全文获取类型
收费全文 | 331篇 |
免费 | 31篇 |
国内免费 | 1篇 |
出版年
2023年 | 2篇 |
2022年 | 3篇 |
2021年 | 9篇 |
2020年 | 8篇 |
2019年 | 5篇 |
2018年 | 3篇 |
2017年 | 9篇 |
2016年 | 8篇 |
2015年 | 14篇 |
2014年 | 15篇 |
2013年 | 20篇 |
2012年 | 18篇 |
2011年 | 20篇 |
2010年 | 16篇 |
2009年 | 12篇 |
2008年 | 15篇 |
2007年 | 12篇 |
2006年 | 14篇 |
2005年 | 11篇 |
2004年 | 14篇 |
2003年 | 17篇 |
2002年 | 13篇 |
2001年 | 7篇 |
2000年 | 8篇 |
1999年 | 7篇 |
1998年 | 5篇 |
1997年 | 4篇 |
1996年 | 5篇 |
1995年 | 2篇 |
1994年 | 3篇 |
1993年 | 5篇 |
1992年 | 5篇 |
1991年 | 8篇 |
1990年 | 3篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 4篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 4篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1969年 | 1篇 |
1966年 | 2篇 |
1964年 | 1篇 |
1963年 | 3篇 |
1962年 | 3篇 |
1958年 | 1篇 |
1946年 | 1篇 |
排序方式: 共有363条查询结果,搜索用时 156 毫秒
71.
72.
Paris Laskaris Sahar Tolba Leo Calvo‐Bado Liz Wellington 《Environmental microbiology》2010,12(3):783-796
We present evidence for the coexistence and coevolution of antibiotic resistance and biosynthesis genes in soil bacteria. The distribution of the streptomycin (strA) and viomycin (vph) resistance genes was examined in Streptomyces isolates. strA and vph were found either within a biosynthetic gene cluster or independently. Streptomyces griseus strains possessing the streptomycin cluster formed part of a clonal complex. All S. griseus strains possessing solely strA belonged to two clades; both were closely related to the streptomycin producers. Other more distantly related S. griseus strains did not contain strA. S. griseus strains with only vph also formed two clades, but they were more distantly related to the producers and to one another. The expression of the strA gene was constitutive in a resistance‐only strain whereas streptomycin producers showed peak strA expression in late log phase that correlates with the switch on of streptomycin biosynthesis. While there is evidence that antibiotics have diverse roles in nature, our data clearly support the coevolution of resistance in the presence of antibiotic biosynthetic capability within closely related soil dwelling bacteria. This reinforces the view that, for some antibiotics at least, the primary role is one of antibiosis during competition in soil for resources. 相似文献
73.
Summary Apterous parthenogentic females of the pea aphid,Acyrthosiphon pisum (Harris), begin to produce alate offspring soon after they have been subjected to crowding. Females which were born early in their
own parent’s reproductive period respond most strongly to crowding, producing much larger numbers of alatae than their late-born
sisters.
In contrast, the early-born daughters of most alate females do not produce winged offspring after being crowded. Some of their
later-born sisters may produce a few winged individuals, resembling in this respect the late-born daughters of the apterous
females.
Control of the production of alatae thus begins in the grandparental generation. Risk-spreading by means of differential dispersal
becomes a less uncertain venture when local populations can modify their responses to environmental changes by utilizing past
as well as present signals from their surroundings. 相似文献
74.
Paleochemistry of manganese in corals from the Galapagos Islands 总被引:7,自引:0,他引:7
Glen T. Shen Todd M. Campbell Robert B. Dunbar Gerard M. Wellington Mitchell W. Colgan Peter W. Glynn 《Coral reefs (Online)》1991,10(2):91-100
Approximately 550 measurements of Mn/Ca ratios in three corals from the western Galapagos Islands have been performed to reconstruct a 380-year history of surface ocean variability with respect to this trace element. The time period studied encompasses 1600 A.D. to 1978. Manganese is inferred to be lattice-bound in coralline aragonite at 10–50% of its seawater proportion to calcium; uncertainty about the distribution coefficient stems from inherent variability of oceanic Mn in nearshore settings. Interannual variations at Urvina Bay, Isabela Island are generally small, with the exception of a few decades during the nineteenth century. A large positive Mn/Ca anomaly found between 1821–1830 is hypothesized to have resulted from a major volcanic eruption on nearby Fernandina Island in 1825. On intrannual timescales a pronounced cycle occurs in response to seasonal upwelling. Quarterly changes in Mn/Ca are six months out-of-phase with Cd/Ca variations-a reflection of the opposite distributions of these metals in the upper waters of the eastern Pacific. High frequency reconstructions over brief time intervals from the 17th, 18th, and 20th century reveal that the seasonal onset of warm and cool phases near Galapagos has persisted for at least 340 years. A quantitative assessment of historical changes in upwelling intensity is complicated by offsets in background Mn levels recorded by different corals. One apparent longterm feature is an overall decline in skeletal Mn concentrations from 1600–1978 which results in a net decrease of 20–30%. Several possible explanations exist for this trend, ranging from accumulation of a persistent diagenetic Mn phase in fossil aragonite to a temporal shift in oceanic/atmospheric Mn fluxes reaching the surface waters of the Galapagos Islands. 相似文献
75.
Wellington Ronildo Clarindo Carlos Roberto Carvalho Maria Andréia Corrêa Mendonça 《Plant Growth Regulation》2012,68(3):533-538
Somatic embryogenesis of Coffea arabica L. has been mainly carried out in liquid medium for clonal and mass propagation of elite lines. This in vitro system involves suspension cultures of embryogenic aggregates, with high multiplication rate and unorganized growth. These characteristics are linked to the occurrence of somaclonal variation (SV), especially considering that cell aggregates are usually maintained for long periods in media supplemented with the synthetic auxin 2,4-dichlorophenoxyacetic acid. Because SV detection has been considered essential in in vitro tissue cultures, flow cytometry (FCM) was applied to verify ploidy instability in embryogenic cell aggregates of C. arabica, throughout successive subcultures. FCM allowed us to detect the occurrence of non-true-to-type aggregates in all samples collected after approximately 4 months in liquid medium. These aggregates showed octaploid and/or aneuploid cells, with DNA ploidy level being corroborated by chromosome counting. Considering this result, we recommend a limit of <4 months for true-to-type mass propagation of C. arabica cell aggregate suspensions. Besides, FCM was an important tool to detect SV at an early stage of tissue culture in this species, proving to be very useful for quality control in clonal propagation and in the introduction of somaclones to breeding programs. 相似文献
76.
Urine preferences of wild (Cavia aperea), domestic (C. porcellus), and F1 adult male guinea pigs were investigated. Males of all three types preferred female urine to male urine regardless of donor type. When given a choice between female urine of each type, males preferred conspecific urine. In choices between male urine of the three types, a conspecific preference was evident for wild and domestic but not f1 subjects. These data indicate that a loss of distinctive male and female odours has not occurred as a result of domestication. However, the urine odours of wild and domestic types have diverged. The possible effects of previous individual experience on the preferential response is discussed. 相似文献
77.
Jiang Z Yu N Kuang P Chen M Shao F Martin G Chui DH Cardoso WV Ai X Lü J 《The Journal of biological chemistry》2012,287(8):5979-5987
Tnrc6 family members (Tnrc6a/b/c) are key components of the RNA-induced silencing complex in microRNA (miRNA)-mediated gene suppression. Here, we show that Tnrc6a, also known as GW182, is selectively expressed in the yolk sac endoderm and that gene trap disruption of GW182 leads to growth arrest and apoptosis. We found that targets of miRNAs highly expressed in the yolk sac are significantly derepressed in GW182(gt/gt) mutant mice, although levels of miRNAs are not altered. Specifically, growth arrest and apoptosis phenotype are associated with significant derepression of Cdkn1a (p21), Cdkn1c (P27), Lats1, Lats2, Rb1, Rbl, Bim, and Pten, known targets of miRNAs from miR-17/20/93/106 clusters highly expressed in yolk sac endoderm. Together, these data strongly suggest that GW182 is an essential functional component in the RNA-induced silencing complex for miRNA-mediated gene silencing in vivo, and selectively regulation of miRNA activity plays an important role in the proper development of yolk sac. 相似文献
78.
Wellington Adriano Moreira Peres Luiz Antonio Carlos Bertollo Paulo Andreas Buckup Daniel Rodrigues Blanco Daniel Luis Zanella Kantek Orlando Moreira-Filho 《Reviews in Fish Biology and Fisheries》2012,22(2):519-526
The Astyanax species of the bimaculatus group include morphologically similar representatives that share the same color pattern, corresponding to at least 20 species.
Specimens from the S?o Francisco River basin and from the Grande River (Upper Paraná basin) are included in this group. In
the early 1960’s, the Piumhi River, an original tributary of the Grande River, was transposed to the S?o Francisco River basin
due to the construction of the Furnas Hydroelectric Power, in the state of Minas Gerais, Brazil. In this context, we characterized
representatives of the bimaculatus group from both basins and from the transposition region of the Piumhi River, using chromosome characteristics obtained by
conventional analyzes and fluorescence in situ hybridization with rDNA probes. All the analyzed specimens presented 2n = 50
chromosomes and similar localization of the nucleolus organizer regions (Ag-NORs), 18S rDNA and 5S rDNA sites. However, diagnostic
chromosome differences were detected concerning the number of submetacentric and acrocentric chromosomes in the karyotypes,
making possible the characterization of five distinct karyotypes (Cytotypes A–E), where the specimens from the S?o Francisco
and from the Grande River correspond to Cytotypes A and B, respectively. In addition, these two Cytotypes were sympatrically
found in the Piumhi River with specimens carrying a clear intermediate karyotype (Cytotype C), as well as with others harboring
different karyotypes (Cytotypes D and E). Our data indicate that Cytotypes A and B probably diverged from each other during
the prior geographic isolation. However, despite their chromosomal differentiations, they still maintain a close relationship
which allows interbreeding and the formation of a secondary hybrid zone, due to the breakdown of the geographic isolation. 相似文献
79.
Chimere O. Agomo Wellington A. Oyibo Rose I. Anorlu Philip U. Agomo 《The Korean journal of parasitology》2009,47(2):179-183
Prevalence rates reported for malaria in pregnancy in Nigeria vary considerably. The accuracy of results of malaria diagnosis is dependent on training, experience, and motivation of the microscopist as well as the laboratory facility available. Results of training programmes on malaria microscopy have shown low levels of sensitivity and specificity of those involved in malaria diagnosis routinely and for research. This study was done to ascertain the true prevalence of malaria in pregnancy in Lagos, South-West Nigeria. A total of 1,084 pregnant women were recruited into this study. Blood smears stained with Giemsa were used for malaria diagnosis by light microscopy. Malaria infection during pregnancy presents mostly as asymptomatic infection. The prevalence of malaria in this population was 7.7% (95% confidence interval; 6.2-9.4%). Factors identified to increase the risk of malaria infection include young maternal age (< 20 years), and gravidity (primigravida). In conclusion, this study exposes the over-diagnosis of malaria in pregnancy and the need for training and retraining of laboratory staffs as well as establishing the malaria diagnosis quality assurance programme to ensure the accuracy of malaria microscopy results at all levels. 相似文献
80.
Men and women at Northwest University (n=751), Xi'an, China were asked to judge the attractiveness of photographs of female patients who had undergone micrograft surgery to reduce their waist-to-hip ratios (WHR). Micrograft surgery involves harvesting adipose tissue from the waist and reshaping the buttocks to produce a low WHR and an 'hourglass' female figure. This gynoid distribution of female body fat has been shown to correlate with measures of fertility and health. Significantly larger numbers of subjects, of both sexes, chose post-operative photographs, with lower WHRs, as more attractive than pre-operative photographs of the same women. Some patients had gained, and some had lost weight, post-operatively, with resultant changes in body mass index (BMI). However, these changes in BMI were not related to judgments of attractiveness. These results show that the hourglass female figure is rated as attractive in China, and that WHR, rather than BMI, plays a crucial role in such attractiveness judgments. 相似文献