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   2474篇
  免费   205篇
  国内免费   207篇
林业   136篇
农学   423篇
基础科学   5篇
  72篇
综合类   976篇
农作物   135篇
水产渔业   376篇
畜牧兽医   453篇
园艺   87篇
植物保护   223篇
  2024年   8篇
  2023年   34篇
  2022年   49篇
  2021年   50篇
  2020年   83篇
  2019年   89篇
  2018年   54篇
  2017年   72篇
  2016年   108篇
  2015年   90篇
  2014年   104篇
  2013年   120篇
  2012年   150篇
  2011年   161篇
  2010年   137篇
  2009年   140篇
  2008年   147篇
  2007年   117篇
  2006年   149篇
  2005年   95篇
  2004年   92篇
  2003年   82篇
  2002年   81篇
  2001年   69篇
  2000年   48篇
  1999年   45篇
  1998年   53篇
  1997年   39篇
  1996年   39篇
  1995年   36篇
  1994年   46篇
  1993年   40篇
  1992年   39篇
  1991年   46篇
  1990年   36篇
  1989年   29篇
  1988年   25篇
  1987年   18篇
  1986年   19篇
  1985年   6篇
  1984年   4篇
  1983年   2篇
  1982年   7篇
  1981年   4篇
  1979年   7篇
  1978年   3篇
  1977年   5篇
  1976年   6篇
  1975年   1篇
  1962年   1篇
排序方式: 共有2886条查询结果,搜索用时 62 毫秒
971.
SOX9基因在脊椎动物性别决定和性腺分化中扮演重要的调控作用。从mRNA和蛋白水平分析中华鳖SOX9基因在不同组织中的差异性表达、在胚胎性腺和成年睾丸中的细胞定位以及在性逆转中的表达变化,研究SOX9基因在中华鳖性别分化中的调控作用。Real-time PCR结果显示,SOX9基因在中华鳖雄性性腺中特异性表达。免疫荧光染色分析显示,SOX9蛋白在雄性18期胚胎性腺中开始表达,随着性腺的发育,SOX9蛋白定位于性腺Sertoli前体细胞细胞核中;而在雌性胚胎性腺并未见其表达。此外,在雌激素诱导的雄性向雌性性逆转胚胎中,SOX9基因显著下调,而在芳香化酶抑制剂诱导的雌性向雄性性逆转胚胎中,SOX9基因表达则显著上升。研究表明,SOX9基因为中华鳖雄性特异性基因,参与雄性性腺的发育过程,可能在中华鳖早期性别分化过程中起调控作用。  相似文献   
972.
The wild relatives of rice (Oryza sativa L.) are useful sources of alleles that have evolved to adapt in diverse environments around the world. Oryza rufipogon, the known progenitor of the cultivated rice, harbors genes that have been lost in cultivated varieties through domestication or evolution. This makes O. rufipogon an ideal source of value-added traits that can be utilized to improve the existing rice cultivars. To explore the potential of the rice progenitor as a genetic resource for improving O. sativa, 33 chromosome segment substitution lines (CSSLs) of O. rufipogon (W0106) in the background of the elite japonica cultivar Koshihikari were developed and evaluated for several agronomic traits. Over 90% of the entire genome was introgressed from the donor parent into the CSSLs. A total of 99 putative QTLs were detected, of which 15 were identified as major effective QTLs that have significantly large effects on the traits examined. Among the 15 major effective QTLs, a QTL on chromosome 10 showed a remarkable positive effect on the number of grains per panicle. Comparison of the putative QTLs identified in this study and previous studies indicated a wide genetic diversity between O. rufipogon accessions.  相似文献   
973.
To study the genetic basis of rice flag leaf morphology, quantitative genetic analysis was conducted in a population of 37 chromosome segment substitution lines (CSSLs) of indica elite variety ‘Habataki’ in the background of japonica cultivar ‘Sasanishiki’ across three different environments. The CSSLs showed normal distribution with transgressive segregation, indicating that these four traits are controlled by polygenes. Moreover, analyses of variance showed that these traits were highly influenced by the growing environment, which are typical for polygenic quantitative traits. Seven quantitative trait loci (QTLs) on four chromosomes were detected in total: four for flag leaf width, one for flag leaf area and two for flag leaf angle. Two key QTLs, qFLW4 and qFLAG5 controlling flag leaf width and angle, respectively, were identified in all three environments. These QTLs could provide useful information for marker‐assisted selection in improving the performance of plant architecture with regard to leaf angle and area. Moreover, developed CSSLs with these QTLs information are also useful research materials to reveal the importance of leaf morphology in relation to grain yield.  相似文献   
974.
【目的】建立苹果DNA纤维荧光原位杂交(Fiber FISH)技术体系,从而为利用该技术确定苹果基因组DNA序列间的位置关系、构建精细物理图谱等方面的应用奠定基础。【方法】以‘Florina’苹果幼叶为试材,通过液氮研磨,尼龙膜过滤和TritonX-100去除叶绿素等步骤提取细胞核。细胞核经碱裂解,采用盖玻片拉伸方法制备DNA纤维。比较细胞核不同裂解时间和在5种不同包被类型的载玻片上DNA纤维拉伸的效果。用碱裂解方法提取来自苹果‘Florina’自交不亲和S9基因座的4个细菌人工染色体(Bacterium Artificial Chromosome)BAC 34G16、 BAC 45M19、BAC 70J19和BAC 69A4。提取的质粒经PEG纯化,用地高辛或生物素标记探针。探针与DNA纤维制片经过80℃变性、37℃杂交2-3 d和洗片,采用“三明治”方法进行信号放大和检测,在荧光显微镜下观察试验结果。【结果】建立了以苹果幼叶为材料,液氮研磨提取细胞核,碱裂解细胞核和盖玻片拉伸制备DNA纤维的试验方法。提取的细胞核纯净、结构完整,细胞核浓度>5×103个/μL。试验结果表明,细胞核裂解4 min,在多聚赖氨酸包被的载玻片上制备的DNA纤维平直、伸展均匀,纤维量多、细长,效果好。经原位杂交和信号检测,获得了清晰的、具有Fiber FISH典型特征的“念珠状”杂交信号。对已知大小和位置关系的两个BAC克隆BAC 34G16和BAC 45M19进行杂交信号测量和分析,得出BAC 克隆大小(Y,Kb)与信号长度(X,μm)的相关方程为Y=3.47X(R2=0.9215),其斜率即为该试验技术体系Fiber FISH的分辨率3.47 kb•μm-1。试验对未知大小和位置关系的两个BAC克隆BAC 70J19和BAC 69A4成功地进行了鉴定,它们大小分别为(112.1±18.4)kb和(133.2±16.3 )kb,之间有(90.2±7.3)kb的重叠区域。【结论】建立了以苹果幼叶提取细胞核,制备DNA纤维和原位杂交的方法,获得了高分辨率的苹果DNA纤维原位杂交试验技术体系。  相似文献   
975.
随机采集体长18~20cm、体重200~250g的红鳍东方鲀(Takifugu rubripes)44尾,解剖观察并记录性腺组织外观,然后利用组织切片技术鉴定个体的性别。结果表明:性腺为细长条状且呈乳白色的个体为雄性个体,而性腺为袋状且呈淡红色的个体为雌性个体,且样本群体中的雌雄性别比例为1∶1。本研究结果为通过性腺外观形状和颜色即可准确鉴定体长为20cm左右的红鳍东方鲀的性别提供了参考依据。  相似文献   
976.
研究了全同胞家系的红鳍东方鲀(Takifugu rubripes)早期幼苗在4个不同温度(15℃、17℃、19℃和正常养殖温度21~25℃)条件下,水温对其生长性状和雌雄性别组合的影响。结果表明,培育温度越低,鱼苗生长速度越慢。对照组(正常养殖温度21~25℃)的幼苗生长最快,其次依次为19℃、17℃和15℃。30~90日龄,不同温度处理组绝对增长率和生长指标具有相似变化趋势。70日龄前,正常养殖温度组的绝对增长率和生长指标最高;80~90日龄,19℃组最高。统计180日龄各温度组雄性和雌性个体比例,利用组织学切片法辨别雄雌。15℃组雄性和雌性比例为86.67%和13.33%,17℃组为70.00%和30.00%,19℃组为56.67%和43.33%,正常养殖温度组为53.33%和46.67%。低温能够有效提高红鳍东方鲀家系雄性个体比例,15℃组为最佳诱导温度处理组。  相似文献   
977.
根据已克隆的田旋花( Convolvulus arvensis L.) EPSPS基因mRNA序列设计引物,通过染色体步移技术获得了1142 bp的EPSPS-P启动子片段( GenBank 登录号:KC107822)。对启动子序列分析显示,该片段富含A/T碱基、TATA-box、CAAT-box及其他作用元件如GATA-motif、TC-rich repeats、sp1等。将该启动子与GUS报告基因连接以构建植物表达载体,利用农杆菌介导法获得转基因拟南芥;PCR和GUS组织化学染色分析证明, EPSPS-P已转化到拟南芥中。  相似文献   
978.
The evolution of sex is one of the most important and controversial problems in evolutionary biology. Although sex is almost universal in higher animals and plants, its inherent costs have made its maintenance difficult to explain. The most famous of these is the twofold cost of males, which can greatly reduce the fecundity of a sexual population, compared to a population of asexual females. Over the past century, multiple hypotheses, along with experimental evidence to support these, have been put forward to explain widespread costly sex. In this review, we outline some of the most prominent theories, along with the experimental and observational evidence supporting these. Historically, there have been 4 classes of theories: the ability of sex to fix multiple novel advantageous mutants (Fisher-Muller hypothesis); sex as a mechanism to stop the build-up of deleterious mutations in finite populations (Muller's ratchet); recombination creating novel genotypes that can resist infection by parasites (Red Queen hypothesis); and the ability of sex to purge bad genomes if deleterious mutations act synergistically (mutational deterministic hypothesis). Current theoretical and experimental evidence seems to favor the hypothesis that sex breaks down selection interference between new mutants, or it acts as a mechanism to shuffle genotypes in order to repel parasitic invasion. However, there is still a need to collect more data from natural populations and experimental studies, which can be used to test different hypotheses.  相似文献   
979.
摘 要 : 利用人工合成大豆食心虫性诱剂进行大豆食心虫田间种群动态调查,结果表明:2006年哈尔滨地区大豆食心虫的发生期大约为1个月,高峰期为8月7日至8月14日左右,与传统的目测惊蛾法调查的结果基本一致。4种不同田间诱蛾方式研究表明:水盆的诱蛾效果好于诱捕器,加入少量杀虫剂后诱捕器防治效果达到43.82%,而水盆加入少量杀虫剂后效果没有提高。2007年大豆食心虫性诱剂在哈尔滨地区开展大面积防治试验,防治效果为45.9%。2008年单用性诱剂+诱捕器防效为50.0%,在诱捕器中加入少量杀虫剂防治效果达62.7%。  相似文献   
980.
Tetrasomic plants with two additional small chromosomes were identified, with a frequency about 2.5%, in the trisomic pea line TRUST-R, which normally contains one extra chromosome covering a sporophyte lethal in the regular chromosome set. As compared to trisomics, tetrasomics exhibited an enhanced expression of the traits resulting from extra chromosome addition: slow growth, enlarged bracts, shortened peduncles, wavy leaflets and stipulae. They were almost sterile, their pollen contained a variable proportion of empty grains and some anomalously large, small or deformed grains. In metaphase I, two extra chromosomes did not form a stable bivalent and only in some cases were situated close to each other. In anaphase I, the extra chromosomes migrated independently to either pole or retarded in the equatorial plain, the same was observed for chromatids in anaphase II. This retardation resulted in anomalous cytokinesis, so that triads, dyads and half-divided or non-divided monads appeared. The retarded extra chromosomes may form small extra nuclei either included into one of the microspores or forming a separate miniature cell; in this way tetrasomics may eliminate extra chromosomes. One of the tetrasomics analysed formed an exceptionally high proportion of microspore pentads. In the regluar TRUST-R trisomics, the sole extra chromosome retarded in the equatorial plain in anaphases I and II. The retardation in anaphase II often makes cytokinesis in trisomics (in general more regular than in tetrasomic) to proceed in two steps: at first cell wall formation separates a pollen mother cell into two dyads and then each of them into two microspores.  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