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Columbian Sharp-tailed Grouse ( Tympanuchus Phasianellus columnianus ) and Sage Grouse ( Centro-cercus urophasianus ) leks were compared in an area of sympatry in south central Wyoming. Sharp-tailed Grouse leks had more (P < .05 shrub cover, taller shrubs, more forb, grass, and shrub species, and less visibility than did Sage Grouse leks. Reduction in shrub cover or the diversity of herbaceous species could potentially have greater influence on the use of lek sites by Columbian Sharp-tailed Grouse than by Sage Grouse in areas of sympatry in Wyoming. 相似文献
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A candidate for the cystic fibrosis locus isolated by selection for methylation-free islands 总被引:10,自引:0,他引:10
X Estivill M Farrall P J Scambler G M Bell K M Hawley N J Lench G P Bates H C Kruyer P A Frederick P Stanier 《Nature》1987,326(6116):840-845
A genomic sequence close to the cystic fibrosis locus with the characteristics of an HTF island has been selectively cloned and characterized. Two markers flanking this sequence, which is conserved throughout mammalian evolution, show a very much greater disequilibrium than that found with any existing marker. A single mutational event accounts for most cases of cystic fibrosis. The sequence is expressed, and is a candidate for the cystic fibrosis gene. 相似文献
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Frederick Beiser 《Studies in history and philosophy of science》2003,34(1):135-147
Against current non-metaphysical interpretations, I argue that Naturphilosophie is central to Hegel’s philosophy. This is so for three reasons. First, it was crucial to Hegel’s program to create a holistic culture. Second, Naturphilosophie is pivotal to absolute idealism, Hegel’s characteristic philosophical doctrine. Third, the idea of organic development, so central to Naturphilosophie, is pervasive throughout Hegel’s system. This idea is essential to Hegel’s concepts of spirit, dialectic, and identity-in-difference. Finally, I take issue with the neo-Kantian critique of Hegel’s Naturphilosophie on the grounds that it fails to appreciate the underlying motive behind Hegel’s system: the attempt to resolve the aporia of Kant’s epistemology. 相似文献
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IgH class switching and translocations use a robust non-classical end-joining pathway 总被引:1,自引:0,他引:1
Yan CT Boboila C Souza EK Franco S Hickernell TR Murphy M Gumaste S Geyer M Zarrin AA Manis JP Rajewsky K Alt FW 《Nature》2007,449(7161):478-482
Immunoglobulin variable region exons are assembled in developing B cells by V(D)J recombination. Once mature, these cells undergo class-switch recombination (CSR) when activated by antigen. CSR changes the heavy chain constant region exons (Ch) expressed with a given variable region exon from Cmu to a downstream Ch (for example, Cgamma, Cepsilon or Calpha), thereby switching expression from IgM to IgG, IgE or IgA. Both V(D)J recombination and CSR involve the introduction of DNA double-strand breaks and their repair by means of end joining. For CSR, double-strand breaks are introduced into switch regions that flank Cmu and a downstream Ch, followed by fusion of the broken switch regions. In mammalian cells, the 'classical' non-homologous end joining (C-NHEJ) pathway repairs both general DNA double-strand breaks and programmed double-strand breaks generated by V(D)J recombination. C-NHEJ, as observed during V(D)J recombination, joins ends that lack homology to form 'direct' joins, and also joins ends with several base-pair homologies to form microhomology joins. CSR joins also display direct and microhomology joins, and CSR has been suggested to use C-NHEJ. Xrcc4 and DNA ligase IV (Lig4), which cooperatively catalyse the ligation step of C-NHEJ, are the most specific C-NHEJ factors; they are absolutely required for V(D)J recombination and have no known functions other than C-NHEJ. Here we assess whether C-NHEJ is also critical for CSR by assaying CSR in Xrcc4- or Lig4-deficient mouse B cells. C-NHEJ indeed catalyses CSR joins, because C-NHEJ-deficient B cells had decreased CSR and substantial levels of IgH locus (immunoglobulin heavy chain, encoded by Igh) chromosomal breaks. However, an alternative end-joining pathway, which is markedly biased towards microhomology joins, supports CSR at unexpectedly robust levels in C-NHEJ-deficient B cells. In the absence of C-NHEJ, this alternative end-joining pathway also frequently joins Igh locus breaks to other chromosomes to generate translocations. 相似文献
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The analysis and forecasting of electricity consumption and prices has received considerable attention over the past forty years. In the 1950s and 1960s most of these forecasts and analyses were generated by simultaneous equation econometric models. Beginning in the 1970s, there was a shift in the modeling of economic variables from the structural equations approach with strong identifying restrictions towards a joint time-series model with very few restrictions. One such model is the vector auto regression (VAR) model. It was soon discovered that the unrestricted VAR models do not forecast well. The Bayesian vector auto regression (BVAR) approach as well the error correction model (ECM) and models based on the theory of co integration have been offered as alternatives to the simple VAR model. This paper argues that the BVAF., ECM, and co integration models are simply VAR models with various restrictions placed on the coefficients. Based on this notion of a restricted VAR model, a four-step procedure for specifying VAR forecasting models is presented and then applied to monthly data on US electricity consumption and prices. 相似文献
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Generation of long-term antibody-mediated immunity depends on the germinal centre reaction, which requires cooperation between antigen-specific T and B lymphocytes. In human X-linked lymphoproliferative disease and its gene-targeted mouse model, loss-of-function mutations in signalling lymphocyte activation molecule-associated protein (SAP, encoded by SH2D1a) cause a profound defect in germinal centre formation by an as yet unknown mechanism. Here, using two-photon intravital imaging, we show that SAP deficiency selectively impairs the ability of CD4(+) T cells to stably interact with cognate B cells but not antigen-presenting dendritic cells. This selective defect results in a failure of antigen-specific B cells to receive adequate levels of contact-dependent T-cell help to expand normally, despite Sap(-/-) T cells exhibiting the known characteristics of otherwise competent helper T cells. Furthermore, the lack of stable interactions with B cells renders Sap(-/-) T cells unable to be efficiently recruited to and retained in a nascent germinal centre to sustain the germinal centre reaction. These results offer an explanation for the germinal centre defect due to SAP deficiency and provide new insights into the bi-directional communication between cognate T and B cells in vivo. 相似文献