40% of the punctata BES on the map, 87% of the punctata FPC map covered 98% of the O. sativa genome sequence. The genome size of O. punctata was estimated to be 8% larger than that of O. sativa with individual chromosome differences of 1.5-16.5%. The sum of expansions and contractions observed in regions >500 kb were similar, suggesting that most of the contractions/expansions contributing to the genome size difference between the two species are small, thus preserving the macro-collinearity between these species, which diverged approximately 2 million years ago.     

Sequencing of Camelina neglecta,a diploid progenitor of the hexaploid oilseed Camelina sativa

Camelina neglecta is a diploid species from the genus Camelina, which includes the versatile oilseed Camelina sativa. These species are closely related to Arabidopsis thaliana and the economically important Brassica crop species, making this genus a useful platform to dissect traits of agronomic importance while providing a tool to study the evolution of polyploids. A highly contiguous chromosome-level genome sequence of C. neglecta with an N50 size of 29.1 Mb was generated utilizing Pacific Biosciences (PacBio, Menlo Park, CA) long-read sequencing followed by chromosome conformation phasing. Comparison of the genome with that of C. sativa shows remarkable coincidence with subgenome 1 of the hexaploid, with only one major chromosomal rearrangement separating the two. Synonymous substitution rate analysis of the predicted 34 061 genes suggested subgenome 1 of C. sativa directly descended from C. neglecta around 1.2 mya. Higher functional divergence of genes in the hexaploid as evidenced by the greater number of unique orthogroups, and differential composition of resistant gene analogs, might suggest an immediate adaptation strategy after genome merger. The absence of genome bias in gene fractionation among the subgenomes of C. sativa in comparison with C. neglecta, and the complete lack of fractionation of meiosis-specific genes attests to the neopolyploid status of C. sativa. The assembled genome will provide a tool to further study genome evolution processes in the Camelina genus and potentially allow for the identification and exploitation of novel variation for Camelina crop improvement.     

Polyploid and hybrid evolution in roses east of the Rocky Mountains

This study investigates the impact of hybridization and polyploidy in the evolution of eastern North American roses. We explore these processes in the Rosa carolina complex (section Cinnamomeae), which consists of five diploid and three tetraploid species. To clarify the status and origins of polyploids, a haplotype network (statistical parsimony) of the glyceraldehyde 3-phosphate dehydrogenase (GAPDH) nuclear gene was estimated for polyploids of the complex and for diploids of section Cinnamomeae in North America. A genealogical approach helped to decipher the evolutionary history of polyploids from noise created by hybridization, incomplete lineage sorting, and allelic segregation. At the diploid level, species west of the Rocky Mountains are distinct from eastern species. In the east, two groups of diploids were found: one consists of R. blanda and R. woodsii and the other of R. foliolosa, R. nitida, and R. palustris. Only eastern diploids are involved in the origins of the polyploids. Rosa arkansana is derived from the blanda-woodsii group, R. virginiana originated from the foliolosa-nitida-palustris group, and R. carolina is derived from a hybrid between the two diploid groups. The distinct origins of these polyploid taxa support the hypothesis that the three polyploids are separate species.     

The evolution pattern of rDNA ITS in Avena and phylogenetic relationship of the Avena species (Poaceae: Aveneae)

Ribosomal ITS sequences are commonly used for phylogenetic reconstruction because they are included in rDNA repeats, and these repeats often undergo rapid concerted evolution within and between arrays. Therefore, the rDNA ITS copies appear to be virtually identical and can sometimes be treated as a single gene. In this paper we examined ITS polymorphism within and among 13 diploid (A and C genomes), seven tetraploid (AB, AC and CC genomes) and four hexaploid (ACD genome) to infer the extent and direction of concerted evolution, and to reveal the phylogenetic and genome relationship among species of Avena. A total of 170 clones of the ITS1-5.8S-ITS2 fragment were sequenced to carry out haplotype and phylogenetic analysis. In addition, 111 Avena ITS sequences retrieved from GenBank were combined with 170 clones to construct a phylogeny and a network. We demonstrate the major divergence between the A and C genomes whereas the distinction among the A and B/D genomes was generally not possible. High affinity among the A(d) genome species A. damascena and the ACD genome species A. fatua was found, whereas the rest of the ACD genome hexaploids and the AACC tetraploids were highly affiliated with the A(l) genome diploid A. longiglumis. One of the AACC species A. murphyi showed the closest relationship with most of the hexaploid species. Both C(v) and C(p) genome species have been proposed as paternal donors of the C-genome carrying polyploids. Incomplete concerted evolution is responsible for the observed differences among different clones of a single Avena individual. The elimination of C-genome rRNA sequences and the resulting evolutionary inference of hexaploid species are discussed.     

Rarely successful polyploids and their legacy in plant genomes

Polyploidy, or whole genome duplication, is recognized as an important feature of eukaryotic genome evolution. Among eukaryotes, polyploidy has probably had the largest evolutionary impact on vascular plants where many contemporary species are of recent polyploid origin. Genomic analyses have uncovered evidence of at least one round of polyploidy in the ancestry of most plants, fueling speculation that genome duplications lead to increases in net diversity. In spite of the frequency of ancient polyploidy, recent analyses have found that recently formed polyploid species have higher extinction rates than their diploid relatives. These results suggest that despite leaving a substantial legacy in plant genomes, only rare polyploids survive over the long term and most are evolutionary dead-ends.     

Subgenome evolution in allotetraploid plants

Polyploidization is a well-known speciation and adaptation mechanism. Traces of former polyploidization events were discovered within many genomes, and especially in plants. Allopolyploidization by interspecific hybridization between two species is common. Among hybrid plants, many are domesticated species of agricultural interest and many of their genomes and of their presumptive parents have been sequenced. Hybrid genomes remain challenging to analyse because of the presence of multiple subgenomes. The genomes of hybrids often undergo rearrangement and degradation over time. Based on 10 hybrid plant genomes from six different genera, with hybridization dating from 10,000 to 5 million years ago, we assessed subgenome degradation, subgenomic intermixing and biased subgenome fractionation. The restructuring of hybrid genomes does not proceed proportionally with the age of the hybrid. The oldest hybrids in our data set display completely different fates: whereas the subgenomes of the tobacco plant Nicotiana benthamiana are in an advanced stage of degradation, the subgenomes of quinoa (Chenopodium quinoa) are exceptionally well conserved by structure and sequence. We observed statistically significant biased subgenome fractionation in seven out of 10 hybrids, which had different ages and subgenomic intermixing levels. Hence, we conclude that no correlation exists between biased fractionation and subgenome intermixing. Lastly, domestication may encourage or hinder subgenome intermixing, depending on the evolutionary context. In summary, comparative analysis of hybrid genomes and their presumptive parents allowed us to determine commonalities and differences between their evolutionary fates. In order to facilitate the future analysis of further hybrid genomes, we automated the analysis steps within manticore , which is publicly available at https://github.com/MatteoSchiavinato/manticore.git .     

Differential Dynamics of Transposable Elements during Long-Term Diploidization of Nicotiana Section Repandae (Solanaceae) Allopolyploid Genomes

Evidence accumulated over the last decade has shown that allopolyploid genomes may undergo drastic reorganization. However, timing and mechanisms of structural diploidization over evolutionary timescales are still poorly known. As transposable elements (TEs) represent major and labile components of plant genomes, they likely play a pivotal role in fuelling genome changes leading to long-term diploidization. Here, we exploit the 4.5 MY old allopolyploid Nicotiana section Repandae to investigate the impact of TEs on the evolutionary dynamics of genomes. Sequence-specific amplified polymorphisms (SSAP) on seven TEs with expected contrasted dynamics were used to survey genome-wide TE insertion polymorphisms. Comparisons of TE insertions in the four allopolyploid species and descendents of the diploid species most closely related to their actual progenitors revealed that the polyploids showed considerable departure from predicted additivity of the diploids. Large numbers of new SSAP bands were observed in polyploids for two TEs, but restructuring for most TE families involved substantial loss of fragments relative to the genome of the diploid representing the paternal progenitor, which could be due to changes in allopolyploids, diploid progenitor lineages or both. The majority of non-additive bands were shared by all polyploid species, suggesting that significant restructuring occurred early after the allopolyploid event that gave rise to their common ancestor. Furthermore, several gains and losses of SSAP fragments were restricted to N. repanda, suggesting a unique evolutionary trajectory. This pattern of diploidization in TE genome fractions supports the hypothesis that TEs are central to long-term genome turnover and depends on both TE and the polyploid lineage considered.     

利用栽培稻C0t-1DNA和基因组DNA比较分析斑点野生稻和短药野生稻基因组

通过荧光原位杂交(FISH)利用来源于A基因组栽培稻的中高度重复序列C0t-1DNA和基因组DNA作为探针,对栽培稻、斑点野生稻和短药野生稻进行了比较基因组分析。结果发现C0t-1DNA杂交信号主要分布在这3种染色体的着丝粒、近着丝粒和端粒区域,在斑点野生稻染色体上的信号多于短药野生稻,与gDNA作为探针FISH的结果相一致,说明A和B基因组间的亲缘关系明显近于A和F基因组。确定了含有中高度重复序列的C0t-1DNA用于属内种间关系研究的可行性,并根据C0t-1DNA的FISH结果进行了染色体核型分析。     

Other tetraploid species and conspecific diploids as sources of genetic variation for an autotetraploid

? Premise of the study: Most plants are polyploid and have more than two copies of the genome. The evolutionary success of polyploids is often attributed to their potential to harbor increased genetic variation, but it is poorly understood how polyploids can attain such variation. Because of their formation bottleneck, newly formed tetraploids start out with little variation. Tetraploids may attain genetic variation through a combination of new mutations, recurrent formation, and gene exchange with diploid ancestors or related tetraploid species. We explore the role of gene exchange and introgression in autotetraploid Rorippa amphibia, a species that harbors more genetic variation than its diploid ancestors. ? Methods: We crossed autotetraploid R. amphibia to diploid conspecifics and tetraploid R. sylvestris and backcrossed resulting F(1) hybrids. We used flow cytometry to determine the ploidy of all progeny. ? Key results: Tetraploids of R. amphibia and R. sylvestris were interfertile; F(1) hybrids were fertile and could backcross. Crosses between diploids and tetraploids yielded a small number of viable, often tetraploid progeny. This indicates that unreduced gametes can facilitate gene flow from diploids to tetraploids. We detected a frequency of unreduced gametes of around 2.7 per 1000, which was comparable between diploids and tetraploids. ? Conclusions: Introgression from tetraploid R. sylvestris provides a realistic source of variation in autotetraploid R. amphibia. Only in a scenario where other compatible partners are absent, for example immediately after tetraploidization, gene flow through unreduced gametes from diploids could be an important source of genetic variation for tetraploids.     

The transcribed 165-bp CentO satellite is the major functional centromeric element in the wild rice species Oryza punctata

Centromeres are required for faithful segregation of chromosomes in cell division. It is not clear what kind of sequences act as functional centromeres and how centromere sequences are organized in Oryza punctata, a BB genome species. In this study, we found that the CentO centromeric satellites in O. punctata share high homology with the CentO satellites in O. sativa. The O. punctata centromeres are characterized by megabase tandem arrays that are flanked by centromere-specific retrotransposons. Immunostaining with an antibody specific to CENH3 indicates that the 165-bp CentO satellites are the major component for functional centromeres. Moreover, both strands of CentO satellites are highly methylated and transcribed and produce small interfering RNA, which may be important for the maintenance of centromeric heterochromatin and centromere function.