全文获取类型
收费全文 | 1737篇 |
免费 | 110篇 |
国内免费 | 2篇 |
专业分类
1849篇 |
出版年
2023年 | 4篇 |
2022年 | 24篇 |
2021年 | 28篇 |
2020年 | 30篇 |
2019年 | 20篇 |
2018年 | 41篇 |
2017年 | 30篇 |
2016年 | 47篇 |
2015年 | 84篇 |
2014年 | 83篇 |
2013年 | 106篇 |
2012年 | 118篇 |
2011年 | 142篇 |
2010年 | 85篇 |
2009年 | 65篇 |
2008年 | 112篇 |
2007年 | 112篇 |
2006年 | 105篇 |
2005年 | 95篇 |
2004年 | 73篇 |
2003年 | 80篇 |
2002年 | 56篇 |
2001年 | 17篇 |
2000年 | 15篇 |
1999年 | 20篇 |
1998年 | 22篇 |
1997年 | 14篇 |
1996年 | 16篇 |
1995年 | 10篇 |
1994年 | 20篇 |
1993年 | 9篇 |
1992年 | 4篇 |
1991年 | 7篇 |
1990年 | 7篇 |
1986年 | 5篇 |
1984年 | 8篇 |
1983年 | 6篇 |
1982年 | 5篇 |
1981年 | 4篇 |
1980年 | 10篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1977年 | 12篇 |
1976年 | 10篇 |
1975年 | 21篇 |
1974年 | 6篇 |
1973年 | 7篇 |
1969年 | 5篇 |
1957年 | 3篇 |
1956年 | 4篇 |
排序方式: 共有1849条查询结果,搜索用时 0 毫秒
51.
52.
Ernesto Maldonado Fabiola Hernandez Carlos Lozano Marta E. Castro Rosa E. Navarro 《Pigment cell & melanoma research》2006,19(4):315-326
Hypopigmentation is a characteristic of several diseases associated with vesicle traffic defects, like the Hermansky–Pudlak, Chediak–Higashi, and Griscelli syndromes. Hypopigmentation is also a characteristic of the zebrafish mutant vps18hi2499A, which is affected in the gene vps18, a component of the homotypic fusion and protein sorting complex that is involved in tethering during vesicular traffic. Vps18, as part of this complex, participates in the formation of early endosomes, late endosomes, and lysosomes. Here, we show that Vps18 is also involved in the formation of melanosomes. In the zebrafish mutant vps18hi2499A the retroviral insertion located at exon 4 of vps18, leads to the formation of two abnormal splicing variants lacking the coding sequence for the clathrin repeat and the RING finger conserved domains. A deficiency of Vps18 in zebrafish larvae results in hepatomegaly and skin hypopigmentation. We also observed a drastic reduction in the number of melanosomes in the eye's retinal pigmented epithelium along with the accumulation of immature melanosomes. A significant reduction in the vps18hi2499A larvae visual system capacity was found using the optokinetic response assay. We propose that the insertional mutant vps18hi2499A can be used as a model for studying hypopigmentation diseases in which vesicle traffic problems exist. 相似文献
53.
Glands play a central role in the social behavior of insects, and comparative analysis of their structure may provide insights about their evolution and their role in insect social biology. Here, we describe the glands of the fifth metasomal sternite of ten polistine wasps with different social behaviors. The glands of foragers of these species were studied using semithin sections of Araldite‐embedded tissue, and scanning electron microscopy. The structure of the glands observed was mostly similar to what has previously been reported for independent‐founder and swarm‐founder species, respectively. Scale‐shaped modifications in the fifth sternite are reported for the first time for members of the genera Synoeca and Epipona. Two previously unreported glands in the fifth sternite are also described. A more restrictive definition of the name Richards' gland is proposed, applying to the class 3 cells associated with scale modifications of the fifth sternite, based on structural differences between the glands observed, homology requirements, and functional evidence. Previous phylogenetic reconstructions of this character suggest that it appeared early in the Vespidae and disappeared four times, with a single reappearance in the genus Vespula. The restricted definition of Richards' gland suggests that this structure is an exclusive trait of the Epiponini, with two reversals. 相似文献
54.
Ernesto Buiza Elena I. Rodríguez-Martínez Catarina I. Barriga-Paulino Antonio Arjona Carlos M. Gómez 《Cognitive neurodynamics》2018,12(3):255-269
The possibility that power-to-power (theta–beta) frequency coupling increases during development was analyzed. Three minutes of spontaneous EEG in an open eyes condition were recorded in a sample of 160 subjects ranging from 6 to 26 years old. Theta (4–7 Hz) and beta band (15–20 Hz) power was calculated in a trial-by-trial basis. Inter-electrode power correlations (IPC) were computed in each subject as the correlation between the power of two frequency bands recorded in two electrodes. An increase in theta–beta IPC with age was obtained. IPCs were higher when theta was seeded in posterior regions than in anterior or central regions. Moreover, the significant correlations between each individual IPC and age were calculated, making it possible to draw IPC versus age correlation maps in order to capture the IPC development topography. An increase was found in significant correlations in the left hemisphere compared to the right hemisphere. There were no differences in the inter-hemispheric versus intra-hemispheric IPC maturation spatial patterns. An increase in power-to-power–frequency coupling in theta–beta occurs during development, suggesting an increase in functional connectivity with age. Frequency coupling between theta and beta rhythms would be one of the mechanisms facilitating integration of long distance functional networks during development. 相似文献
55.
de Navas LF Reed H Akam M Barrio R Alonso CR Sánchez-Herrero E 《Development (Cambridge, England)》2011,138(1):107-116
Although most metazoan genes undergo alternative splicing, the functional relevance of the majority of alternative splicing products is still unknown. Here we explore this problem in the Drosophila Hox gene Ultrabithorax (Ubx). Ubx produces a family of six protein isoforms through alternative splicing. To investigate the functional specificity of the Ubx isoforms, we studied their role during the formation of the Drosophila halteres, small dorsal appendages that are essential for normal flight. Our work shows that isoform Ia, which is encoded by all Ubx exons, is more efficient than isoform IVa, which lacks the amino acids coded by two small exons, in controlling haltere development and regulating Ubx downstream targets. However, our experiments also demonstrate that the functional differences among the Ubx isoforms can be compensated for by increasing the expression levels of the less efficient form. The analysis of the DNA-binding profiles of Ubx isoforms to a natural Ubx target, spalt, shows no major differences in isoform DNA-binding activities, suggesting that alternative splicing might primarily affect the regulatory capacity of the isoforms rather than their DNA-binding patterns. Our results suggest that to obtain distinct functional outputs during normal development genes must integrate the generation of qualitative differences by alternative splicing to quantitative processes affecting isoform protein expression levels. 相似文献
56.
Paula C. Rodríguez-Flores Jorge Gutiérrez-Rodríguez Ernesto F. Aguirre-Ruiz Mario García-París 《ZooKeys》2016,(561):63-103
The tiger beetle assemblage of the wetlands of La Mancha (central Spain) comprises nine species: Calomera
littoralis
littoralis, Cephalota
maura
maura, Cephalota
circumdata
imperialis, Cephalota
dulcinea, Cicindela
campestris
campestris, Cicindela
maroccana, Cylindera
paludosa, Lophyra
flexuosa
flexuosa, and Myriochila
melancholica
melancholica. This assemblage represents the largest concentration of tiger beetles in a single 1º latitude / longitude square in Europe. General patterns of spatial and temporal segregation among species are discussed based on observations of 1462 specimens registered during an observation period of one year, from April to August. The different species of Cicindelini appear to be distributed over space and time, with little overlapping among them. Three sets of species replace each other phenologically as the season goes on. Most of the species occupy drying or dried salt lakes and salt marshes, with sparse vegetation cover. Spatial segregation is marked in terms of substrate and vegetation use. Calomera
littoralis and Myriochila
melancholica have been observed mainly on wet soils; Cephalota
circumdata on dry open saline flats; Cephalota
dulcinea and Cylindera
paludosa in granulated substrates with typical halophytic vegetation; Cephalota
maura is often present in man-modified areas. Cephalota
circumdata and Cephalota
dulcinea are included as species of special interest in the list of protected species in Castilla–La Mancha. Conservation problems for the Cicindelini assemblage arise from agricultural activities and inadequate use of sport vehicles. Attempts at restoring the original habitat, supressing old semi-industrial structures, may affect the spatial heterogeneity of the lakes, and have an effect on Cicindelinae diversity. 相似文献
57.
Colonies of honeybees (Apis mellifera L.) were established thatvaried in the proportions of their workers that were of Europeanand hybrid (Africanized x European) descent. Colony defensiveresponses increased with higher proportions of hybrid workers.Colonies consisting exclusively of hybrid workers did not differin their response from "pure" Africanized colonies, suggestingthat the strong defensive behavior of Africanized workers isgenetically dominant. European workers became more defensivein colonies that also contained hybrid workers, whereas hybridworkers became less defensive in the same mixed colonies. Inmixed colonies hybrid workers were individually more likelythan Europeans to sting a leather target but not more likelyto guard the entrance. 相似文献
58.
Valeriano Parravicini Sébastien Villéger Tim R. McClanahan Jesus Ernesto Arias‐González David R. Bellwood Jonathan Belmaker Pascale Chabanet Sergio R. Floeter Alan M. Friedlander François Guilhaumon Laurent Vigliola David Mouillot 《Ecology letters》2014,17(9):1101-1110
The impact of anthropogenic activity on ecosystems has highlighted the need to move beyond the biogeographical delineation of species richness patterns to understanding the vulnerability of species assemblages, including the functional components that are linked to the processes they support. We developed a decision theory framework to quantitatively assess the global taxonomic and functional vulnerability of fish assemblages on tropical reefs using a combination of sensitivity to species loss, exposure to threats and extent of protection. Fish assemblages with high taxonomic and functional sensitivity are often exposed to threats but are largely missed by the global network of marine protected areas. We found that areas of high species richness spatially mismatch areas of high taxonomic and functional vulnerability. Nevertheless, there is strong spatial match between taxonomic and functional vulnerabilities suggesting a potential win–win conservation‐ecosystem service strategy if more protection is set in these locations. 相似文献
59.
Ernesto Shimizu Analía Macías Fernando Paolicchi Gabriel Magnano Laura Zapata Analía Fernández Ana Canal Sergio Garbaccio Angel Cataldi Karina Caimi Martín Zumárraga 《Memórias do Instituto Oswaldo Cruz》2014,109(2):236-245
Mycobacterium bovis is the causative agent of bovine tuberculosis
(TB), a disease that affects approximately 5% of Argentinean cattle. Among the
molecular methods for genotyping, the most convenient are spoligotyping and variable
number of tandem repeats (VNTR). A total of 378 samples from bovines with visible
lesions consistent with TB were collected at slaughterhouses in three provinces,
yielding 265 M. bovis spoligotyped isolates, which were distributed
into 35 spoligotypes. In addition, 197 isolates were also typed by the VNTR method
and 54 combined VNTR types were detected. There were 24 clusters and 27 orphan types.
When both typing methods were combined, 98 spoligotypes and VNTR types were observed
with 27 clusters and 71 orphan types. By performing a meta-analysis with previous
spoligotyping results, we identified regional and temporal trends in the population
structure of M. bovis. For SB0140, the most predominant spoligotype
in Argentina, the prevalence percentage remained high during different periods,
varying from 25.5-57.8% (1994-2011). By contrast, the second and third most prevalent
spoligotypes exhibited important fluctuations. This study shows that there has been
an expansion in ancestral lineages as demonstrated by spoligotyping. However, exact
tandem repeat typing suggests dynamic changes in the clonal population of this
microorganism. 相似文献
60.
Ernesto G. Verga Gerardo C. Leynaud Julián N. Lescano Laura M. Bellis 《European Journal of Wildlife Research》2012,58(5):823-832
The High Mountains of Córdoba, Argentina have a long evolutionary history of grazing by large herbivores. However, about 400?years ago, European livestock were introduced and gradually replaced native herbivores. Since the 1920s, domestic herbivores have been the only large herbivores present in the area, causing severe soil erosion and a threat to the system diversity. The endemic fauna of the region includes four amphibian species. We evaluated the effect of livestock rearing on amphibian diversity and water bodies in woodlands and grasslands of the High Mountains of Córdoba. The work was conducted on stream stretches and ponds in two contrasting grazing situations: an area with livestock presence and another area where livestock was excluded 14?years ago. The following variables were recorded at each sampling site: amphibian richness and abundance, percentage of emergent, submerged and peripheral vegetation in areas surrounding the water bodies, water pH, and water dissolved O2. No significant differences were detected in amphibian diversity between streams of both grazing situations, whereas a greater diversity (p?<?0.01) was observed in ponds in grazed grasslands. Our results suggest that livestock rearing, qualitatively measured as grazing and 14?years of livestock exclusion, in the study area would not have negative effects on amphibian diversity. This finding might be due to the long evolutionary grazing history of the area, large-scale livestock exclusion exhibiting a novel scenario. The ongoing reintroduction of native grazers may provide the benefits of grazing without the consequent soil erosion and habitat degradation associated with domestic livestock. 相似文献