人工合成异源六倍体小麦的表观遗传稳定性分析
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摘要
人们认为,在植物异源多倍体早期世代中,基因组是不稳定的。然而,已有研究表明,在不同的物种当中,由异源多倍化所引起的基因组不稳定性并不是一种普遍现象。对于植物进化而言,此种不稳定性可能具有极其重要的作用,抑或只是随机地出现在某些特定的物种或植物个体当中。本研究利用四倍体小麦Triticum turgidum ssp. Durum (基因组BBAA)和二倍体物种Aegilops tauschii (基因组DD)杂交后染色体加倍,获得新合成异源六倍体小麦(Allo-AT6)。通过多色基因组原位杂交分析表明,新合成异源六倍体小麦(Allo-AT6)与普通小麦(T. aestivum基因组BBAADD)在基因组构成上相似,且大部分的个体在染色体水平上呈现稳定的状态。但是, S2代的某一单株为非整倍体,且发生了基因组间染色体明显易位。同时,与其它个体相比,此植株在分子水平上也呈现出很大的差异,包括DNA片段的丢失以及获得新的DNA片段。本实验基于这些研究结果,采用MSAP的方法进一步对新合成异源六倍体小麦Allo-AT6及其直系自交后代(S2~S8)的表观遗传(DNA甲基化)稳定性的变化进行分析。分析结果表明,后代甲基化总体水平高于亲本;同时,后代中发生了较高频率的甲基化变异并且能够稳定遗传。我们的研究结果表明,在小麦的进化历程当中,发生在某些特定个体当中短暂且快速的基因组变化,很可能对普通小麦物(T. aestivum)的物种形成并不具有极其重要的作用。
Rampant genomic instability can be elicited by nascent allopolyploidization in plants. Most previous studies however have not endeavored to define whether and to what extent the allopolyploidy-incurred rapid genomic instability represents a general response, and hence likely consequential to evolution, or merely anomalous and incidental events occurring stochastically in limited individuals. We report here that in a newly formed allohexaploid wheat line (Allo-AT6) between tetraploid wheat Triticum turgidum ssp. durum (genome BBAA) and Aegilops tauschii (genome DD), the great majority of individual plants showed chromosomal stability, and exhibiting a genomic constitution similar to that of the present-day T. aestivum (genome BBAADD). In contrast, a single individual plant was identified at S2, which exhibited chromosomal instability in both number and structure based on multicolor genomic in situ hybridization (mc-GISH) analysis. Accordingly, this plant also manifested extensive changes at the molecular level including loss and gain of DNA segments. Based on these evidences, we take the MSAP method to further analysis on the synthetic Allohexaploid wheat (Allo-AT6), as well as its consecutive progenies (S2~S8), to unveil its epigenetic stability (DNA methylation) in this experiment. The results show that the progenies are higher than their parents in terms of methylation level. Meanwhile, there is a high frequency of methylation variation among the progenies and it can be stably inherited. Our results suggest that these ephemeral and individual-specific rapid genomic changes, albeit interesting, probably have not played a major role in the speciation and evolution of common wheat, T. aestivum.
Rampant genomic instability can be elicited by nascent allopolyploidization in plants. Most previous studies however have not endeavored to define whether and to what extent the allopolyploidy-incurred rapid genomic instability represents a general response, and hence likely consequential to evolution, or merely anomalous and incidental events occurring stochastically in limited individuals. We report here that in a newly formed allohexaploid wheat line (Allo-AT6) between tetraploid wheat Triticum turgidum ssp. durum (genome BBAA) and Aegilops tauschii (genome DD), the great majority of individual plants showed chromosomal stability, and exhibiting a genomic constitution similar to that of the present-day T. aestivum (genome BBAADD). In contrast, a single individual plant was identified at S2, which exhibited chromosomal instability in both number and structure based on multicolor genomic in situ hybridization (mc-GISH) analysis. Accordingly, this plant also manifested extensive changes at the molecular level including loss and gain of DNA segments. Based on these evidences, we take the MSAP method to further analysis on the synthetic Allohexaploid wheat (Allo-AT6), as well as its consecutive progenies (S2~S8), to unveil its epigenetic stability (DNA methylation) in this experiment. The results show that the progenies are higher than their parents in terms of methylation level. Meanwhile, there is a high frequency of methylation variation among the progenies and it can be stably inherited. Our results suggest that these ephemeral and individual-specific rapid genomic changes, albeit interesting, probably have not played a major role in the speciation and evolution of common wheat, T. aestivum.
引文
[1] Steimer A., Schob H., Grossniklaus U. Epigenetic control of plant development: New layers of complexity[J]. Curr. Opin. Plant Biol. 2004, 7 (1): 11-19.
[2] Chan S W L, Henderson I R, Jacobsen S E. Gardening the genome: DNA methylation in Arabidopsis thaliana[J]. Nat. Rev. Genet. 2005, 6: 351-360.
[3] Yoder J A, Walsh C P, Bestor T H. Cytosine methylation and the ecology of intragenomic parasites[J]. Trends Genet. 1997, 13 (8): 335-340.
[4] Miura A, Yonebayashi S, Watanabe K, et al. Mobilization of transposons by a mutation abolishing full DNA methylation in Arabidopsis[J]. Nature 2001, 411: 212-214.
[5] Zilberman D, Gehring M, Tran R.K, et al. Genome-wide analysis of Arabidopsis thaliana DNA methylation uncovers an interdependence between methylation and transcription[J]. 2007, 39:61-69.
[6] Gehring M, Henikoff S. DNA methylation dynamics in plant genomes[J]. Biochim. Biophys. Acta 2007, 1769 (5-6): 276-286.
[7] Fransz P F, de Jong J H. Chromatin dynamics in plants[J]. Curr. Opin. Genet. Dev. 2002, 5 (6): 560-567.
[8] Chen T, Li E. Structure and function of eukaryotic DNA methyltransferases[J]. Curr. Top. Dev. Biol.2004, 60: 55-89.
[9] Bird A. . DNA methylation patterns and epigenetic memory[J]. Genes Dev. 16: 6-21.
[10] Henderson I R, Jacobsen S E. Epigenetic inheritance inplants[J]. Nature 2007, 447: 418-424.
[11] Gonzalo S, Jaco I, Fraga M F, et al. DNA methyltransferases control telomere length and telomere recombination in mammalian cells[J]. Nat. Cell Biol.2006, 8: 416-424.
[12] Zhu J, Kapoor A, Sridhar V V, et al. The DNA glycosylase/lyase ROS1 functions in pruning DNA methylation patterns in Arabidopsis[J]. Curr. Biol. 2007, 17: 54-59.
[13] Zhang M S, Yan H Y, Zhao N, et al. Endosperm-specific hypomethylation, and meiotic inheritance and variation of DNA methylation level and pattern in sorghum (Sorghum bicolor L.) inter-strain hybrids[J]. Theor. Appl. Genet. 2007,115: 195-207.
[14] Cokus S J, Feng S, Zhang X, et al. Shotgun bisulphite sequencing of the Arabidopsis genome reveals DNA methylation patterning[J]. Nature 2008,452: 215-219.
[15] Dai Y, Ni Z, Dai J, et al. Isolation and expression analysis of genes encoding DNA methyltransferase in wheat (Triticum aestivum L.) [J]. Biochim. Biophys. Acta 2005,1729 (2): 118-125.
[16] Tran R K, Henikoff J G, Zilberman D, et al. DNA methylation profiling identifies CG methylation clusters in Arabidopsis genes[J]. Curr. Biol. 2005,15: 154-159.
[17] Li X, Wang X, He K, et al. High-resolution mapping of epigenetic modifications of the rice genome uncovers interplay between DNA methylation, histone methylation, and gene expression[J]. Plant Cell 2008,20: 259-276.
[18] Rabinowicz P D, Palmer L E, May B P, et al. Genes and transposons are differentiallymethylated in plants, but not in mammals[J]. Genome Res. 2003,13: 2658-2664.
[19] Wang X, Elling A A, Li X, et al. Genome-wide and organ-specific landscapes of epigenetic modifications and their relationships to mRNA and small RNA transcriptomes in maize[J]. Plant Cell 2009,21: 1053-1069.
[20] Ruiz-Garcia L, Cervera M T, Martinez-Zapater, J.M. DNA methylation increases throughout Arabidopsis development[J]. Planta 2005, 222: 301-306.
[21] Cao X, Springer N M, Muszynski M G, et al. Conserved plant genes with similarity to mammalian de novo DNA methyltransferases[J]. Proc. Natl. Acad. Sci. USA 2000,97: 4979-4984.
[22]Cao X, Jacobsen S E. Role of the Arabidopsis DRM methyltransferases in de novo DNA methylation and gene silencing[J]. Curr. Biol. 2002,12: 1138-1144.
[23] Finnegan E J, Kovac K A. Plant DNA methyltransferases[J]. Plant Mol. Biol. 2000,43: 189-201.
[24] Zhang X, Yazaki J, Sundaresan A, et al. Genome-wide high-resolution mapping and functional analysis of DNA methylation in Arabidopsis[J]. Cell 2006,126: 1189-1201.
[25] Hsieh T F, Ibarra C A, Silva P, et al. Genome-wide demethylationof Arabidopsis endosperm[J]. Science 2009, 324: 1451-1454.
[26] Cao X, Jacobsen S E. Locus-specific control of asymmetric and CpNpG methylation by the DRM and CMT3 methyltransferase genes[J]. Proc. Natl. Acad. Sci. USA 2002, 99: 16491-16498.
[27] Pavlopoulou A, Kossida S. Plant cytosine-5 DNA methyltransferases: structure, function, and molecular evolution[J]. Genomics 2007, 90: 530-541.
[28] Chandler V L, Stam M. Chromatin conversations: mechanisms and implications of paramutation[J]. Nat. Rev. Genet. 2004,5: 532-544.
[29] Zhang X, Henderson I R, Lu C, et al. Role of RNA polymerase IV in plant small RNA metabolism[J]. Proc. Natl. Acad Sci. USA 2007,104: 4536-4541.
[30] Xiong L Z, Xu C G, Saghai Maroof, et al. Patterns of cytosine methylation in an elite rice hybrid and its parental lines, detected by a methylation-sensitive amplification polymorphism technique[J]. Mol. Gen. Genet. 1999,261: 439-446.
[31] Matzke A J M, Matzke M. RNA-directed DNA methylation mediated by DRD1 and Pol IVb: a versatile pathway for transcriptional gene silencing in plants[J]. Biochim. Biophys. Acta 2007, 1769: 358-374.
[32] Igarashi J, Muroi S, Kawashima H, et al. Quantitative analysis of human tissue-specific differences in methylation[J]. Biochem. Biophys. Res. Commun. 2008, 376: 658-664.
[33] Cao ., Aufsatz W, Zilberman D, et al. Role of the DRM and CMT3 methyltransferases in RNA-directed DNA methylation[J]. Curr. Biol. 2003, 13: 2212-2217.
[34] Huettel B, Kanno T, Daxinger L, et al. RNA-directed DNA methylation mediated by DRD1 and Pol IVb: a versatile pathway for transcriptional gene silencing in plants[J]. Biochim. Biophys. Acta 2007, 1769: 358-374.
[35] Zhu J, Kapoor A, Sridhar V V, et al. The DNA glycosylase/lyase ROS1 functions in pruning DNA methylation patterns in Arabidopsis[J]. Curr. Biol. 2007, 17: 54-59.
[36] Choi Y, Gehring M, Johnson L, et al. DEMETER, a DNA glycosylase domain protein, is required for endosperm gene imprinting and seed viability in Arabidopsis[J]. Cell 2002, 110: 33-42.
[37] Gong Z, Morales-Ruiz T, Ariza R R, et al. ROS1, a repressor of transcriptional gene silencing in Arabidopsis, encodes a DNA glycosylase/lyase[J]. Cell 2002, 111: 803-814.
[38] Dai Y, Ni Z, Dai J, et al. Isolation and expression analysis of genes encoding DNA methyltransferase in wheat (Triticum aestivum L.) [J]. Biochim. Biophys. Acta 2005, 1729: 118-125.
[39] Kinoshita T, Miura A, Choi Y, et al. One-way control of FWA imprinting in Arabidopsis endosperm by DNA methylation[J]. Science 2004, 303: 521-523.
[40] Soppe W J J, Jacobsen S E, Alonso-Blanco C, et al. The late flowering phenotype of fwa mutants is caused by gain-of-function epigenetic alleles of a homeodomain gene[J]. Mol. Cell 2000, 6: 791-802.
[41] Kitamura E, Oinuma T, Nemoto N, et al. Quantitative analysis of human tissue- specific differences in methylation[J]. Biochem. Biophys. Res. Commun. 2008, 376: 658-664.
[42] Reik W, Walter J. Imprinting mechanisms in mammals[J]. Curr. Opin. Genet. Dev. 1998, 8: 154-164.
[43] Jullien P E, Mosquna A, Ingouff M, et al. Retinoblastoma and its binding partner MSI1 control imprinting in Arabidopsis[J]. PLoS Biol 2008, 6: e194.
[44] Berger F, Chaudhury A. Parental memories shape seeds[J]. Trends Plant Sci.2009, 14: 550-556.
[45]Zilberman D. The evolving function of DNA methylation[J]. Curr. Opin. Plant Biol. 2008, 11: 554-559.
[46] Finnegan E J, Peacock W J, Dennis E S. Reduced DNA methylation in Arabidopsis thaliana results in abnormal plant development[J]. Proc. Natl. Acad. Sci. USA 1996, 93: 8449-8454.
[47] Nakano Y, Steward N, Sekine M, et al. A tobacco NtMET1 cDNA encoding a DNA methyltransferase: molecular characterization and abnormal phenotypes of transgenic tobacco plants[J]. Plant Cell Physiol. 2000, 41: 448-457.
[48] Kankel M W, Ramsey D E, Stokes T L, et al. Arabidopsis MET1 cytosine methyltransferase mutants[J]. Genetics 2003, 163: 1109-1122.
[49] Lindroth A M, Cao X., Jackson J P, et al. Requirement of CHROMOMETHYLASE3 for maintenance of CpXpG methylation[J]. Science 2001, 292: 2077-2080.
[50] Carrozza M J, Li B, Florens L, et al. Histone H3 methylation by Set2 directs deacetylation of coding regions by Rpd3S to suppress spurious intragenic transcription[J]. Cell 2005, 123: 581-592.
[51] Xiao W, Custard R D, Brown R C, et al. DNA methylation is critical for Arabidopsis embroyogenesis and seed viability[J]. Plant Cell 2006, 18: 805-814.
[52]Adams K L, Cronn R, Percifield R, et al.. Genes duplicated by polyploidy show unequal contributions to the transcriptome and organ-specific reciprocal silencing[J]. Proc Natl Acad Sci USA, 2003,100:4649-4654.
[53]Feldman M, Liu B, Segal G, et al. Rapid elimination of low-copy DNA sequences in polyploid wheat: A possible mechanism for differentiation of homoeolegous chromosomes[J]. Genetics, 1997, 147(4): 1381-1387.
[54]Gupta P K, Reddy VRK. Cytogenetics of Triticale - a man-made cereal. In: Gupta PK, Tsuchiya T, editors. Chromosome Engineering in Plants: Genetics, Breeding, Evolution (Part A). Elsevier [J]. 1991, 335-359 p.
[55]Song K, Lu P, Tang K, et al. Rapid genome change in synthetic polyploids of Brassica and its implications for polyploidy evolution [J]. Proc Natl Acad Sci USA .1995, 92:7719-7723.
[56]Ramsey J, Schemske D W. Pathways, mechanisms, and rates of polyploid formation inflowering plants [J]. Annu Rev Ecol Syst, 1998, 29: 467-501
[57]Vigfusson E. On polyspermy in the sunflower. Hereditas, 1970, 64: 1-52
[58]Hagerup O. The spontaneous formation of haploid, polyploid and aneuploid embryos in some orchids. Kongel Danshe Videnskab Selskab Biol Meddelelser, 1947, 20:1-22
[59]Thompson J D, Lumaret R. The evolutionary dynamics of polyploid plants: origins, establishment and persistence[J]. Trends Ecol Evol.1992, 7: 303-306
[60]De Haan A, Maceira NO, Lumaret R, et al. Production of 2n gametes in diploid subspecies of Dactylis glomerata L 2. Occurrence and frequency of 2n eggs. Ann Bot, 1992, 69: 345-350
[61]Grant V. Plant Speciation. New York: Columbia University Press. 1971.
[62]Stebbins G L. Chromosomal Evolution in Higher Plants. London:Edward Arnold. 1971
[63]Lewis W H. Polyploidy: Biological Relevance. New York: Plenum. 1980,583 p.
[64]Wang J, Tian L, Madlung A, et al. Stochastic and epigenetic changes of gene expression in Arabidopsis polyploids[J]. Genetics.2004,167:1961-1973.
[65]Madlung A, Masuelli R W, Watson B, et al. Remodeling of DNA methylation and phenotypic and transcriptional changes in synthetic Arabidopsis allotetraploids[J]. Plant Physiol. 2002,129:733-746.
[2] Chan S W L, Henderson I R, Jacobsen S E. Gardening the genome: DNA methylation in Arabidopsis thaliana[J]. Nat. Rev. Genet. 2005, 6: 351-360.
[3] Yoder J A, Walsh C P, Bestor T H. Cytosine methylation and the ecology of intragenomic parasites[J]. Trends Genet. 1997, 13 (8): 335-340.
[4] Miura A, Yonebayashi S, Watanabe K, et al. Mobilization of transposons by a mutation abolishing full DNA methylation in Arabidopsis[J]. Nature 2001, 411: 212-214.
[5] Zilberman D, Gehring M, Tran R.K, et al. Genome-wide analysis of Arabidopsis thaliana DNA methylation uncovers an interdependence between methylation and transcription[J]. 2007, 39:61-69.
[6] Gehring M, Henikoff S. DNA methylation dynamics in plant genomes[J]. Biochim. Biophys. Acta 2007, 1769 (5-6): 276-286.
[7] Fransz P F, de Jong J H. Chromatin dynamics in plants[J]. Curr. Opin. Genet. Dev. 2002, 5 (6): 560-567.
[8] Chen T, Li E. Structure and function of eukaryotic DNA methyltransferases[J]. Curr. Top. Dev. Biol.2004, 60: 55-89.
[9] Bird A. . DNA methylation patterns and epigenetic memory[J]. Genes Dev. 16: 6-21.
[10] Henderson I R, Jacobsen S E. Epigenetic inheritance inplants[J]. Nature 2007, 447: 418-424.
[11] Gonzalo S, Jaco I, Fraga M F, et al. DNA methyltransferases control telomere length and telomere recombination in mammalian cells[J]. Nat. Cell Biol.2006, 8: 416-424.
[12] Zhu J, Kapoor A, Sridhar V V, et al. The DNA glycosylase/lyase ROS1 functions in pruning DNA methylation patterns in Arabidopsis[J]. Curr. Biol. 2007, 17: 54-59.
[13] Zhang M S, Yan H Y, Zhao N, et al. Endosperm-specific hypomethylation, and meiotic inheritance and variation of DNA methylation level and pattern in sorghum (Sorghum bicolor L.) inter-strain hybrids[J]. Theor. Appl. Genet. 2007,115: 195-207.
[14] Cokus S J, Feng S, Zhang X, et al. Shotgun bisulphite sequencing of the Arabidopsis genome reveals DNA methylation patterning[J]. Nature 2008,452: 215-219.
[15] Dai Y, Ni Z, Dai J, et al. Isolation and expression analysis of genes encoding DNA methyltransferase in wheat (Triticum aestivum L.) [J]. Biochim. Biophys. Acta 2005,1729 (2): 118-125.
[16] Tran R K, Henikoff J G, Zilberman D, et al. DNA methylation profiling identifies CG methylation clusters in Arabidopsis genes[J]. Curr. Biol. 2005,15: 154-159.
[17] Li X, Wang X, He K, et al. High-resolution mapping of epigenetic modifications of the rice genome uncovers interplay between DNA methylation, histone methylation, and gene expression[J]. Plant Cell 2008,20: 259-276.
[18] Rabinowicz P D, Palmer L E, May B P, et al. Genes and transposons are differentiallymethylated in plants, but not in mammals[J]. Genome Res. 2003,13: 2658-2664.
[19] Wang X, Elling A A, Li X, et al. Genome-wide and organ-specific landscapes of epigenetic modifications and their relationships to mRNA and small RNA transcriptomes in maize[J]. Plant Cell 2009,21: 1053-1069.
[20] Ruiz-Garcia L, Cervera M T, Martinez-Zapater, J.M. DNA methylation increases throughout Arabidopsis development[J]. Planta 2005, 222: 301-306.
[21] Cao X, Springer N M, Muszynski M G, et al. Conserved plant genes with similarity to mammalian de novo DNA methyltransferases[J]. Proc. Natl. Acad. Sci. USA 2000,97: 4979-4984.
[22]Cao X, Jacobsen S E. Role of the Arabidopsis DRM methyltransferases in de novo DNA methylation and gene silencing[J]. Curr. Biol. 2002,12: 1138-1144.
[23] Finnegan E J, Kovac K A. Plant DNA methyltransferases[J]. Plant Mol. Biol. 2000,43: 189-201.
[24] Zhang X, Yazaki J, Sundaresan A, et al. Genome-wide high-resolution mapping and functional analysis of DNA methylation in Arabidopsis[J]. Cell 2006,126: 1189-1201.
[25] Hsieh T F, Ibarra C A, Silva P, et al. Genome-wide demethylationof Arabidopsis endosperm[J]. Science 2009, 324: 1451-1454.
[26] Cao X, Jacobsen S E. Locus-specific control of asymmetric and CpNpG methylation by the DRM and CMT3 methyltransferase genes[J]. Proc. Natl. Acad. Sci. USA 2002, 99: 16491-16498.
[27] Pavlopoulou A, Kossida S. Plant cytosine-5 DNA methyltransferases: structure, function, and molecular evolution[J]. Genomics 2007, 90: 530-541.
[28] Chandler V L, Stam M. Chromatin conversations: mechanisms and implications of paramutation[J]. Nat. Rev. Genet. 2004,5: 532-544.
[29] Zhang X, Henderson I R, Lu C, et al. Role of RNA polymerase IV in plant small RNA metabolism[J]. Proc. Natl. Acad Sci. USA 2007,104: 4536-4541.
[30] Xiong L Z, Xu C G, Saghai Maroof, et al. Patterns of cytosine methylation in an elite rice hybrid and its parental lines, detected by a methylation-sensitive amplification polymorphism technique[J]. Mol. Gen. Genet. 1999,261: 439-446.
[31] Matzke A J M, Matzke M. RNA-directed DNA methylation mediated by DRD1 and Pol IVb: a versatile pathway for transcriptional gene silencing in plants[J]. Biochim. Biophys. Acta 2007, 1769: 358-374.
[32] Igarashi J, Muroi S, Kawashima H, et al. Quantitative analysis of human tissue-specific differences in methylation[J]. Biochem. Biophys. Res. Commun. 2008, 376: 658-664.
[33] Cao ., Aufsatz W, Zilberman D, et al. Role of the DRM and CMT3 methyltransferases in RNA-directed DNA methylation[J]. Curr. Biol. 2003, 13: 2212-2217.
[34] Huettel B, Kanno T, Daxinger L, et al. RNA-directed DNA methylation mediated by DRD1 and Pol IVb: a versatile pathway for transcriptional gene silencing in plants[J]. Biochim. Biophys. Acta 2007, 1769: 358-374.
[35] Zhu J, Kapoor A, Sridhar V V, et al. The DNA glycosylase/lyase ROS1 functions in pruning DNA methylation patterns in Arabidopsis[J]. Curr. Biol. 2007, 17: 54-59.
[36] Choi Y, Gehring M, Johnson L, et al. DEMETER, a DNA glycosylase domain protein, is required for endosperm gene imprinting and seed viability in Arabidopsis[J]. Cell 2002, 110: 33-42.
[37] Gong Z, Morales-Ruiz T, Ariza R R, et al. ROS1, a repressor of transcriptional gene silencing in Arabidopsis, encodes a DNA glycosylase/lyase[J]. Cell 2002, 111: 803-814.
[38] Dai Y, Ni Z, Dai J, et al. Isolation and expression analysis of genes encoding DNA methyltransferase in wheat (Triticum aestivum L.) [J]. Biochim. Biophys. Acta 2005, 1729: 118-125.
[39] Kinoshita T, Miura A, Choi Y, et al. One-way control of FWA imprinting in Arabidopsis endosperm by DNA methylation[J]. Science 2004, 303: 521-523.
[40] Soppe W J J, Jacobsen S E, Alonso-Blanco C, et al. The late flowering phenotype of fwa mutants is caused by gain-of-function epigenetic alleles of a homeodomain gene[J]. Mol. Cell 2000, 6: 791-802.
[41] Kitamura E, Oinuma T, Nemoto N, et al. Quantitative analysis of human tissue- specific differences in methylation[J]. Biochem. Biophys. Res. Commun. 2008, 376: 658-664.
[42] Reik W, Walter J. Imprinting mechanisms in mammals[J]. Curr. Opin. Genet. Dev. 1998, 8: 154-164.
[43] Jullien P E, Mosquna A, Ingouff M, et al. Retinoblastoma and its binding partner MSI1 control imprinting in Arabidopsis[J]. PLoS Biol 2008, 6: e194.
[44] Berger F, Chaudhury A. Parental memories shape seeds[J]. Trends Plant Sci.2009, 14: 550-556.
[45]Zilberman D. The evolving function of DNA methylation[J]. Curr. Opin. Plant Biol. 2008, 11: 554-559.
[46] Finnegan E J, Peacock W J, Dennis E S. Reduced DNA methylation in Arabidopsis thaliana results in abnormal plant development[J]. Proc. Natl. Acad. Sci. USA 1996, 93: 8449-8454.
[47] Nakano Y, Steward N, Sekine M, et al. A tobacco NtMET1 cDNA encoding a DNA methyltransferase: molecular characterization and abnormal phenotypes of transgenic tobacco plants[J]. Plant Cell Physiol. 2000, 41: 448-457.
[48] Kankel M W, Ramsey D E, Stokes T L, et al. Arabidopsis MET1 cytosine methyltransferase mutants[J]. Genetics 2003, 163: 1109-1122.
[49] Lindroth A M, Cao X., Jackson J P, et al. Requirement of CHROMOMETHYLASE3 for maintenance of CpXpG methylation[J]. Science 2001, 292: 2077-2080.
[50] Carrozza M J, Li B, Florens L, et al. Histone H3 methylation by Set2 directs deacetylation of coding regions by Rpd3S to suppress spurious intragenic transcription[J]. Cell 2005, 123: 581-592.
[51] Xiao W, Custard R D, Brown R C, et al. DNA methylation is critical for Arabidopsis embroyogenesis and seed viability[J]. Plant Cell 2006, 18: 805-814.
[52]Adams K L, Cronn R, Percifield R, et al.. Genes duplicated by polyploidy show unequal contributions to the transcriptome and organ-specific reciprocal silencing[J]. Proc Natl Acad Sci USA, 2003,100:4649-4654.
[53]Feldman M, Liu B, Segal G, et al. Rapid elimination of low-copy DNA sequences in polyploid wheat: A possible mechanism for differentiation of homoeolegous chromosomes[J]. Genetics, 1997, 147(4): 1381-1387.
[54]Gupta P K, Reddy VRK. Cytogenetics of Triticale - a man-made cereal. In: Gupta PK, Tsuchiya T, editors. Chromosome Engineering in Plants: Genetics, Breeding, Evolution (Part A). Elsevier [J]. 1991, 335-359 p.
[55]Song K, Lu P, Tang K, et al. Rapid genome change in synthetic polyploids of Brassica and its implications for polyploidy evolution [J]. Proc Natl Acad Sci USA .1995, 92:7719-7723.
[56]Ramsey J, Schemske D W. Pathways, mechanisms, and rates of polyploid formation inflowering plants [J]. Annu Rev Ecol Syst, 1998, 29: 467-501
[57]Vigfusson E. On polyspermy in the sunflower. Hereditas, 1970, 64: 1-52
[58]Hagerup O. The spontaneous formation of haploid, polyploid and aneuploid embryos in some orchids. Kongel Danshe Videnskab Selskab Biol Meddelelser, 1947, 20:1-22
[59]Thompson J D, Lumaret R. The evolutionary dynamics of polyploid plants: origins, establishment and persistence[J]. Trends Ecol Evol.1992, 7: 303-306
[60]De Haan A, Maceira NO, Lumaret R, et al. Production of 2n gametes in diploid subspecies of Dactylis glomerata L 2. Occurrence and frequency of 2n eggs. Ann Bot, 1992, 69: 345-350
[61]Grant V. Plant Speciation. New York: Columbia University Press. 1971.
[62]Stebbins G L. Chromosomal Evolution in Higher Plants. London:Edward Arnold. 1971
[63]Lewis W H. Polyploidy: Biological Relevance. New York: Plenum. 1980,583 p.
[64]Wang J, Tian L, Madlung A, et al. Stochastic and epigenetic changes of gene expression in Arabidopsis polyploids[J]. Genetics.2004,167:1961-1973.
[65]Madlung A, Masuelli R W, Watson B, et al. Remodeling of DNA methylation and phenotypic and transcriptional changes in synthetic Arabidopsis allotetraploids[J]. Plant Physiol. 2002,129:733-746.