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171.
172.
Wolfe JH Mihalov JD Collard HR McKibbin DD Frank LA Intriligator DS 《Science (New York, N.Y.)》1980,207(4429):403-407
The Ames Research Center Pioneer 11 plasma analyzer experiment provided measurements of the solar wind interaction with Saturn and the character of the plasma environment within Saturn's magnetosphere. It is shown that Saturn has a detached bow shock wave and magnetopause quite similar to those at Earth and Jupiter. The scale size of the interaction region for Saturn is roughly one-third that at Jupiter, but Saturn's magnetosphere is equally responsive to changes in the solar wind dynamic pressure. Saturn's outer magnetosphere is inflated, as evidenced by the observation of large fluxes of corotating plasma. It is postulated that Saturn's magnetosphere may undergo a large expansion when the solar wind pressure is greatly diminished by the presence of Jupiter's extended magnetospheric tail when the two planets are approximately aligned along the same solar radial vector. 相似文献
173.
In the absence of access to formal credit, informal contracts with independent investors give the small ranchers of the Lower Amazon an acceptable means through which to surmount the high investment hurdle of starting a cattle herd. These contracts – called sociedades – allow small ranchers to raise an outside investor's cattle in return for a portion of the offspring and are commonplace in the cattle production systems of the Amazon. But, notwithstanding a vast literature on cattle production in the Amazon, informal contracts have been largely overlooked. This paper presents the results of a field survey and financial analysis for informal contracts on small ranches in the Lower Amazon. In the results, we suggest that informal contracts are an important means of cattle herd start-up and herd production for small ranchers. Internal rates of returns in cattle production under these contracts are in the range of –7 to –12% for the small rancher and 12% for the investor. The net present value of the contract to the small rancher ranges from –R$1219 to –R$8599 and from R$1681 to R$8845 for the investor, for a 10-year period, depending on herd size. Financial returns contracts are sensitive to the contract design – e.g., to who pays health costs – and to beef prices. The small ranchers have a negative IRR, lose money, and bear all the risk of loss, yet persist in using this form of herd development. We surmise that this is due to the non-financial benefits of cattle ownership and the lack of access to formal credit structures. In conclusion, although outside the formal economy and apparently financially unrewarding, these contracts are an important mechanism by which the small ranchers on the Amazon Floodplains create cattle herds. 相似文献
174.
Feely RA Sabine CL Lee K Berelson W Kleypas J Fabry VJ Millero FJ 《Science (New York, N.Y.)》2004,305(5682):362-366
Rising atmospheric carbon dioxide (CO2) concentrations over the past two centuries have led to greater CO2 uptake by the oceans. This acidification process has changed the saturation state of the oceans with respect to calcium carbonate (CaCO3) particles. Here we estimate the in situ CaCO3 dissolution rates for the global oceans from total alkalinity and chlorofluorocarbon data, and we also discuss the future impacts of anthropogenic CO2 on CaCO3 shell-forming species. CaCO3 dissolution rates, ranging from 0.003 to 1.2 micromoles per kilogram per year, are observed beginning near the aragonite saturation horizon. The total water column CaCO3 dissolution rate for the global oceans is approximately 0.5 +/- 0.2 petagrams of CaCO3-C per year, which is approximately 45 to 65% of the export production of CaCO3. 相似文献
175.
Zdobnov EM von Mering C Letunic I Torrents D Suyama M Copley RR Christophides GK Thomasova D Holt RA Subramanian GM Mueller HM Dimopoulos G Law JH Wells MA Birney E Charlab R Halpern AL Kokoza E Kraft CL Lai Z Lewis S Louis C Barillas-Mury C Nusskern D Rubin GM Salzberg SL Sutton GG Topalis P Wides R Wincker P Yandell M Collins FH Ribeiro J Gelbart WM Kafatos FC Bork P 《Science (New York, N.Y.)》2002,298(5591):149-159
Comparison of the genomes and proteomes of the two diptera Anopheles gambiae and Drosophila melanogaster, which diverged about 250 million years ago, reveals considerable similarities. However, numerous differences are also observed; some of these must reflect the selection and subsequent adaptation associated with different ecologies and life strategies. Almost half of the genes in both genomes are interpreted as orthologs and show an average sequence identity of about 56%, which is slightly lower than that observed between the orthologs of the pufferfish and human (diverged about 450 million years ago). This indicates that these two insects diverged considerably faster than vertebrates. Aligned sequences reveal that orthologous genes have retained only half of their intron/exon structure, indicating that intron gains or losses have occurred at a rate of about one per gene per 125 million years. Chromosomal arms exhibit significant remnants of homology between the two species, although only 34% of the genes colocalize in small "microsyntenic" clusters, and major interarm transfers as well as intra-arm shuffling of gene order are detected. 相似文献
176.
Herd CD Blinova A Simkus DN Huang Y Tarozo R Alexander CM Gyngard F Nittler LR Cody GD Fogel ML Kebukawa Y Kilcoyne AL Hilts RW Slater GF Glavin DP Dworkin JP Callahan MP Elsila JE De Gregorio BT Stroud RM 《Science (New York, N.Y.)》2011,332(6035):1304-1307
The complex suite of organic materials in carbonaceous chondrite meteorites probably originally formed in the interstellar medium and/or the solar protoplanetary disk, but was subsequently modified in the meteorites' asteroidal parent bodies. The mechanisms of formation and modification are still very poorly understood. We carried out a systematic study of variations in the mineralogy, petrology, and soluble and insoluble organic matter in distinct fragments of the Tagish Lake meteorite. The variations correlate with indicators of parent body aqueous alteration. At least some molecules of prebiotic importance formed during the alteration. 相似文献
177.
M Mer?ep A M Weissman S J Frank R D Klausner J D Ashwell 《Science (New York, N.Y.)》1989,246(4934):1162-1165
Activation of spontaneously dividing T cell hybridomas induces interleukin-2 (IL-2) production, a cell cycle block, and programmed cell death. T cell hybridomas that express the T cell antigen receptor (TCR) zeta homodimer (zeta 2), but not the TCR zeta eta heterodimer, were studied. The zeta eta- cells produced little or no inositol phosphates (IP) when stimulated with antigen. In most cases the hydrolysis of phosphoinositides was also impaired after stimulation with antibody to CD3, although one zeta eta- cell produced normal concentrations of IP. The zeta eta- cells slowed their growth and secreted IL-2 in response to both stimuli. However, the zeta eta- cells did not die after activation with antigen. Since activated thymocytes also undergo programmed cell death, these results may have important implications for the role of the zeta eta.TCR in negative selection. 相似文献
178.
Armbrust EV Berges JA Bowler C Green BR Martinez D Putnam NH Zhou S Allen AE Apt KE Bechner M Brzezinski MA Chaal BK Chiovitti A Davis AK Demarest MS Detter JC Glavina T Goodstein D Hadi MZ Hellsten U Hildebrand M Jenkins BD Jurka J Kapitonov VV Kröger N Lau WW Lane TW Larimer FW Lippmeier JC Lucas S Medina M Montsant A Obornik M Parker MS Palenik B Pazour GJ Richardson PM Rynearson TA Saito MA Schwartz DC Thamatrakoln K Valentin K Vardi A Wilkerson FP Rokhsar DS 《Science (New York, N.Y.)》2004,306(5693):79-86
Diatoms are unicellular algae with plastids acquired by secondary endosymbiosis. They are responsible for approximately 20% of global carbon fixation. We report the 34 million-base pair draft nuclear genome of the marine diatom Thalassiosira pseudonana and its 129 thousand-base pair plastid and 44 thousand-base pair mitochondrial genomes. Sequence and optical restriction mapping revealed 24 diploid nuclear chromosomes. We identified novel genes for silicic acid transport and formation of silica-based cell walls, high-affinity iron uptake, biosynthetic enzymes for several types of polyunsaturated fatty acids, use of a range of nitrogenous compounds, and a complete urea cycle, all attributes that allow diatoms to prosper in aquatic environments. 相似文献
179.
Stephanie Kramer-Schadt Tobias S. Kaiser Karin Frank Thorsten Wiegand 《Landscape Ecology》2011,26(4):501-513
With habitat loss and fragmentation having become two of the major threats to the viability of species, the question of how
to manage landscapes for species conservation has attracted much attention. In this context, the planning of stepping stones
has been proposed to increase connectivity in fragmented landscapes. We present a simulation study with a neutral landscape
approach to assess the effects of stepping stones on colonization success. To that end, we used a spatially explicit, calibrated
population model of the European lynx (Lynx lynx) coupled with structured landscapes, in which we could control the landscape parameters of dispersal habitat coverage and
contagion, as well as the number and size of stepping stones available for breeding. In general, we found that colonization
success increased with increasing habitat coverage but decreased with increasing habitat contagion, while the introduction
of stepping stones had significant effects in critical situations. Especially at low to medium dispersal habitat coverage
and high disperser mortality, stepping stones had a positive effect on colonization success when they were large enough to
produce new dispersers, but negative effects when they were small and located in a way that dispersers would be distracted
from more suitable breeding habitat patches. The latter clearly constituted a shading effect and argues for a thorough consideration
of the trade-offs related to stepping stone size and location when implementing stepping stones as a conservation measure,
especially when the number of individuals of conservation concern is low. 相似文献
180.
Beef authentication and retrospective dietary verification using stable isotope ratio analysis of bovine muscle and tail hair 总被引:2,自引:0,他引:2
Osorio MT Moloney AP Schmidt O Monahan FJ 《Journal of agricultural and food chemistry》2011,59(7):3295-3305
Stable isotope ratio analysis (SIRA) was used as an analytical tool to verify the preslaughter diet of beef cattle. Muscle and tail hair samples were collected from animals fed either pasture (P), a barley-based concentrate (C), silage followed by pasture (SiP), or silage followed by pasture with concentrate (SiPC) for 1 year (n = 25 animals per treatment). The (13)C/(12)C, (15)N/(14)N, (2)H/(1)H, and (34)S/(32)S isotope ratios in muscle clearly reflected those of the diets consumed by the animals. By applying a stepwise canonical discriminant analysis, a good discrimination of bovine meat according to dietary regimen was obtained. On the basis of the classification success rate, the (13)C/(12)C and (34)S/(32)S ratios in muscle were the best indicators for authentication of beef from animals consuming the different diets. Analysis of (13)C/(12)C and (15)N/(14)N in tail hair sections provided an archival record of changes to the diet of the cattle for periods of over 1 year preslaughter. 相似文献