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101.
102.
Many bioactive products from benthic invertebrates mediating ecological interactions have proved to reduce predation, but their mechanisms of action, and their molecular identities, are usually unknown. It was suggested, yet scarcely investigated, that nutritional quality interferes with defensive metabolites. This means that antifeedants would be less effective when combined with energetically rich prey, and that higher amounts of defensive compounds would be needed for predator avoidance. We evaluated the effects of five types of repellents obtained from Antarctic invertebrates, in combination with diets of different energetic values. The compounds came from soft corals, ascidians and hexactinellid sponges; they included wax esters, alkaloids, a meroterpenoid, a steroid, and the recently described organic acid, glassponsine. Feeding repellency was tested through preference assays by preparing diets (alginate pearls) combining different energetic content and inorganic material. Experimental diets contained various concentrations of each repellent product, and were offered along with control compound-free pearls, to the Antarctic omnivore amphipod Cheirimedon femoratus. Meridianin alkaloids were the most active repellents, and wax esters were the least active when combined with foods of distinct energetic content. Our data show that levels of repellency vary for each compound, and that they perform differently when mixed with distinct assay foods. The natural products that interacted the most with energetic content were those occurring in nature at higher concentrations. The bioactivity of the remaining metabolites tested was found to depend on a threshold concentration, enough to elicit feeding repellence, independently from nutritional quality.  相似文献   
103.
In the subtropical highlands of Central Mexico, where the main crop is maize (Zea mays), the conventional practice (CP) involves tillage, monoculture and residue removal, leading to soil degradation and unsustainable use of natural resources and agricultural inputs. Conservation agriculture (CA) has been proposed as a viable alternative in the region, based on reduction in tillage, retention of adequate levels of crop residues and soil surface cover and use of crop rotation. This study began in 2009 when the highlands of Central Mexico suffered from a prolonged drought during vegetative maize growth in July-August, providing an opportunity for the on-farm comparison of CA with CP under severe drought conditions which 21 climate change models projected to become more frequent. Under dry conditions, CA resulted in higher yields and net returns per hectare as early as the first and second years after adoption by farmers. As an average of 27 plots under farmers' management in 2009, the maize yields were 26% higher under CA (6.3 t ha-1) than under CP (5.0 t ha-l). 2010 was close to a normal year in terms of rainfall so yields were higher than in 2009 for both practices; in addition, the yield difference between the practices was reduced to 19% (6.8 t ha-1 for CA vs. 5.7 t ha-1 for CP). When all the 2009 and 2010 observations were analyzed in a modified stability analysis, CA had an overall positive effect of 3 838 Mexican Pesos ha-1 (320 $US ha-1) on net return and 1.3 t ha-1 on yield. After only one to two years of adoption by farmers on their fields, CA had higher yields and net returns under dry conditions that were even drier than those predicted by the analyzed 21 climate change models under a climate change scenario, emission scenario A2.  相似文献   
104.
Using data collected at the Pierre Auger Observatory during the past 3.7 years, we demonstrated a correlation between the arrival directions of cosmic rays with energy above 6 x 10(19) electron volts and the positions of active galactic nuclei (AGN) lying within approximately 75 megaparsecs. We rejected the hypothesis of an isotropic distribution of these cosmic rays with at least a 99% confidence level from a prescribed a priori test. The correlation we observed is compatible with the hypothesis that the highest-energy particles originate from nearby extragalactic sources whose flux has not been substantially reduced by interaction with the cosmic background radiation. AGN or objects having a similar spatial distribution are possible sources.  相似文献   
105.
Soft x-ray contact microscopy, utilizing single-shot exposures of approximately 60 nanoseconds duration in polymethyl methacrylate, has been realized with a resolution of 300 angstroms. The radiation spectrum is intense in the "window" between 23 and 44 angstroms where water is transparent compared to biological materials, and therefore permits viewing of wet samples.  相似文献   
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Abstract

AIMS

To determine the frequency of the FAS-ligand gene (FASLG) variant associated with feline autoimmune lymphoproliferative syndrome (FALPS) and the proportion of carriers of the variant in three British shorthair (BSH) breeding catteries in New Zealand.  相似文献   
108.
羌塘自然保护区湖泊变化及其原因分析   总被引:2,自引:0,他引:2  
选取西藏羌塘自然保护区MSS、TM、ETM+数据和1[JP18]∶[JP]100 000地形图,以85° E为中心线,将研究区划分为东西两部分,解译该区域面积大于1 km2的所有湖泊。结果表明:1975-1990年,羌塘自然保护区湖泊呈退缩状态,湖泊个数由388个减少为277个,面积由6 603.3 km2减少到5 539.7 km2,面积减少达16.1%;2005年,湖泊个数比1990年增加164个,达441个,湖泊总面积9 652.2 km2,比1990年增加4 112.5 km2,增幅74.2%。在区域上,表现为东部变化幅度大于西部。同时,重点分析了研究区附近的狮泉河、改则、班戈、申扎、那曲、安多6个气象站1978-2007年的数据。结果显示:6个代表气象站年平均气温、年平均最高气温、年降水量均呈上升趋势,其中东部上升幅度较大,西部变化不明显;年蒸发量呈减少趋势。降水是影响湖泊变化最主要的因素。  相似文献   
109.
玉树藏族自治州农牧业机械化稳步发展,但在当地特殊环境下,农牧业生产出现了脱轨式的裂变,农牧业机械化发展渐趋下滑态势,针对这种扭变现象,文章提出了一些探索农牧业“特色”发展的措施.  相似文献   
110.
In Chile Vasconcellea pubescens is cropped to produce canned fruit, juice, jam and processed sweets. Additionally this species produces latex with a high level of papain, an important and valuable proteolytic enzyme with industrial applications. In this investigation seven ISSR primers were used to study the level and organization of genetic diversity in 333 samples of V. pubescens. Out of the 114 bands recorded, 63 proved to be polymorphic (P = 55.3%). At the species level, the genetic diversity was rather low (h = 0.01 ± 6,80188E-05, Shannon’s Index I = 0.16 ± 0,000148). The major portion of the genetic diversity was found within groups (65%). The genetic differentiation between the different groups was significant, as the AMOVA analysis suggested (Φpt = 0.35). When analysing the Northern area alone, the differentiation increased to Φpt = 0.40. When only the Southern area was analysed, Φpt decreased to 0.18, indicating greater genetic similarity among the samples. The results generated from Structure and Bayesian Analysis of Population Structure distinguished 8 genetically different groups, five of them located in the north and three in the south. The results are discussed in the light of the growers’ practices.  相似文献   
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