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991.
992.
Determinants of woody cover in African savannas 总被引:8,自引:0,他引:8
Sankaran M Hanan NP Scholes RJ Ratnam J Augustine DJ Cade BS Gignoux J Higgins SI Le Roux X Ludwig F Ardo J Banyikwa F Bronn A Bucini G Caylor KK Coughenour MB Diouf A Ekaya W Feral CJ February EC Frost PG Hiernaux P Hrabar H Metzger KL Prins HH Ringrose S Sea W Tews J Worden J Zambatis N 《Nature》2005,438(7069):846-849
Savannas are globally important ecosystems of great significance to human economies. In these biomes, which are characterized by the co-dominance of trees and grasses, woody cover is a chief determinant of ecosystem properties. The availability of resources (water, nutrients) and disturbance regimes (fire, herbivory) are thought to be important in regulating woody cover, but perceptions differ on which of these are the primary drivers of savanna structure. Here we show, using data from 854 sites across Africa, that maximum woody cover in savannas receiving a mean annual precipitation (MAP) of less than approximately 650 mm is constrained by, and increases linearly with, MAP. These arid and semi-arid savannas may be considered 'stable' systems in which water constrains woody cover and permits grasses to coexist, while fire, herbivory and soil properties interact to reduce woody cover below the MAP-controlled upper bound. Above a MAP of approximately 650 mm, savannas are 'unstable' systems in which MAP is sufficient for woody canopy closure, and disturbances (fire, herbivory) are required for the coexistence of trees and grass. These results provide insights into the nature of African savannas and suggest that future changes in precipitation may considerably affect their distribution and dynamics. 相似文献
993.
Lindblad-Toh K Wade CM Mikkelsen TS Karlsson EK Jaffe DB Kamal M Clamp M Chang JL Kulbokas EJ Zody MC Mauceli E Xie X Breen M Wayne RK Ostrander EA Ponting CP Galibert F Smith DR DeJong PJ Kirkness E Alvarez P Biagi T Brockman W Butler J Chin CW Cook A Cuff J Daly MJ DeCaprio D Gnerre S Grabherr M Kellis M Kleber M Bardeleben C Goodstadt L Heger A Hitte C Kim L Koepfli KP Parker HG Pollinger JP Searle SM Sutter NB Thomas R Webber C Baldwin J Abebe A Abouelleil A Aftuck L Ait-Zahra M Aldredge T 《Nature》2005,438(7069):803-819
Here we report a high-quality draft genome sequence of the domestic dog (Canis familiaris), together with a dense map of single nucleotide polymorphisms (SNPs) across breeds. The dog is of particular interest because it provides important evolutionary information and because existing breeds show great phenotypic diversity for morphological, physiological and behavioural traits. We use sequence comparison with the primate and rodent lineages to shed light on the structure and evolution of genomes and genes. Notably, the majority of the most highly conserved non-coding sequences in mammalian genomes are clustered near a small subset of genes with important roles in development. Analysis of SNPs reveals long-range haplotypes across the entire dog genome, and defines the nature of genetic diversity within and across breeds. The current SNP map now makes it possible for genome-wide association studies to identify genes responsible for diseases and traits, with important consequences for human and companion animal health. 相似文献
994.
Mutations in NOTCH1 cause aortic valve disease 总被引:2,自引:0,他引:2
Garg V Muth AN Ransom JF Schluterman MK Barnes R King IN Grossfeld PD Srivastava D 《Nature》2005,437(7056):270-274
995.
Orme CD Davies RG Burgess M Eigenbrod F Pickup N Olson VA Webster AJ Ding TS Rasmussen PC Ridgely RS Stattersfield AJ Bennett PM Blackburn TM Gaston KJ Owens IP 《Nature》2005,436(7053):1016-1019
Biodiversity hotspots have a prominent role in conservation biology, but it remains controversial to what extent different types of hotspot are congruent. Previous studies were unable to provide a general answer because they used a single biodiversity index, were geographically restricted, compared areas of unequal size or did not quantitatively compare hotspot types. Here we use a new global database on the breeding distribution of all known extant bird species to test for congruence across three types of hotspot. We demonstrate that hotspots of species richness, threat and endemism do not show the same geographical distribution. Only 2.5% of hotspot areas are common to all three aspects of diversity, with over 80% of hotspots being idiosyncratic. More generally, there is a surprisingly low overall congruence of biodiversity indices, with any one index explaining less than 24% of variation in the other indices. These results suggest that, even within a single taxonomic class, different mechanisms are responsible for the origin and maintenance of different aspects of diversity. Consequently, the different types of hotspots also vary greatly in their utility as conservation tools. 相似文献
996.
Light-induced shape-memory polymers 总被引:4,自引:0,他引:4
Materials are said to show a shape-memory effect if they can be deformed and fixed into a temporary shape, and recover their original, permanent shape only on exposure to an external stimulus. Shape-memory polymers have received increasing attention because of their scientific and technological significance. In principle, a thermally induced shape-memory effect can be activated by an increase in temperature (also obtained by heating on exposure to an electrical current or light illumination). Several papers have described light-induced changes in the shape of polymers and gels, such as contraction, bending or volume changes. Here we report that polymers containing cinnamic groups can be deformed and fixed into pre-determined shapes--such as (but not exclusively) elongated films and tubes, arches or spirals--by ultraviolet light illumination. These new shapes are stable for long time periods, even when heated to 50 degrees C, and they can recover their original shape at ambient temperatures when exposed to ultraviolet light of a different wavelength. The ability of polymers to form different pre-determined temporary shapes and subsequently recover their original shape at ambient temperatures by remote light activation could lead to a variety of potential medical and other applications. 相似文献
997.
Plectasin is a peptide antibiotic with therapeutic potential from a saprophytic fungus 总被引:4,自引:0,他引:4
Mygind PH Fischer RL Schnorr KM Hansen MT Sönksen CP Ludvigsen S Raventós D Buskov S Christensen B De Maria L Taboureau O Yaver D Elvig-Jørgensen SG Sørensen MV Christensen BE Kjaerulff S Frimodt-Moller N Lehrer RI Zasloff M Kristensen HH 《Nature》2005,437(7061):975-980
Animals and higher plants express endogenous peptide antibiotics called defensins. These small cysteine-rich peptides are active against bacteria, fungi and viruses. Here we describe plectasin-the first defensin to be isolated from a fungus, the saprophytic ascomycete Pseudoplectania nigrella. Plectasin has primary, secondary and tertiary structures that closely resemble those of defensins found in spiders, scorpions, dragonflies and mussels. Recombinant plectasin was produced at a very high, and commercially viable, yield and purity. In vitro, the recombinant peptide was especially active against Streptococcus pneumoniae, including strains resistant to conventional antibiotics. Plectasin showed extremely low toxicity in mice, and cured them of experimental peritonitis and pneumonia caused by S. pneumoniae as efficaciously as vancomycin and penicillin. These findings identify fungi as a novel source of antimicrobial defensins, and show the therapeutic potential of plectasin. They also suggest that the defensins of insects, molluscs and fungi arose from a common ancestral gene. 相似文献
998.
Stochastic resonance is a counterintuitive concept: the addition of noise to a noisy system induces coherent amplification of its response. First suggested as a mechanism for the cyclic recurrence of ice ages, stochastic resonance has been seen in a wide variety of macroscopic physical systems: bistable ring lasers, superconducting quantum interference devices (SQUIDs), magnetoelastic ribbons and neurophysiological systems such as the receptors in crickets and crayfish. Although fundamentally important as a mechanism of coherent signal amplification, stochastic resonance has yet to be observed in nanoscale systems. Here we report the observation of stochastic resonance in bistable nanomechanical silicon oscillators. Our nanomechanical systems consist of beams that are clamped at each end and driven into transverse oscillation with the use of a radiofrequency source. Modulation of the source induces controllable switching of the beams between two stable, distinct states. We observe that the addition of white noise causes a marked amplification of the signal strength. Stochastic resonance in nanomechanical systems could have a function in the realization of controllable high-speed nanomechanical memory cells, and paves the way for exploring macroscopic quantum coherence and tunnelling. 相似文献
999.
1000.
Nusbaum C Zody MC Borowsky ML Kamal M Kodira CD Taylor TD Whittaker CA Chang JL Cuomo CA Dewar K FitzGerald MG Yang X Abouelleil A Allen NR Anderson S Bloom T Bugalter B Butler J Cook A DeCaprio D Engels R Garber M Gnirke A Hafez N Hall JL Norman CH Itoh T Jaffe DB Kuroki Y Lehoczky J Lui A Macdonald P Mauceli E Mikkelsen TS Naylor JW Nicol R Nguyen C Noguchi H O'Leary SB O'Neill K Piqani B Smith CL Talamas JA Topham K Totoki Y Toyoda A Wain HM Young SK Zeng Q Zimmer AR Fujiyama A Hattori M 《Nature》2005,437(7058):551-555
Chromosome 18 appears to have the lowest gene density of any human chromosome and is one of only three chromosomes for which trisomic individuals survive to term. There are also a number of genetic disorders stemming from chromosome 18 trisomy and aneuploidy. Here we report the finished sequence and gene annotation of human chromosome 18, which will allow a better understanding of the normal and disease biology of this chromosome. Despite the low density of protein-coding genes on chromosome 18, we find that the proportion of non-protein-coding sequences evolutionarily conserved among mammals is close to the genome-wide average. Extending this analysis to the entire human genome, we find that the density of conserved non-protein-coding sequences is largely uncorrelated with gene density. This has important implications for the nature and roles of non-protein-coding sequence elements. 相似文献