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91.
Vegetation trends due to climatic changes are difficult to separate from disturbances caused by varying land uses. To separate climatic influences from livestock grazing and fire disturbances within sagebrush steppe, we compared vegetation structure and productivity during 2 periods (10-year sequences of data from the late 1950s to the late 1960s and 3 years in the early 1990s) at 12 stands within 3 relict areas in or near the Great Rift of southern Idaho. Yearto- year fluctuations in annual net aboveground phytomass accumulation (ANAPA) were considerable in response to varying climate during both periods. More importantly, an apparently significant increase in ANAPA was observed during the latter study period. However, this apparent increase may have been due to the unusually wet conditions of 1993. Throughout both study periods, ANAPA was even more positively correlated with species richness than with precipitation, although both influences were statistically significant. Aggregation of these data by plant growth forms showed that shrubs, particularly sagebrushes, have progressively increased their proportion relative to herbaceous understory. This directional (successional) change in the proportional contributions of growth forms to ANAPA was probably due more to long-term absence of fire at these isolated locations than to any detectable climate change or lack of grazing. This finding reduces our confidence in the use of such areas as reference sites for monitoring or as living examples for restoration efforts, but it does increase our understanding of vegetation dynamics within sagebrush steppe. ACRONYMS. ANAPA—annual net aboveground phytomass accumulations, ANOVA—analysis of variance, ANPP—aboveground net primary production, COV—coefficient of variation, CY—crop year, CYP—crop-year precipitation, ES—ecological site, MANOVA—multivariate analysis of variance, MAT—mean annual temperature, NRCS—Natural Resources Conservation Service, PDSI—Palmer Drought Severity Index, RUE—rainfall use efficiency. 相似文献
92.
A successful ground nesting by Swainson’s Hawks, Buteo swainsoni, is reported for central Utah. Unusual aggressive nest defense behavior is described for this ground nesting pair. 相似文献
93.
Smith UM Consugar M Tee LJ McKee BM Maina EN Whelan S Morgan NV Goranson E Gissen P Lilliquist S Aligianis IA Ward CJ Pasha S Punyashthiti R Malik Sharif S Batman PA Bennett CP Woods CG McKeown C Bucourt M Miller CA Cox P Algazali L Trembath RC Torres VE Attie-Bitach T Kelly DA Maher ER Gattone VH Harris PC Johnson CA 《Nature genetics》2006,38(2):191-196
Meckel-Gruber syndrome is a severe autosomal, recessively inherited disorder characterized by bilateral renal cystic dysplasia, developmental defects of the central nervous system (most commonly occipital encephalocele), hepatic ductal dysplasia and cysts and polydactyly. MKS is genetically heterogeneous, with three loci mapped: MKS1, 17q21-24 (ref. 4); MKS2, 11q13 (ref. 5) and MKS3 (ref. 6). We have refined MKS3 mapping to a 12.67-Mb interval (8q21.13-q22.1) that is syntenic to the Wpk locus in rat, which is a model with polycystic kidney disease, agenesis of the corpus callosum and hydrocephalus. Positional cloning of the Wpk gene suggested a MKS3 candidate gene, TMEM67, for which we identified pathogenic mutations for five MKS3-linked consanguineous families. MKS3 is a previously uncharacterized, evolutionarily conserved gene that is expressed at moderate levels in fetal brain, liver and kidney but has widespread, low levels of expression. It encodes a 995-amino acid seven-transmembrane receptor protein of unknown function that we have called meckelin. 相似文献
94.
Neil Ashby 《系统工程与电子技术(英文版)》1995,(4)
l'INTRo~TloNThepurposeofthisworkistoexplainhowrelativisticeffectsareaccountedforintheGlobalPositioningSystem(GPS).AlthoughclockvelocitiesaresmallandgravitationalfieldsareweakneartheEarth,theygiverisetosignificantrelativisticeffects.Theseeffectsincludesecond--orderDopplerfrequencyshiftsofclocksduetotheirrelativevelocities,gravitationalfrequencyshifts,andtheSagnaceffectduetotheEarth'srotation.Ifsucheffectsarenotaccountedforproperly,theGPSsimplywouldnotworkatallwell.Thesuccessandself--con… 相似文献
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The invention of the laser has resulted in many innovations, and the device has become ubiquitous. However, the maser, which amplifies microwave radiation rather than visible light, has not had as large an impact, despite being instrumental in the laser's birth. The maser's relative obscurity has mainly been due to the inconvenience of the operating conditions needed for its various realizations: atomic and free-electron masers require vacuum chambers and pumping; and solid-state masers, although they excel as low-noise amplifiers and are occasionally incorporated in ultrastable oscillators, typically require cryogenic refrigeration. Most realizations of masers also require strong magnets, magnetic shielding or both. Overcoming these various obstacles would pave the way for improvements such as more-sensitive chemical assays, more-precise determinations of biomolecular structure and function, and more-accurate medical diagnostics (including tomography) based on enhanced magnetic resonance spectrometers incorporating maser amplifiers and oscillators. Here we report the experimental demonstration of a solid-state maser operating at room temperature in pulsed mode. It works on a laboratory bench, in air, in the terrestrial magnetic field and amplifies at around 1.45 gigahertz. In contrast to the cryogenic ruby maser, in our maser the gain medium is an organic mixed molecular crystal, p-terphenyl doped with pentacene, the latter being photo-excited by yellow light. The maser's pumping mechanism exploits spin-selective molecular intersystem crossing into pentacene's triplet ground state. When configured as an oscillator, the solid-state maser's measured output power of around -10 decibel milliwatts is approximately 100 million times greater than that of an atomic hydrogen maser, which oscillates at a similar frequency (about 1.42 gigahertz). By exploiting the high levels of spin polarization readily generated by intersystem crossing in photo-excited pentacene and other aromatic molecules, this new type of maser seems to be capable of amplifying with a residual noise temperature far below room temperature. 相似文献
100.
Broadhead R Dawe HR Farr H Griffiths S Hart SR Portman N Shaw MK Ginger ML Gaskell SJ McKean PG Gull K 《Nature》2006,440(7081):224-227
The 9 + 2 microtubule axoneme of flagella and cilia represents one of the most iconic structures built by eukaryotic cells and organisms. Both unity and diversity are present among cilia and flagella on the evolutionary as well as the developmental scale. Some cilia are motile, whereas others function as sensory organelles and can variously possess 9 + 2 and 9 + 0 axonemes and other associated structures. How such unity and diversity are reflected in molecular repertoires is unclear. The flagellated protozoan parasite Trypanosoma brucei is endemic in sub-Saharan Africa, causing devastating disease in humans and other animals. There is little hope of a vaccine for African sleeping sickness and a desperate need for modern drug therapies. Here we present a detailed proteomic analysis of the trypanosome flagellum. RNA interference (RNAi)-based interrogation of this proteome provides functional insights into human ciliary diseases and establishes that flagellar function is essential to the bloodstream-form trypanosome. We show that RNAi-mediated ablation of various proteins identified in the trypanosome flagellar proteome leads to a rapid and marked failure of cytokinesis in bloodstream-form (but not procyclic insect-form) trypanosomes, suggesting that impairment of flagellar function may provide a method of disease control. A postgenomic meta-analysis, comparing the evolutionarily ancient trypanosome with other eukaryotes including humans, identifies numerous trypanosome-specific flagellar proteins, suggesting new avenues for selective intervention. 相似文献