首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   446篇
  免费   45篇
  国内免费   24篇
  515篇
  2024年   3篇
  2023年   10篇
  2022年   2篇
  2021年   5篇
  2020年   6篇
  2019年   19篇
  2018年   11篇
  2017年   16篇
  2016年   20篇
  2015年   20篇
  2014年   28篇
  2013年   49篇
  2012年   13篇
  2011年   25篇
  2010年   23篇
  2009年   24篇
  2008年   24篇
  2007年   19篇
  2006年   9篇
  2005年   8篇
  2004年   21篇
  2003年   19篇
  2002年   21篇
  2001年   18篇
  2000年   20篇
  1999年   18篇
  1998年   18篇
  1997年   16篇
  1996年   11篇
  1995年   6篇
  1994年   3篇
  1993年   3篇
  1992年   2篇
  1991年   2篇
  1989年   1篇
  1988年   1篇
  1986年   1篇
排序方式: 共有515条查询结果,搜索用时 15 毫秒
21.
North-Atlantic records of Schizymenia dubyi extend along the eastern shores of the North Atlantic from Morocco to southern Britain and Ireland, and the species is also recorded from Iceland. A study was undertaken to confirm the identity of the specimens from Iceland that were geographically separate from the main distribution of S. dubyi and in contrast to other species of the genus did not have gland cells. We analyzed rbcL and COI molecular sequence data from Icelandic specimens and compared the results with those for Schizymenia specimens available in GenBank. For both markers, Schizymenia was shown to be a monophyletic genus. The Icelandic specimens were clearly genetically distinct from S. dubyi and formed a well-supported clade with Schizymenia species from the Northern Pacific. Based on these results, we have described a new species, Schizymenia jonssonii, which can be distinguished by molecular phylogeny, its lack of gland cells and by being strictly intertidal. Crustose tetrasporophytes with identical COI and rbcL sequences were found at the same locations as foliose plants. Schizymenia apoda is reported for the first time in the UK, its identity confirmed by rbcL sequence data. In light of these findings, it is likely that by further molecular analysis of the genus Schizymenia in the north-eastern Atlantic and the Mediterranean, a higher diversity of Schizymenia spp. will be discovered in this region.  相似文献   
22.
23.
Abstract Bambusoideae is an important subfamily of the grass family Poaceae that has considerable economic, ecologic and cultural value. In addition, Bambusoideae species are important constituents of the forest vegetation in China. Because of the paucity of flower‐bearing specimens and homoplasies of morphological characters, it is difficult to identify species of Bambusoideae using morphology alone, especially in the case of temperate woody bamboos (i.e. Arundinarieae). To this end, DNA barcoding has shown great potential in identifying species. The present study is the first attempt to test the feasibility of four proposed DNA barcoding markers (matK, rbcL, trnHpsbA, and internal transcribed spacer [ITS]) in identifying 27 species of the temperate woody bamboos. Three plastid markers showed high levels of universality, whereas the universality of ITS was comparatively low. A single plastid marker provided low levels of discrimination success at both the genus and species levels (<12%). Among the combinations of plastid markers, the highest discriminatory power was obtained using the combination of rbcL+matK (14.8%). Using a combination of three markers did not increase species discrimination. The nuclear region ITS alone could identify 66.7% of species, although fewer taxa were included in the ITS analyses than in the plastid analyses. When ITS was integrated with a single or combination of plastid markers, the species discriminatory power was significantly improved. We suggest that a combination of rbcL+ ITS, which exhibited the highest species identification power of all combinations in the present study, could be used as a potential DNA barcode for temperate woody bamboos.  相似文献   
24.
利用聚合酶链式反应扩增胶州湾表层海水浮游植物核酮糖1,5-二磷酸羧化/氧化酶大亚基基因(rbcL)片段,建立了该基因片段变异类型文库.随机测定了28个rbcL片段序列,依此初步分析了胶州湾表层海水浮游植物rbcL基因分子遗传多样性.结果表明,春季胶州湾表层海水浮游植物优势种群为D类rbcL代表的浮游植物,其中隐藻占28.6%、Stramenopies占32.1%、定鞭藻占28.6%、红藻占3.6%.B类rbcL代表的浮游植物为绿藻,占7.1%.根据各操作分类单元丰度计算的分子遗传多样性指数为2.85,根据逆翻译成的氨基酸序列计算的序列多样性为0.20,  相似文献   
25.
The canal-bearing diatom genus Nagumoea, described based on only morphological evidence, was tentatively assigned to the order Bacillariales, although its phylogenetic position remained unclear. Because three isolates of Nagumoea (SK002, SK024 and SK053) were successfully established from Japanese coasts, we performed their morphological observations and molecular phylogenetic analyses to discuss the phylogeny and taxonomic position of this genus. Strains SK002 and SK024 were identified as Nagumoea africana, whereas SK053 conformed with Nagumoea serrata. There was high interspecific divergence between N. africana and N. serrata in the rbcL sequences (8.03–8.17%), indicating their distinctness. Furthermore, intraspecific variations were detected within N. africana (2.35%) in the rbcL, implying its cryptic diversity. The maximum likelihood and Bayesian phylogenetic trees inferred from the plastid rbcL, psbC and nuclear 18S rDNA genes recovered Nagumoea as monophyletic with strong statistical support and embedded within an unresolved, poorly supported lineage containing Achnanthes, Craspedostauros, Staurotropis and Undatella in the canal-bearing order Bacillariales (= the family Bacillariaceae). Although the constrained tree based on the monophyly of Nagumoea and the other canal-bearing clade (Surirellales and Rhopalodiales) was statistically rejected by the topology tests, the phylogenetic position of Nagumoea with other Bacillarialean members remains equivocal. The possession of two plastids positioned fore and aft, observed in the present study, and lack of keel, typical of the Bacillariales, indicate the possibility of Nagumoea being part of the ingroup of the Bacillariales or its closely related outgroup.  相似文献   
26.
The chemical constituents of the root extracts and the nucleotide sequences of the atpB-rbcL intergenic region of Ligularia latihastata and L. villosa, collected in northwestern Yunnan Province, were studied. In the twelve collected samples of L. latihastata, two major benzofurans, 5,6-dimethoxy-2-(1-methylethenyl)-1-benzofuran (1) and euparin (2) were detected as major components. The minor compound (2R*,3S*)-5-acetyl-2,3-dihydro-6-hydroxy-2-(1-methylethenyl)-1-benzofuran-3-yl (2Z)-2-[(acetoxy)methyl]but-2-enoate (4) was found to be susceptible to artifact formation upon extraction with EtOH. The intra-specific diversity in chemical composition of the samples was small, but the diversity in the atpB-rbcL sequence was fairly large. Compounds 1 and 2 were also found in the three collected samples of L. villosa, indicating that the two species are chemically close to each other, in agreement with morphological taxonomy.  相似文献   
27.
Primer sequences are described for amplifying and sequencing a large fragment (approximately 2500 b.p.) of the nuclear-encoded large-subunit ribosomal RNA gene (LSU) from red algae. In comparison to RuBisCo large-subunit gene (rbcL) and nuclear-encoded small-subunit ribosomal RNA gene (SSU) sequence data, LSU sequence data was intermediate in the number of phylogenetically informative positions and sequence divergence. Parsimony analysis of LSU sequences for 16 Gelidiales species resolved some nodes unresolved in rbcL and SSU parsimony trees. An analysis of LSU sequences from 13 species of red algae classified in 11 orders suggests that this gene may be useful in studies of higher-level relationships of red algae.  相似文献   
28.
29.
The simple and convergent morphologies of many red algae make these species difficult to identify using traditional morphological characters. Many cryptic species have been described in recent years based on molecular datasets, and this has led to the application of an integrative taxonomy approach in species delimitation. Here, we performed several species delimitation methods (mBGD, ABGD, SPN, PTP, GMYCs and GMYCm) based on two different loci (COI-5P and rbcL) in species of the Hypnea cornuta complex. These methods were combined with morphological and phylogenetic data, extensive sampling, analysis of topotype material, and historically relevant herbarium samples. Our findings demonstrate that the groups morphologically assigned to H. cornuta and H. stellulifera consist of five different cryptic species. H. cornuta is a polyphyletic taxon composed of three well-separated lineages, thus requiring sequencing of type or topotype specimens to determine which one is Hypnea cornuta sensu stricto. We have revealed that the distribution of H. stellulifera is limited to Asia, while the Brazilian specimens initially assigned to this species were clarified as a new endemic species: Hypnea cryptica sp. nov. Our results indicated that only an integrative approach combining several lines of evidence, including morphology, nomenclature history, molecular data, biogeography and ecology can correctly solve the taxonomic status of widely distributed cryptic species.  相似文献   
30.
The intraspecific diversity in L. tsangchanensis collected in the Chinese Provinces Yunnan and southwestern Sichuan was studied by chemical and genetic approaches. The samples collected in Yunnan were found to contain cacalol (1) as the sole major component, while samples from Sichuan contained 7alpha- and 7beta-eremophila-9,11-dien-8-one (5 and 6) as well as the 3alpha-angeloyloxy derivative 7 as major components. In addition, the sequences of the internal transcribed spacers (ITSs) of the ribosomal RNA gene indicated that the Yunnan and the Sichuan samples constitute separate clades. These results demonstrate that L. tsangchanensis in Yunnan and Sichuan are distinct.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号