Ciba Foundation symposium最新文献

Drosophila melanogaster has an unusual telomere elongation mechanism. Instead of short repeats that are synthesized by telomerase, long retrotransposons, HeT-A and TART, transpose to the ends of chromosomes. This mechanism generates tandem arrays of these elements at the chromosome ends, in which all elements are oriented with their oligo(A) tails towards the centromere. Structural features of HeT-A and TART elements may provide clues as to their transposition mechanism. Drosophila telomere length polymorphism is mainly due to terminal retrotransposon arrays that differ between chromosome tips and that change with time. In addition, stable terminal chromosome deletions can be generated that do not contain terminal HeT-A and TART arrays, suggesting that, unlike the equivalent terminal repeats in yeast and humans, the presence and length of terminal arrays in Drosophila may not be critical for cell cycle progression.

Precision agriculture is based on the concept of soil-specific management, which aims to adapt management within a field according to specific site conditions in order to maximize production and minimize environmental damage. This paper examines how the nature and sources of variation in soil moisture regimes affect our ability to simulate soil water behaviour within a field with adequate precision in order to advise optimal soil-specific management. Field examples of variation in soil moisture regimes are described to illustrate the difficulties involved. A discussion identifies three main points. First, it is recognized that the current modelling approaches to soil moisture regimes do not sufficiently account for local heterogeneities in soil and crop characteristics such as soil morphology and rooting patterns. Second, the estimation of within-field variation of soil hydraulic properties is difficult because of large short-range variation of the properties and general lack of observed data; one way to overcome this problem is to seek new measurement techniques or to find easy-to-measure auxiliary variables spatially correlated to the variables of interest. Last, as pollution impacts often become noticeable to society at scales larger than the scale of agricultural management, hydrological modelling can serve for linking both scales and advising agricultural practices that minimize undesirable pollution effects.

Accurate interpolation of soil and climate variables at fine spatial scales is necessary for precise field management. Interpolation is needed to produce the input variables necessary for crop modelling. It is also important when deciding on regulations to limit environmental impacts from processes such as nitrate leaching. Non-stationarity may arise due to many factors, including differences in soil type, or heterogeneity in chemical concentrations. Many geostatistical methods make stationarity assumptions. Substantial improvements in interpolation or in the estimation of standard errors may be obtained by using non-stationary models of spatial covariances. This paper presents recent methodological developments for an approach to modelling non-stationary spatial covariance structure through deformations of the geographic coordinate system. This approach was first introduced by Sampson & Guttorp, although the estimation approach is updated in more recent papers. They compute a deformation of the geographic plane so that the spatial covariance structure can be considered stationary in terms of a new spatial coordinate system. This provides a non-stationary model for the spatial covariances between sampled locations and prediction locations. In this paper, we present a cross-validation procedure to avoid over-fitting of the sample dispersions. Results concerning the variability of the spatial covariance estimates are also presented. An example of the modelling of the spatial correlation field of rainfall at small regional scale is presented. Other directions in methodological development, including modelling temporally varying spatial correlation, and approaches to model temporal and spatial correlation are mentioned. Future directions for methodological development are indicated, including the modelling of multivariate processes and the use of external spatially dense covariables. Such covariates are frequently available in precision agriculture.

Application of the theories of precision agriculture to the practicalities of broad-acre farming relies on successful handling of the ramifications of uncertainty in information, i.e. information pertaining to the spatial and temporal variation of those factors which determine yield components and/or environmental losses. This paper discusses the uncertainty of yield and related variables as measured by their spatial and temporal variance. The magnitude of these two components gives a suggestion as to the appropriate scale of management. Simultaneous reporting on spatial and temporal variation is rare and the theory of these types of process is still in its infancy. Some brief theory is presented, followed by several examples from the Rothamsted classic experiments, yield-monitoring experiments in Australia, a long-term barley trial in Denmark, and a soil moisture monitoring network. It is clear that annual temporal variation is much larger than the spatial variation within single fields. This leads to the conclusion that if precision agriculture is to have a sound scientific basis and ultimately a practical outcome then the null hypothesis that still remains to be seriously researched is: 'given the large temporal variation in yields relative to the scale of a single field, then the optimal risk aversion strategy is uniform management.'

Methicillin-resistant Staphylococcus aureus (MRSA) is an intractable nosocomial pathogen. The chemotherapeutic intransigence of this organism stems from its predilection to antimicrobial resistance as a consequential response to selective pressures prevailing in the clinical environment. MRSA isolates are frequently resistant to all practicable antimicrobials except the glycopeptide, vancomycin. Although antimicrobial resistance sometimes arises via chromosomal mutation, the emergence of multiply-antibiotic-resistant staphylococci is primarily due to the acquisition of pre-existent resistance genes; such determinants can be encoded chromosomally or by plasmids and are often associated with transposons or insertion sequences. Clinical staphylococci commonly carry one or more plasmids, ranging from small replicons that are phenotypically cryptic or contain only a single resistance gene, to larger episomes that possess several such determinants and sometimes additionally encode systems that mediate their own conjugative transmission and the mobilization of other plasmids. The detection of closely related plasmids, elements and/or genes in other hosts, including coagulase-negative staphylococci and enterococci, attests to interspecific and intergeneric genetic exchange facilitated by mobile genetic elements and DNA transfer mechanisms. The extended genetic reservoir accessible to staphylococci afforded by such horizontal gene flux is fundamental to the acquisition, maintenance and dissemination of staphylococcal antimicrobial resistance in general, and multiresistance in particular.

Cell nuclei contain several abundant enzymes that bind rapidly and avidly to exposed termini of DNA. The properties and physiological roles of such factors are described; they include poly (ADP-ribose) polymerase, DNA-dependent protein kinase, several DNA ligases and excision-repair enzymes. Telomeres normally seem shielded from these activities by telomere-binding proteins. If incomplete protection of telomeres occurred, the functions of the DNA end-specific enzymes would be relevant for processing of telomeres. This could include alternative pathways for telomere propagation in telomerase-negative cells.

The Tc1 and Tc2 subsets of CD8+ T effector cells secrete different patterns of cytokines, but have similar functions, including perforin- and Fas-dependent cytotoxicity, and induction of delayed type hypersensitivity (DTH) reactions involving oedema and granulocytic infiltration. The characteristic cytokines of Tc1 (gamma-interferon) and Tc2 (interleukins 4 and 5) are expressed in vivo during the DTH reaction. Tc1 cells that are deficient in cytokine synthesis also induce similar levels of DTH, supporting the lack of correlation between CD8+ T cell cytokine patterns and DTH. CD8+ T cells often produce lower cytokine levels than CD4 cells because the CD8 cells kill their antigen-presenting cells before full stimulation can occur. This effect can be counteracted by increasing the frequency of stimulation, or using perforin-deficient T cells. A multiparameter analysis of cytokine effects on CD8+ T cell differentiation has been initiated, on the basis of the principle that normal immune responses involve complex cytokine mixtures. All combinations of seven cytokines were tested. In some combinations, the combined effect could not have been predicted from individual cytokine functions. Conditions were identified in which each of interleukins 4, 10 and 12 could have opposite effects on CD8+ T cell differentiation.