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1.
By modifying habitats and creating bridges and barriers between landmasses, climate change and tectonic events are believed to have important consequences for diversification of terrestrial organisms. Such consequences should be most evident in phylogenetic histories of groups that are ancient, widespread, and diverse. The squirrel family (Sciuridae) is one of very few mammalian families endemic to Eurasia, Africa, and North and South America and is ideal for examining these issues. Through phylogenetic and molecular-clock analyses, we infer that arrival and diversification of squirrels in Africa, on Sunda Shelf islands, across Beringea, and across the Panamanian isthmus coincide in timing and location with multiple well-documented sea-level, tectonic, and paleontological events. These precise correspondences point to an important role for global change in the diversification of a major group of mammals. 相似文献
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The success of forward genetic (from phenotype to gene) approaches to uncover genes that drive the molecular mechanism of circadian clocks and control circadian behavior has been unprecedented. Links among genes, cells, neural circuits, and circadian behavior have been uncovered in the Drosophila and mammalian systems, demonstrating the feasibility of finding single genes that have major effects on behavior. Why was this approach so successful in the elucidation of circadian rhythms? This article explores the answers to this question and describes how the methods used successfully for identifying the molecular basis of circadian rhythms can be applied to other behaviors such as anxiety, addiction, and learning and memory. 相似文献
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家庭生命周期、社会资本与农户生计策略研究 总被引：5，自引：1，他引：4
基于家庭生命周期视角,采用农户实地调查数据,运用Mlogit回归模型,分析社会资本对农户生计策略产生的影响。结果表明:湖北农村家庭的兼业化程度较高,家庭结构呈现"纺锤形"分布,社会资本积累会促使农户选择偏非农型的生计策略。通过检验社会资本的影响机制,发现社会资本在家庭生命周期对生计策略的影响中既存在调节作用,又存在部分中介作用,其中介效应的大小为35.2%。为促进农户生计多样化,加强农户抗风险能力,防止生命周期阶段两端的家庭遭受生计风险,建议政府通过职业教育或者技能培训提高农户的人力资本,加强对农户资金、贷款方面的支持,扩大农户的社会网络。 相似文献
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Environmental temperature cycles are a universal entraining cue for all circadian systems at the organismal level with the exception of homeothermic vertebrates. We report here that resistance to temperature entrainment is a property of the suprachiasmatic nucleus (SCN) network and is not a cell-autonomous property of mammalian clocks. This differential sensitivity to temperature allows the SCN to drive circadian rhythms in body temperature, which can then act as a universal cue for the entrainment of cell-autonomous oscillators throughout the body. Pharmacological experiments show that network interactions in the SCN are required for temperature resistance and that the heat shock pathway is integral to temperature resetting and temperature compensation in mammalian cells. These results suggest that the evolutionarily ancient temperature resetting response can be used in homeothermic animals to enhance internal circadian synchronization. 相似文献
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Ramsey NF 《Science (New York, N.Y.)》1990,248(4963):1612-1619
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Ranson H Claudianos C Ortelli F Abgrall C Hemingway J Sharakhova MV Unger MF Collins FH Feyereisen R 《Science (New York, N.Y.)》2002,298(5591):179-181
The emergence of insecticide resistance in the mosquito poses a serious threat to the efficacy of many malaria control programs. We have searched the Anopheles gambiae genome for members of the three major enzyme families- the carboxylesterases, glutathione transferases, and cytochrome P450s-that are primarily responsible for metabolic resistance to insecticides. A comparative genomic analysis with Drosophila melanogaster reveals that a considerable expansion of these supergene families has occurred in the mosquito. Low gene orthology and little chromosomal synteny paradoxically contrast the easily identified orthologous groups of genes presumably seeded by common ancestors. In A. gambiae, the independent expansion of paralogous genes is mainly a consequence of the formation of clusters among locally duplicated genes. These expansions may reflect the functional diversification of supergene families consistent with major differences in the life history and ecology of these organisms. These data provide a basis for identifying the resistance-associated enzymes within these families. This will enable the resistance status of mosquitoes, flies, and possibly other holometabolous insects to be monitored. The analyses also provide the means for identifying previously unknown molecules involved in fundamental biological processes such as development. 相似文献
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Megabase-scale mapping of the HLA gene complex by pulsed field gel electrophoresis 总被引：18，自引：0，他引：18
S K Lawrance C L Smith R Srivastava C R Cantor S M Weissman 《Science (New York, N.Y.)》1987,235(4794):1387-1390
In the study of the genetic structure of mammalian chromosomes, there exists a "resolution gap" between molecular cloning experiments and meiotic linkage analyses. This gap has discouraged attempts to construct full-scale genetic maps of mammalian chromosomes. The organization of the human major histocompatibility complex was examined within this range by pulsed field gel electrophoresis. The data obtained indicate that the complex spans over 3000 kilobases and enable the construction of a megabase-scale molecular map. These results indicate that the techniques employed in DNA extraction, enzymatic digestion, electrophoresis, and hybridization are suitable for the efficient analysis of megabase regions of mammalian chromosomes and effectively bridge the resolution gap between molecular cloning and classical genetics. 相似文献
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Immunity-related genes and gene families in Anopheles gambiae 总被引：2，自引：0，他引：2
Christophides GK Zdobnov E Barillas-Mury C Birney E Blandin S Blass C Brey PT Collins FH Danielli A Dimopoulos G Hetru C Hoa NT Hoffmann JA Kanzok SM Letunic I Levashina EA Loukeris TG Lycett G Meister S Michel K Moita LF Müller HM Osta MA Paskewitz SM Reichhart JM Rzhetsky A Troxler L Vernick KD Vlachou D Volz J von Mering C Xu J Zheng L Bork P Kafatos FC 《Science (New York, N.Y.)》2002,298(5591):159-165
We have identified 242 Anopheles gambiae genes from 18 gene families implicated in innate immunity and have detected marked diversification relative to Drosophila melanogaster. Immune-related gene families involved in recognition, signal modulation, and effector systems show a marked deficit of orthologs and excessive gene expansions, possibly reflecting selection pressures from different pathogens encountered in these insects' very different life-styles. In contrast, the multifunctional Toll signal transduction pathway is substantially conserved, presumably because of counterselection for developmental stability. Representative expression profiles confirm that sequence diversification is accompanied by specific responses to different immune challenges. Alternative RNA splicing may also contribute to expansion of the immune repertoire. 相似文献
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Hatten ME 《Science (New York, N.Y.)》2002,297(5587):1660-1663
Over the past decade, genetic analyses have yielded a more molecular view of neuronal migration and its role in central nervous system development. We now realize that many of the molecular mechanisms that guide migrations in invertebrates are recapitulated in the vertebrate nervous system. These mechanisms guide dorsoventral and anterior-posterior migrations and merge with radial migratory pathways that are prominent in the development of the mammalian cortex. This review discusses the choreography of these different migratory mechanisms within the context of genetic approaches that have defined their molecular mechanisms. 相似文献
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对北京市百花山自然保护区的野生花卉进行实地调查,共调查了142种植物,属于50个科。该地区野生花卉资源具有资源丰富、品种多样,抗逆性强、适应性广,应用成本低、经济价值高等特点。结合其开发利用途径,提出明确发展方向、多样化应用、开展生态旅游、保护环境等建议,可供园林应用参考。 相似文献
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A comprehensive phylogeny of beetles reveals the evolutionary origins of a superradiation 总被引：1，自引：0，他引：1
Hunt T Bergsten J Levkanicova Z Papadopoulou A John OS Wild R Hammond PM Ahrens D Balke M Caterino MS Gómez-Zurita J Ribera I Barraclough TG Bocakova M Bocak L Vogler AP 《Science (New York, N.Y.)》2007,318(5858):1913-1916
Beetles represent almost one-fourth of all described species, and knowledge about their relationships and evolution adds to our understanding of biodiversity. We performed a comprehensive phylogenetic analysis of Coleoptera inferred from three genes and nearly 1900 species, representing more than 80% of the world's recognized beetle families. We defined basal relationships in the Polyphaga supergroup, which contains over 300,000 species, and established five families as the earliest branching lineages. By dating the phylogeny, we found that the success of beetles is explained neither by exceptional net diversification rates nor by a predominant role of herbivory and the Cretaceous rise of angiosperms. Instead, the pre-Cretaceous origin of more than 100 present-day lineages suggests that beetle species richness is due to high survival of lineages and sustained diversification in a variety of niches. 相似文献
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A cluster analysis of the stratigraphic distribution of all Ordovician trilobite families, based on a comprehensive taxonomic database, identified two major faunas with disjunct temporal diversity trends. The Ibex Fauna behaved as a cohort, declining through the Ordovician and disappearing at the end-Ordovician mass extinction. In contrast, the Whiterock Fauna radiated rapidly during the Middle Ordovician and gave rise to all post-Ordovician trilobite diversity. Its pattern of diversification matches that of the Paleozoic Evolutionary Fauna; hence, trilobites were active participants in the great Ordovician radiations. Extinction patterns at the end of the Ordovician are related to clade size: Surviving trilobite families show higher genus diversity than extinguished families. 相似文献
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Angela Steward 《Agriculture and Human Values》2007,24(1):75-92
Over the past 15 years income sources in the Amazonian community of Carvão have diversified to include government salaries, retirement and welfare benefits, and wages from an evolving informal service sector. These non-farm incomes are now more important to household incomes than the sale of agricultural products. Out of 80 households only three families were found to depend almost entirely on the sale of agricultural goods for cash income. Agriculture is still a part of most families’ livelihoods; however, production today is mainly a subsistence activity. Recent changes in Carvão are consistent with trends of livelihood diversification observed in rural societies across the globe. However, current research reveals that Carvão is different from other case studies in a number of ways. A history of livelihoods illustrates that residents in Carvão have always engaged in a range of activities, including farming, extractive activities, and wage labor. New incomes are the result of new jobs in the public sector and social policies benefiting the rural poor. In contrast to the literature on livelihood diversification, the decentralization of the federal government in Brazil has resulted in greater opportunities for rural income and employment. Consistent with recorded trends, research shows that small farmers in Carvão have down-sized agricultural production. Farmers cite low market prices (the result of vertical integration of local markets) as one cause of this decline. Residents, especially small farmers, interested in diversifying agricultural production are limited by inadequate extension services and credit, and younger residents seek public sector employment. Income diversification has increased livelihoods security; however, future livelihoods will depend on new economic growth. Given the stagnating public sector and a weak industrial sector, production geared toward growing urban markets is a viable means for further income generation in Carvão. 相似文献
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基于线粒体16S rDNA序列的蝽总科系统发育研究(异翅亚目:蝽次目) 总被引：2，自引：1，他引：2
以线粒体16S rDNA基因作为分子标记,对蝽总科8个科23种昆虫进行了系统发育的分析,并采用最大简约法、最大似然法和邻接法构建了分子系统树。结果表明:利用16S rDNA分子研究蝽总科的系统发育关系是适合的,能够重建蝽总科的单系性;土蝽科的位置不稳定,在蝽总科各科进化过程中处于中间位置;龟蝽科与蝽科在MP树和ML树中都形成姊妹群关系,在NJ树中2个科的亲缘关系也比较近;异蝽科是最早分支出来的,表明异蝽科是所研究类群中最为原始的类群。本研究从分子水平上支持了将荔蝽亚科、盾蝽亚科和兜蝽亚科从蝽科中分立出来提升为科的观点。 相似文献
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江雪茹 《安徽农业大学学报》2004,(6):41-44
世界多样性，这是一个客观事实;国际关系民主化，这是时代发展的必然要求。尊重世界的多样性是实现国际关系民主化所应遵循的一条基木原则，但是多样性所伴生的差异性和矛盾性会阻碍国际关系的民主进程，因此木文立足于多极化趋势的背景下具体分析世界多样性和国际关系民主化之间的内在联系，并对在尊重世界多样性的基础上如何实现国际关系民主化作出初步的探索。 相似文献
18.
"Inordinate Fondness" explained: why are there So many beetles? 总被引：1，自引：0，他引：1
BD Farrell 《Science (New York, N.Y.)》1998,281(5376):555-559
The phylogeny of the Phytophaga, the largest and oldest radiation of herbivorous beetles, was reconstructed from 115 complete DNA sequences for the 18S nuclear ribosomal subunit and from 212 morphological characters. The results of these analyses were used to interpret the role of angiosperms in beetle diversification. Jurassic fossils represent basal lineages that are still associated with conifers and cycads. Repeated origins of angiosperm-feeding beetle lineages are associated with enhanced rates of beetle diversification, indicating a series of adaptive radiations. Collectively, these radiations represent nearly half of the species in the order Coleoptera and a similar proportion of herbivorous insect species. 相似文献
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During October 1971, four cesium beam atomic clocks were flown on regularly scheduled commercial jet flights around the world twice, once eastward and once westward, to test Einstein's theory of relativity with macroscopic clocks. From the actual flight paths of each trip, the theory predicts that the flying clocks, compared with reference clocks at the U.S. Naval Observatory, should have lost 40 +/- 23 nanoseconds during the eastward trip, and should have gained 275 +/- 21 nanoseconds during the westward trip. The observed time differences are presented in the report that follows this one. 相似文献
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Due to differences in morphology, ecology and biology, the phylogenesis of Coniferopsida and Taxaceae remains controversial, and the phylogenetic relationship of Pseudotaxus is yet to be confirmed. Three rDNA regions, ITS1, 5.8S rDNA, and ITS2, were prepared for 77 species belonging to seven families of Coniferopsida, including Taxaceae. Maximum parsimony(MP)and Neighbor-joining(NJ) analyses were performed. Results showed that:(1) the Coniferopsida species formed three branches:(Taxaceae, Cephalotaxaceae, Cupressaceae, Taxodiaceae),(Araucariaceae, Podocarpaceae), and(Pinaceae);(2) Taxaceae included Taxus, Pseudotaxus, Amentotaxus, and Torreya; and(3) Pseudotaxus belonged to Taxaceae, with a support rate of 97%in MP analyses and 99% in NJ analyses. 相似文献