Multiple copies of a gene that encodes human U1 small nuclear RNA were introduced into mouse C127 cells with bovine papilloma virus as the vector. For some recombinant constructions, the human U1 gene copies were maintained extrachromosomally on the viral episome in an unrearranged fashion. The relative abundance of human and mouse U1 small nuclear RNA varied from one cell line to another, but in some lines human U1 RNA accounted for as much as one-third of the total U1. Regardless of the level of human U1 expression, the total amount of U1 RNA (both mouse and human) in each cell line was nearly the same relative to endogenous mouse 5S or U2 RNA. This result was obtained whether measurements were made of total cellular U1 or of only the U1 in small nuclear ribonucleoprotein particles that could be precipitated with antibody directed against the Sm antigen. The data suggest that the multigene families encoding mammalian U1 RNA are subject to some form of dosage compensation. 相似文献
Farm-level simulators such as the Agroforestry Estate Model use as inputs either yield tables or outputs from forest modeling
tools. Forest models rely upon assumptions on site index, stem diameter (DBH) distribution, wood production and tree mortality,
which may or may not apply to agroforestry practices. Differences may arise because of the effects on tree growth of unusual
spacings and configurations, fertilizer, pruning and grazing regimes, and tree-understory relationships as well. We examined
data from published or existing field trials to determine mid- and long-term trends in tree growth and understory yields in
silvopastoral practices with southern pines (Pinus spp.) in the United States. Tree DBH and height were greater in practices with improved pastures than in those with spontaneous
grasses. Understory affected DBH more than height and, therefore, DBH-height relationships differed among practices. Sigmoidal
models predicted that tree height will peak at different age depending on tree spacing and understory type. These changes
may affect the accuracy of site indices and wood yield predictions. Livestock gains decrease linearly with increasing stand
basal area and stand age, although forage yields sometimes decay exponentially. In one of the experiments, livestock gains
decreased to almost zero at age 19 but with stand basal areas at that age markedly differing (14 and 25 m2 ha–1) for two different spacings. Additional data would allow to generate empirical algorithms to obtain farm-level simulations
of broad application, improve economic analysis and generate hypotheses to guide future experimental work.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
Few studies have focused on the toxicological risks of dairy fat intake. A standard dairy fat (SDF) with a 70% SFA content and a naturally enriched dairy fat (EDF) in vaccenic, rumenic and α-linolenic acids and low in SFA (54%) have been examined in a 4-week repeated dose oral toxicity study as a daily dose of 2000 mg/kg bw by gavage in rats. Comparisons were established with a third group of rats (control) which did not receive fat administration. Both fats were well tolerated, and no adverse events or mortality were observed during the treatment nor after a 2-week observation period. EDF and SDF did not cause significant differences with respect to a control group in body weight gain, food consumption, clinical observations, organ weight ratios, histopathological findings and most of the hematological and biochemical parameters including total cholesterol and cholesterol fractions in plasma. In rats treated with SDF, a significant increase of triglycerides was observed as compared to the control group. By contrast, in rats treated with EDF, a significant decrease in triglycerides was detected. EDF orally administered to rats was safe, and no treatment-related toxicity was detected. The results also suggest that EDF could protect against the increase of triglyceride concentrations in plasma. 相似文献
We developed metrics at a landscape scale to evaluate the costs and rewards experienced by large herbivores while foraging
in natural vegetation with patchy anti-herbivore plant structures. We show an application of these metrics to the analysis
of 16,000 records of positions at successive 1 min intervals of free-ranging ewes (Ovis aries) harnessed with Global-Positioning-System (GPS) loggers, in a large paddock of the Patagonian Monte shrublands (Argentina).
Dominant shrubs in the area display numerous anti-herbivore defenses (spiny-resinous leaves, thorny stems, etc.) protecting
them from grazing and herbivore trampling. Preferred grasses and forbs constitute a minor part of aboveground plant biomass
and grow in relatively open areas among or around shrub patches. We mapped the movement speed of ewes onto high-resolution
aerial photographs of the grazed paddocks and estimated costs and rewards along their paths based on algorithms of surface
cost theory. Ewes explored areas of sparse vegetation at low speeds compatible with predominant grazing, and increased their
speed when crossing denser shrubby patches. The cost algorithm was applied to evaluate daily searching costs as well as grazing
rewards in relation to the length of daily searching paths. The observed path lengths and search speeds were consistent with
those that compensate costs and rewards of the grazing activities as estimated by the surface cost analysis. We conclude that
the technique presented here constitutes a valuable tool to quantify the effect of landscape characteristics on behavioral
traits of grazing animals in similar environments. 相似文献
Most world drylands are used as graziny lands and undergo degradation of their vegetation cover. The plant cover is typically structured in patchy arrangements, inducing fertility islands critical to maintenance of ecosystem properties. The characteristics of patch structure (size of patches, connectivity-continuity of patch units, etc.) are indicators of the degree of dryland deterioration. We characterized changes in patch structure induced by sheep grazing at a landscape scale using monochromatic low-altitude imagery digitized to a spatial resolution of about 1 m with standard techniques of harmonic analysis applied to develop Fourier signatures. The signatures developed on image line transects were tested with ground samples and mathematical models of plant cover in several dryland fields where spatial deterioration gradients existed. The sensitivity and errors associated to long-wave noise introduced by the geometry of the camera-field-sun spatial arrangement and to high frequency noise introduced by the digitizing process were evaluated by applying suitable filters in the frequency domain. Fourier signatures developed on monochromatic low-altitude imagery proved to be indicative of changes in the patching arrangements of plant cover. We concluded that adequately filtered, high spatial resolution monochromatic images can be used to evaluate the degree of deterioration of dryland landscapes through the computation of selected Fourier signatures in their frequency domain. At comparable cost, aerial photography allows inspecting the landscape at higher spatial resolutions than those attainable with satellite imagery. Also, aerial photos of many areas are available for earlier dates than images from remote sensors, which would allow better inspection of long-term ecosystem changes.This revised version was published online in May 2005 with corrections to the Cover Date. 相似文献
Shiitake mushrooms (Lentinus edodes) were packaged under air in two macroperforated packages (A—9.0 × 103 perforations/m2, 0.1 mm2 surface and B—17 perforations/m2, 0.1 mm2 surface) and under two gas mixtures (15% and 25% O2) in polyethylene packages, and stored at 5 °C for 18 d. The sensory quality of the mushrooms was evaluated using a trained assessors panel.
Mushrooms packaged in macroperforated packages A showed the smallest deterioration rate. However, weight loss after 6 d of storage reached 15%, which is unacceptable. Besides, shelf life of shiitake mushrooms in active modified atmosphere was limited by off-odour development. Mushrooms in these packages developed off-odour after 12 d of storage due to the fact that O2 concentrations fall bellow 5%.
On the other hand, the shelf life of mushrooms packaged in macroperforated packages B was limited by their sensory deterioration, particularly by changes in the colour and uniformity of their gills. In this film, mushrooms could be stored for approximately 10 d with a weight loss lower than 2%.
Results from the present work suggest that during the first 6 d of storage all the evaluated packaging conditions were useful for reducing mushroom deterioration rate. 相似文献
Riverine silvopastoral practices with native pecan (Carya illinoinensis) are a suitable land use for areas subjected to seasonal flooding in southern and central regions of the United States. Nut,
timber and forage production, and the economics of managed pecan silvopastures were examined in southeastern Kansas. During
1981–2000, annual hulled nut production varied between 50 and 1600 kg ha−1 in stands averaging 72 years of age, and ranging in density between 35 and 74 trees ha−1. The nut crop had a pattern of biennial bearing with some exceptions. Tree stem diameter and stand basal area increased linearly
with time. Nut production was not related to stand age or tree density, however, suggesting that nut production had reached
a steady state level. Merchantable timber yield ranged between 0.25 and 1.35 m3 ha−1 year−1. In pecan silvopastures with a mean tree age of 37 years, forage production varied between 1500 and 4600 kg DM ha−1 in 2001 and 2002. In 2001 only, grass production decreased with decreasing solar radiation within the range of 0.25–0.83
of fraction light transmitted. In both years, the grass understory had acceptable quality for cow-calf production with average
crude protein content between 9 and 11.8%, and no evidence of excessive levels of ergoalkaloids from tall fescue. Twenty-seven
vascular plants were identified in the understory of which nut sedge (Cyperus esculentus), tall fescue (Festuca arundinacea), wild oat (Avena fatua) and Canadian wild rye (Elymus canadensis) were the most abundant. Economic simulations obtained with the U.S. Agroforestry Estate Model indicated that pecan nut price
is the main variable driving economic outputs under current production conditions. Annual cash flows from nut sales had smaller
fluctuations than nut yields because of an inverse relation between nut price and yield. Improved timber production appears
an option for increasing profitability of pecan silvopastures. 相似文献
Many landowners in the United States have little knowledge of the potential economic returns from agroforestry practices.
Economic simulators for temperate agroforestry practices have been generated; yet, there are few data sets on yields of timber
and other products to validate and refine such models. The objectives of this study were to characterize variations in nut
yields among open canopy eastern black walnut (Juglans nigra L.) trees and apply this information to the development of predictive equations between tree diameter at breast height (DBH)
and nut yields. Three data sets were analyzed that included results from Tennessee; Chetopa, Kansas; and Mt Vernon, Missouri.
Tree-to-tree variation in nut yields was high within each data set, with coefficients of variation for nut yields typically
exceeding 50%. Averaging nut yields over several consecutive years reduced coefficients of variation. Nearly half of the high
nut producing trees exhibited an alternate, biennial nut bearing pattern. Trees with low average nut yields had either sporadic
or irregular patterns of nut bearing. The regression coefficients for equations relating stem diameter and nut yields varied
considerably. Averaging nut yields over consecutive years, and averaging stem diameter and nut yields over a number of trees
increased regression coefficients of such equations. These results indicate that predicting nut yields of a tree stand over
a several year-period will be easier than predicting yields for a specific tree in a specific year.
Deceased 2002 相似文献
Computational approaches are breaking new ground in understanding how embryos form. Here, we discuss recent studies that couple precise measurements in the embryo with appropriately matched modeling and computational methods to investigate classic embryonic patterning strategies. We include signaling gradients, activator-inhibitor systems, and coupled oscillators, as well as emerging paradigms such as tissue deformation. Parallel progress in theory and experiment will play an increasingly central role in deciphering developmental patterning. 相似文献