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1.
CIMMYT新型人工合成小麦Pina和Pinb基因等位变异 总被引：4，自引：0，他引：4
六倍体人工合成小麦由硬粒小麦（Triticum turgidum subsp. durum）与粗山羊草（Aegilops tauschii Coss.）杂交产生,是研究小麦进化过程中基因变异的重要材料。以国际玉米小麦改良中心（CIMMYT）提供的57份由野生二粒小麦（T. turgidum subsp. dicoccoides）与粗山羊草杂交产生的新型人工合成六倍体小麦为材料，用单籽粒特性测定仪和Pina、Pinb特异性PCR引物对其籽粒硬度变异以及控制籽粒硬度的主效基因Pina和Pinb的分布情况进行了研究。结果表明，这些材料的SKCS硬度值变异较大，从10.5到42.6，其中15~30的占78%。共有Pina-D1a、Pina-D1c、Pinb-D1h和Pinb-D1j 4种等位变异型，基因型为Pina-D1a/Pinb-D1j的8个，占14%；基因型为Pina-D1c/Pinb-D1h的49个，占86%。方差分析表明，基因型Pina-D1a/Pinb-D1j与Pina-D1c/Pinb-D1h对籽粒硬度的影响差异不显著，但父本粗山羊草和母本野生二粒小麦以及二者间的互作对籽粒硬度有显著影响，说明除Pina和Pinb外，还有其他微效基因影响籽粒硬度的形成。 相似文献
2.
Characterization of dinucleotide and trinucleotide EST-derived microsatellites in the wheat genome 总被引：6，自引：0，他引：6
Over the past decade microsatellites or simple sequence repeats (SSRs) have attracted a considerable amount of attention from
researchers. The aim of the present paper was to analyse expressed sequence tag-derived SSR (EST-SSR) marker variability in
wheat and to investigate the relationships between the number and type of repeat units and the level of microsatellite polymorphism.
Two hundred and forty-one new EST-SSR markers available in a public database () were characterized in eight durum wheat cultivars (Svevo, Ciccio, Primadur, Duilio, Meridiano, Claudio, Latino, Messapia),
two accessions of Triticum turgidum var. dicoccoides (MG4343, MG29896), one accession of T. turgidum var. dicoccum (MG5323) and in the common wheat cv. Chinese Spring. Of these, 201 primer pairs (83.4%) amplified PCR products successfully,
while the remaining 40 (16.6%) failed to amplify any product. Of the EST-SSRs analysed, 45.2% of the primer pairs amplified
one or two PCR products. Multiple discrete PCR products were observed among both di- and trinucleotide EST-SSR markers (31.2
and 40.5%, respectively). Markers based on dinucleotide microsatellites were more polymorphic than those based on trinucleotide
SSRs in the 12 wheat genotypes tested (68.9 and 52.7%, respectively). An average of 2.5 alleles for dinucleotide and 2.0 alleles
for trinucleotide SSRs was observed. The data reported in the present work indicate the presence of a significant relationship
between motif sequence types and polymorphism. The primer set based on the AG repeat motif showed the lowest percentage of
polymorphism (55.0%), while the primer set based on the AC repeat motif showed t he highest percentage (85.0%). Among trinucleotide
SSRs, the AGG microsatellite markers showed the highest percentage of polymorphism (70.0%), and the ACG motif the lowest value
(25.0%). The characterization of these new EST-SSR markers and the results of our studyon the effect of repeat number and
type of motifs could have important applications in the genetic analysis of agronomically important traits, quantitative trait
locus discovery and marker-assisted selection. 相似文献
3.
For breeding to increase amylose content in the starch of hexaploid wheat, either the introduction of genes from Aegilops
squarrosa (DD genome) or the development of high-amylose amphiploids between Triticum durum (AABB genome) and Ae. squarrosa
is one of the most efficient ways. The high-amylose genes have not been detected in any species of Triticum and Aegilops.
Therefore, we assessed the apparent amylose content in endosperm starch of T. durum and Ae. squarrosa accessions supplied
from the major gene banks from world wide, and found several accessions with higher amylose content out of 665 T. durum accessions
and out of 732 Ae. squarrosa accessions. Genetic analysis indicated that a high amylose trait in Ae. squarrosa is monogenically
inherited by a recessive major gene.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
4.
籽粒硬度是小麦加工品质的重要影响因素.籽粒硬度生化标记Friabilin蛋白主要由Puroindoline A（PinA）和Puroindoline B（PinB）构成,它们的编码基因puroindoline a（Pina）和puroindoline b（Pinb）紧密连锁,位于Ha（hardness）位点,是控制籽粒硬度的主效基因.根据小麦Pina、Pinb的保守序列,设计合成了2对特异性引物,对粘果山羊草Aegilops kotschyi（UUSS）3个材料的基因组DNA和胚乳RNA进行Pina、Pinb基因克隆、序列测定和表达分析,发现了3个新型Pina等位基因和4个新型Pinb等位基因.新型puroindoline等位基因全长447bp,编码148个氨基酸残基,具有麦类作物puroindoline基因特有的信号肽序列和色氨酸结构域.与Pina-D1a相比较,新型Pina等位基因核苷酸同源性分别为98.7%、98.4%、97.5%,氨基酸同源性分别为96.6%、95.9%、93.9%.等位基因Pina-allele-2包含一个位于色氨酸结构域内的突变位点（Lys70Arg）.新型Pinb等位基因与Pinb-D1a相比较,其核苷酸同源性分别为93.3%、94.6%、94.6%、94.4%,氨基酸同源性分别为90.5%、93.2%、93.2%、92.6%.等位基因Pinb-allele-1含有一个紧邻色氨酸结构域的突变位点（Val66Phe）.Southern blot分析结果表明,3个材料中Pina和Pinb基因都含有2个拷贝.RT-PCR和Western blot都证实了Pina、Pinb基因在籽粒胚乳中的表达.研究结果显示山羊草中包含着与小麦差异较大的籽粒硬度控制基因,为小麦分子育种提供了丰富的的遗传资源. 相似文献
5.
A.F. Merezhko 《Euphytica》1998,100(1-3):295-303
This article describes the impact of plant genetic resources on wheat breeding. It defines the important contribution of N.I.
Vavilov Institute (St. Petersburg, Russia) to broadening the genetic diversity of new wheat cultivars. Special attention is
given to conducting a comprehensive evaluation of intraspecific variability for valuable characters, including: 1) formation
of special subcollections, consisting of accessions with useful characters and accessions representing intraspecies diversity
for a given character; 2) revealing genetic differences among phenotypically superior accessions; 3) determining a genetic
system of intraspecific variability for a given character; 4) formation of a genetic collection; 5) revealing and developing
the donors of useful characters. An example using plant height shows that the proposed research approach adequately determines
the genetic potential of species and reveals the most effective genes for practical uses. It is assumed that the main sources
of genes for breeding Triticum aestivum L. in order of importance will remain: 1) intraspecific diversity of T. aestivum itself;
2) other Triticum species; 3) other genera of the Triticeae Dum. tribe (particularly Aegilops L.); 4) more remote genera of
the Poaceae Barnh. botanical family. It is stressed that existing diversity of T. aestivum has been poorly investigated genetically
and only partially used by breeders. Properly evaluated, it can provide multiple solutions for traditional and new problems
of wheat improvement.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
6.
Lamiae Amallah Mouna Taghouti Keltoum Rhrib Fatima Gaboun Moustapha Arahou Rachida Hassikou Ghizlane Diria 《Journal of Crop Science and Biotechnology》2016,19(2):137-150
Over recent years, quality has become an important commercial issue for durum wheat breeders. Modern breeding methods are most efficient for producing and supplying the best quality raw materials to the pasta industry. Here we assessed the effectiveness of molecular marker-assisted selection of quality traits in durum wheat. To this end, DNA and quality trait markers were jointly used to analyze quality-related traits in a durum wheat collection. A total of 132 durum wheat (Triticum turgidum ssp. durum) Mediterranean landraces, international lines, and Moroccan cultivars were analyzed for seven important qualityrelated traits including thousand-kernel weight (TKW), test weight (TW), gluten strength, yellow pigment (YP), and grain protein content (GPC). Additionally, 18 simple sequence repeat (SSR) markers previously reported to be associated with different quality traits were analyzed. Of these, 14 (78%) were polymorphic and four were monomorphic. There were between two and seven alleles per locus, with an average of four alleles per locus. The average phenotypic variation value (R2) ranged from 2.81 to 20.43%. Association analysis identified nine markers significantly associated with TKW, TW, and YP, followed by eight markers associated with GPC, six markers associated with yellow index b, four markers associated with brightness L, and three markers associated with SDS-sedimentation volume. This study highlights the efficiency of SSR technology, which holds promise for a wide range of applications in marker-assisted wheat breeding programs. 相似文献
7.
M. G. Mosaad G. Ortiz-Ferrara V. Mahalakshmi 《Journal of Agronomy and Crop Science》1995,174(3):173-180
The number of productive tillers is an important yield component in wheat and is affected by water stress and genetic factors. A greenhouse experiment was conducted during spring 1992 at ICARDA, Tel Hadya, Syria, with eight genotypes representing two Triticum species ( Triticum turgidum L. ssp. durum and Triticum aestivum L. ssp. aestivum ) under four soil-moisture regimes (95 %, 75 %, 55 %, and 35 % field capacity) to study the effect of water deficit on tiller development and tiller contribution to grain yield. In the highest watering regime appearance of Tiller 1 was delayed in both species. Also Tiller 2 was suppressed in this treatment in durum , while its appearance was delayed in aestivum. In the driest treatment, a majority of the tillers were suppressed and the ones which emerged were delayed. In durum , the heat units required to produce successive leaves on the tillers were higher than that for the main stem and increased with increasing water stress, causing high rate of tiller abortion. In aestivum , each tiller, once produced, developed leaves at the same rate as on the main stem. Phyllochron of tillers as well as main stem was not affected by water stress in aestivum. In aestivum , contribution by tillers to yield was higher than that of durum in all treatments. Results indicate that early appearance of tillers and faster rate of leaf appearance under water stress result in higher tiller survival and greater tiller contribution to final yield. Hence, tiller dynamics under water stress can be used as a selection criterion for breeding for drought tolerance. 相似文献
8.
小麦6-SFT是果聚糖合成的关键酶基因。以23份六倍体普通小麦(AABBDD)、5份D基因组材料(DD)为多样性代表群体材料,通过测序分析小麦6-SFT-D基因的序列多态性,根据多态性开发6-SFT-D基因的功能标记,分析由154份六倍体普通小麦构成的自然群体的6-SFT-D基因单倍型(haplotype)与表型性状的关联特性和基因累加效应。在28份多样性代表群体中,共检测到6-SFT-D基因的4个多态性位点,均为单核苷酸多态性(SNP)位点,构成3种6-SFT-D基因单倍型;而在自然群体中只检测到6-SFT-D的两种单倍型。根据6-SFT-D基因2850 bp位点的T/C变异开发等位变异特异PCR标记。关联分析表明,6-SFT-D单倍型分别与灌溉条件下的千粒重和穗长显著关联,单倍型Hap I是提高千粒重的优异等位变异;在雨养和灌溉条件下,同时具有6-SFT-D与6-SFT-A2优异等位变异小麦材料的千粒重显著高于其他基因型材料,说明6-SFT-D和6-SFT-A2优异等位变异对于提高千粒重表现累加效应。 相似文献
9.
The purpose of this study was to identify the species of local landraces of wheat (Triticum spp.), held in the Israel Gene
Bank, to evaluate them for basic characters and to assess their response to infection by two rust fungi under artificial inoculation
conditions. One-hundred-thirty one seed samples were collected from local or Beduin farmers during 1978–1981 throughout the
Galilee, Mt. Gilboa. Judean Desert and the south Negev. The samples were collected and stored in the Israel Gene Bank without
any characterization or evaluation. Each accession was planted in a 1 m row at Bet Dagan and grown under favorable conditions
for plant growth and rust development. Determination of the species, data of plant height, days to heading and reaction of
the landraces to artificial inoculation with a composite inoculum of Puccinia recondita and P. striiformis were collected
from each row. A small part of the landraces collection consisted of mixed populations of T. durum and T. aestivum plants,
where one of the two species was predominant. One-hundred-fourteen and 17 accessions from this collection represented, respectively,
Triticum durum and T. aestivum Israel landraces. Large variations were found for all the characters examined. Of the total
accessions, 6.5% (8 accessions) and 32% (42 accessions) were resistant, respectively, to yellow- and leaf-rust. It was concluded
that the diversified populations of the local landraces of wheat can be used as a source not only for genes affecting basic
characters such as plant height and heading date, but also for resistance to leaf rust and yellow rust.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
10.
P. Codianni G. Ronga N. DI Fonzo A. Troccoli 《Journal of Agronomy and Crop Science》1996,176(1):15-21
The aim of this study was to evaluate the performance of three species of unhulled-wheat (Triticum monococcum L., Triticum dicoccum Schubler and Triticum spelta L.) with respect to durum wheat (Triticum durum Desf. cv. Trinakria) in southern Italy's flat environment.
As far as yield is concerned, the mean yield was significantly different among species. Durum wheat yielded 16.1 %, 37.6% and 69.5 % more than dicoccum, spelta and monococcum species, respectively.
Among unhulled-wheat species, only Triticum dicoccum showed some similarity with durum wheat, such as one-thousand kernels weight, plant height and number of spikes per m2 .
Among the factors studied, the days to reach heading represented the factor that mostly influenced yield 相似文献
As far as yield is concerned, the mean yield was significantly different among species. Durum wheat yielded 16.1 %, 37.6% and 69.5 % more than dicoccum, spelta and monococcum species, respectively.
Among unhulled-wheat species, only Triticum dicoccum showed some similarity with durum wheat, such as one-thousand kernels weight, plant height and number of spikes per m
Among the factors studied, the days to reach heading represented the factor that mostly influenced yield 相似文献
11.
小麦区试品系DUS测试的分子标记 总被引：10，自引：1，他引：9
为了确定测试小麦(Triticum aestivum L.)区试品系特异性、一致性、稳定性(DUS)的分子标记,采用156个来自我国不同麦区的品种对SSR、EST-SSR和AFLP-SCAR标记的1334对引物进行筛选,根据在染色体上分布均匀、多态性信息指数较高、带型清晰、不同等位变异的带型易于区分及PCR产物稳定的原则,筛选出105对小麦品种DUS测试的分子标记引物,包括63对SSR引物、21对EST-SSR引物和21对AFLP-SCAR引物,可以检测122个位点的754个等位变异,平均每条染色体上被检测位点5.8个,平均每个位点包含7.2个等位变异。根据DUS测试的需求、引物的染色体分布、PIC值大小和带型特点,将105对引物分为21对核心引物、29对一级备用引物和55对二级备用引物。核心引物分辨力较高,可以完成约80%品系的特异性检测,约95%品系的种子纯度检测和约60%品系的一致性、稳定性检测;备用引物用于确定品系DNA位点纯合率和相似品种(品系)之间的遗传相似系数,以判断DNA指纹相同或相似的品种(品系)之间的相似性和特异性,评价核心标记中具有非纯位点的品系的DNA位点纯合度,同时完成核心引物未能完成的少数品系的种子纯度检测。通过在2006-2007、2007-2008、2008-2009年度对464个冬小麦区试品系DUS测试中的应用,证明105对引物具有很好的代表性和实用性,可以完成90%以上参试品系的DUS检测。 相似文献
12.
Three major types of B subunits of glutenin patterns were detected among 240 durum wheat lines collected from eleven countries by the one-step one-dimensional SDS-PAGE procedure. Most commercial durum lines had the LMW-2 type while extensive variation of other banding patterns was found for lines particularly from North African and Mediterranean region. A total of 281 wild and less-cultivated tetraploid wheat lines (var. dicoczoides, Triticum dicoccum and T. polonicum) were also studied for the variation of low molecular weight glutenin subunits and extensive variation was found. The complexity of banding patterns observed among the T. turgidum species indicate a rich source of glutenin variation with a potential value to be used for quality improvement of cultivated wheat when their functional properties have been tested. 相似文献
13.
A. Hadjichristodoulou 《Euphytica》1989,44(1-2):143-150
Summary Environmental correlation coefficients were computed among all pairs of five traits, namely grain yield, heading date, number of tillers per m2, plant height and 1000-grain weight (grain size) using 30 Triticum durum and 30 Triticum aestivum varieties grown in 18 environments. Grain yield was significantly correlated with the other four traits in almost all of the varieties. The mean correlation coefficient over all varieties ranged from 0.58 to –0.83 for durum wheat and 0.66 to 0.88 for aestivum wheat. The correlation coefficients between heading date and the other traits were also significant, ranging from –0.45 to –0.79 in durum wheat and –0.61 to –0.85 in aestivum wheat. The correlation coefficient between number of tillers with plant height and 1000-grain weight were the smallest, 0.19–0.32 in durum wheat and 0.39–0.60 in aestivum wheat. It was concluded that agronomic practices favouring early and good stand establishment in the dry regions will favour the yield components and important adaptive traits, which contribute towards larger yields. Significant differences were found among genotypes in the environmental correlation coefficients and the associated changes in one trait as a result of changes in other traits. 相似文献
14.
Penko Spetsov Dominique Mingeot Jean Marie Jacquemin Krassiana Samardjieva Elena Marinova 《Euphytica》1997,93(1):49-54
Winter hexaploid wheat (Triticum aestivum L.) was crossed with Aegilops variabilis to transfer resistance to powdery mildew into wheat. Following two backcrosses to wheat and from 5 to 9 generations of selfing, several disomic addition and substitution lines of hexaploid wheat resistant to the mildew pathogen were isolated. A pair of short satellited chromosomes was always observed in the resistant lines. Further evidence utilizing as markers for homoeologous group 1 HMW glutenin subunits and DNA hybridization with probe pGBX 3076 showed that an alien substitution involved this homoeologous group. 相似文献
15.
Morphological and physiological differences in the response of cereals to zinc deficiency 总被引：8，自引：0，他引：8
I. Cakmak B. Torun B. Erenoğlu L. Öztürk H. Marschner M. Kalayci H. Ekiz A. Yilmaz 《Euphytica》1998,100(1-3):349-357
Greenhouse and growth chamber experiments were carried out using seven bread wheat (Triticum aestivum), three durum wheat
(T. durum), two rye (Secale cereale), three barley (Hordeum vulgare), two triticale (x Triticosecale Wittmack) and one oat
(Avena sativa) cultivars to study response to zinc (Zn) deficiency and Zn fertilisation in nutrient solution and in a severely
Zn deficient calcareous soil. Visual Zn deficiency symptoms, such as whitish-brown necrotic patches on leaf blades, developed
rapidly and severely in the durum wheat and oat cultivars. Bread wheat showed great genotypic differences in sensitivity to
Zn deficiency. In triticale and rye, visual deficiency symptoms were either absent or appeared only slightly, while barley
showed a moderate sensitivity. When grown in soil, average decreases in shoot dry matter production due to Zn deficiency were
15% for rye, 25% for triticale, 34% for barley, 42% for bread wheat, 63% for oat and 65% for durum wheat. Differential Zn
efficiency among and within cereal species was better related to the total amount of Zn per shoot, but not to the Zn concentration
in the shoot dry matter. However, in leaves of Zn efficient rye and bread wheat cultivars, the activity of Zn-containing superoxide
dismutase was greater than in Zn inefficient bread and durum wheat cultivars, suggesting higher amounts of physiologically
active Zn in leaf tissue of efficient genotypes. When grown in nutrient solution, there was a poor relationship between Zn
efficiency and release rate of Zn-chelating phytosiderophores from roots, but uptake of labelled Zn (65Zn) and its translocation to the shoot was higher in the Zn efficient rye and bread wheat cultivars than in inefficient bread
and durum wheat cultivars. The results demonstrate that susceptibility of cereals to Zn deficiency decline in the order durum
wheat > oat > bread wheat > barley > triticale > rye. The results also show that expression of high Zn efficiency in cereals
was causally related to enhanced capability of genotypes to take up Zn from soils and use it efficiently in tissues.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
16.
O.F. Mamluk 《Euphytica》1998,100(1-3):45-50
Bunts [common bunt (Tilletia laevis and T. tritici) and dwarf bunt (T. controversa)] and smuts [loose smut (Ustilago tritici)
and flag smut (Urocystis agropyri)] of wheat are important cereal diseases in most countries of north Africa and the Near
East. There are no peculiarities in the occurrence of bunts and smuts, except for dwarf bunt, which is limited to high-altitude
areas of Turkey, Iran and Iraq. Host adaptation, rather than topography, determines the distribution of the common bunt pathogens:
T. laevis predominates in bread wheat, whereas T. tritici attacks both bread and durum wheat non-preferentially. Incidence
of bunt- and smut-affected wheat heads is generally low, however the frequency of their occurrence in fields is high, indicating
their potential threat. Some bunts and smuts occur on hosts other than wheat, for example, flag smut on Aegilops crassa, loose
smut on Ae. geniculata and rye, and dwarf bunt on Hordeum and Aegilops spp. Genetic variability of the pathogens causing common
bunt and loose smut was investigated. Chemical seed treatment is the most widely used control for bunts and smuts. Organic
nutrients as seed treatments to control common bunt of wheat show considerable promise. Sources of resistance to loose smut,
and common and dwarf bunts of wheat, are available in wheat and its wild relatives. Triticum boeoticum, T. dicoccoides, and
Aegilops species represent excellent sources of resistance to common bunt. There are three major sources of resistance in
durum wheats, Senatore Cappelli and Haurani, Jenneh Khetifa, and Mindum. Common bunt resistance genes Bt5, Bt6, Bt8, Bt9,
Bt10, and Bt11, and several undescribed resistances remain effective in the screening field at ICARDA, Syria.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
17.
V. Vallega 《Plant Breeding》1988,100(4):241-246
Variation in high-molecular-weight glutenin subunit composition amongst Triticum durum cvs. of different origins was investigated by SDS-PAGE and compared with that reported for T. dicoccum and T. aestivum. Tetraploid wheat collections (408 cvs.) were found to carry nearly twice as many Glu-A1 and Glu-B1 alleles as a hexaploid wheat sample of comparable size. In each of the taxa considered, allelic variation at the Glu-B1 locus was markedly greater than that observed for Glu-A1. However, because all the Glu-B1 subunits so far discovered exhibit a restricted and distinctive mobility range during. SDS-PAGE, it is suggested that they are derived from a single source, possibly from Aegilops searsii. Most durum cvs. carried a ‘null’Glu-A1 allele and therefore fewer subunits than dicoccums and common wheats. It is argued that differences in the frequency of occurrence of ‘null’Glu-1 alleles between taxa probably resulted from random samplings made by early agriculturalists and breeders, rather than from an inherent tendency of polyploid wheats to suppress the activity of “redundant” genes. 相似文献
18.
Waxy proteins in diploid, tetraploid and hexaploid wheats 总被引：4，自引：0，他引：4
Electrophoretic analyses of waxy proteins, encoded by genes present at the Wx‐1 loci, present in several cultivars and accessions of hexaploid wheat, Triticum aestivum, have permitted the detection of null alleles at the Wx‐B1 and Wx‐D1 loci. Polymorphism at the Wx‐A1 and Wx‐B1 loci was also investigated in several accessions of tetraploid wheats, Triticum durum, Triticum dicoccoides and Triticum timopheevi, and in diploid species, Triticum urartu, Triticum boeoticum and Triticum monococcum. One null allele at the Wx‐A1 locus and three polymorphic alleles at Wx‐B1 locus were detected in T. durum; a new allele at one of the two waxy loci was identified in the tetraploid wheat T. timopheevi; no polymorphism was detected in diploid species. Polymerase chain reaction techniques made possible the detection of further polymorphism existing at the Wx‐1 loci and the reason for the lack of expression of the null genotypes to be investigated. The null forms detected at each locus have been used to produce complete sets of partial and total waxy lines in durum and bread wheat. 相似文献
19.
Excised ears of Triticum durum (HD 4502 and B 449) and T. aestivum (Kalyansona and Kundan) varieties were cultured in 14 C-sucrose, and the uptake and distribution of 14 C within the ear was examined. Species-level differences in the distribution of 14 C to spikelets at basal, middle and apical positions in the wheat ear (vertical distribution) were observed. T. aestivum var. Kalyansona and Kundan showed no limitation in vertical translocation of 14 C-sucrose, whereas in T. durum there was a decrease in the distribution of 14 C to apical spikelets. Within a spikelet, the distribution of 14 C-sucrose to distal grains was significantly less than that to proximal grains in all the genotypes. 相似文献