o,p′-DDT induction of vitellogenesis and its inhibition by tamoxifen in Nile tilapia (Oreochromis niloticus)     

Olga Leaos-Castaeda  Glen Van Der Kraak  Andrea Lister  Raúl Sim-Alvarez  Gerardo Gold-Bouchot 《Marine environmental research》2002,54(3-5)

In order to investigate the mechanism by which o,p′-DDT disrupts endocrine functioning of Nile tilapia in vivo, the estrogenicity of o,p′-DDT was investigated in conjunction with 17β-estradiol (E₂) and tamoxifen. Mature, male tilapia were treated intraperitoneally with o,p′-DDT (60 mg/kg, one dose) or E₂ (5 mg/kg, four doses) in the presence or absence of tamoxifen (5 mg/kg, six doses) for 12 days and then plasma vitellogenin (Vtg) (measured as alkaline-labile phosphorous), E₂, and testosterone (T) were measured. Vtg levels were increased dramatically by E₂ (1744±171 μg/ml) and moderately by o,p′-DDT (82±15 μg/ml) compared with controls (23±3.5 μg/ml). Tamoxifen alone had no effect on Vtg production, but inhibited both E₂ and o,p′-DDT stimulated vitellogenesis. T levels were reduced with E₂ administration (1688±383 pg/ml) and declined further with the combined treatment of E₂ and tamoxifen (281±70 pg/ml), compared with controls (6558±1438 pg/ml). Tamoxifen or o,p′-DDT alone did not affect T levels, but their combined treatment did (2069±647 pg/ml). The results of this study suggest that o,p′-DDT is weakly estrogenic in male tilapia, and that this activity may be mediated through the estrogen receptor.  相似文献   

Spatial variability of phytoplankton absorption coefficients and pigments off Baja California during November 2002     

Óscar A. Barocio-León  Roberto Millán-Núñez  Eduardo Santamaría-del-Ángel  Adriana González-Silvera  Charles C. Trees 《Journal of Oceanography》2006,62(6):873-885

We have estimated the spatial variability of phytoplankton specific absorption coefficients (a* _ph ) in the water column of the California Current System during November 2002, taking into account the variability in pigment composition and phytoplankton community structure and size. Oligotrophic conditions (surface Chl < 0.2 mg m⁻³) dominated offshore, while mesotrophic conditions (surface Chl 0.2 to 2.0 mg m⁻³) where found inshore. The specific absorption coefficient at 440 [a* _ph (440)] ranged from 0.025–0.281 m²mg⁻¹ while at 675 nm [a* _ph (675)] it varied between 0.014 and 0.087 m²mg⁻¹. The implementation of a size index based on HPLC data showed the community structure was dominated by picoplankton. This would reduce the package effect in the variability of a* _ph (675). Normalized a _ph curves were classified in two groups according to their shape, separating all spectra with peaks between 440 and 550 nm as the second group. Most samples in the first group were from surface layers, while the second group were from the deep chlorophyll maximum or deeper. Accessory photoprotective pigments (APP) tended to decrease with depth and accessory photosynthetic pigments (APS) to increase, indicating the importance of photoprotective mechanisms in surface layers and adaptation to low light at depth. Samples with higher ratios of APP:APS (>0.4) were considered as phytoplankton adapted to high irradiances, and lower ratios (<0.26) as adapted to low irradiances. We found a good relationship between APP:APS and a* _ph (440) for the deeper layer (DCM and below), but no clear evidence of the factors causing the variability of a* _ph (440) in the upper layer.  相似文献   

Cosmological parameters and redshift periodicity     

Ágnes Holba  I. Horváth  B. Lukács  G. Paál 《Astrophysics and Space Science》1992,198(1):111-120

This work is the continuation of the search for such a cosmological model using which the observed redshift distribution of galaxies in the sample of Broadhurstet al. (1990) turns out to be maximally periodic in the calculated spatial distance. In a previous work, Paálet al. (1992) have demonstrated that among theflat models with non-negative cosmological constant (e.e., vacuum density) the one with a vacuum: dust ratio 2:1 provides the optimum. Now we extend that study to the case of arbitrary space curvature and find equally good periodicity in a surprisingly wide range of models. By use of the dimensionless parameters ₀= ₀/ _crit and ₀=/3H ₀ ² acceptable periodicity is obtained forall points of the parameter plane within the strip between the parallel lines 0.83₀–0.30< ₀(₀)<0.83₀+0.85(₀<1.8), whilst the best periodicities appear along the line ₀=0.83₀+0.39 fitting to the previous optimum at ₀=1/3, ₀=2/3. Any nonpositive value of ₀ gives bad periodicity unless the space curvature is strongly negative and ₀<0.4. Fairly good periodicity is observed only in the range of the deceleration parameter –1.2q ₀<0.2, corresponding to a small or even negative total gravitational attraction and an expansion time-scale longer than usually expected.  相似文献   

Bipolar molecular outflows     

R. Bachiller  J. Gómez-González 《Astronomy and Astrophysics Review》1992,3(3-4):257-287

Summary Bipolar molecular outflows are a ubiquitous phenomenon in the process of star formation. We review the main observational properties of the outflows around young stellar objects, highlighting the recent wealth of information provided by the new generation of large radiotelescopes operating at millimeter wavelengths (in particular the IRAM 30-m, the NRO 45-m, and the JCMT 15-m dishes). The observations of outflows containing molecular, jet-like flows and bullets are discussed in detail, as they provide key information for understanding origin and evolution of the outflows. We also discuss a number of closely related issues: the evidence for dense shocked gas associated with the flows, the interaction of the outflows with the ambient dense cores, the evolutionary status of the sources driving the outflows, the properties of circumstellar disks, and theoretical models of the origin of the neutral winds. All these areas are important for developing a plausible scenario for the formation and evolution of the bipolar molecular outflows.This article was processed by the author using the Springer-Verlag T_EX AAR macro package 1991.  相似文献   

The Yale/San Juan Southern Proper Motion program     

William F. van Altena  Terrence Girard  Carlos E. López 《Astrophysics and Space Science》1991,177(1-2):59-63

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CS observations of selected star-forming regions     

J. Buj  R. Estalella  R. López  G. Anglada  P. Planesas 《Astrophysics and Space Science》1989,156(1-2):251-256

Maps of the CS (J=10) emission at 49 GHz of L43 (RNO 91), NGC 2068 (HH 19-27) and L1524 (Haro 6-10) are presented. Physical parameters of these clouds are derived from the observations. The CS emission is compared with ammonia emission maps obtained with a similar angular resolution. Both CS and ammonia molecules are tracers of the high-density gas in molecular clouds. Thus, a similar distribution of the emission is expected. However, our observations show evidence of some remarkable morphological differences.  相似文献   

Physical parameters of reflection nebulae in the galaxy     

M. Centurión  G. Vladilo 《Astrophysics and Space Science》1989,156(1-2):51-56

We show how, given observed equivalent widths of Mgii and Mgi absorptions due to an interstellar cloud in which a late-B star is embedded, the basic physical parameters: kinetic temperature, mean density, electron density, and radius can be constrained. Hydrogen ionization by means of cosmic rays and the effect of the stellar radiation field on the magnesium ionization equilibrium are taken into account.The method is applied to the reflection nebula surrounding the star HD 26676. The resulting solutions for the radius and temperature of the nebulosity are comparable to the typical values derived for diffuse interstellar clouds from optical and 21-cm measurements, if a cosmic-ray ionization rate 10^–16s^–1 — in agreement with recent determinations — is assumed. The results are not strongly dependent on the gas pressureP forP varying in a range of values typical of interstellar clouds.  相似文献   

Historical Landscape Change in Yellowstone National Park: Demonstrating the Value of Intensive Field Observation and Repeat Photography          下载免费PDF全文

Judith l. Meyer  Yolonda Youngs 《Geographical review》2018,108(3):387-409

Although transportation and outdoor recreation are well‐recognized aspects of national parks, few studies have explored these aspects from the perspective of human geography as a means of analyzing historical landscape change. This paper offers an innovative synthesis of methods for studying cultural landscape change over time through a case study of the historical geography of transportation, tourism, and outdoor recreation along the Howard Eaton Trail (HET) in Yellowstone National Park. We conducted research through a field course that combined repeat photography, archival research, Geographic Information Systems (GIS), and traditional field methods. Results indicate that a combination of repeat photography and other methods can create an effective means of evaluating cultural landscape change; even short field courses provide opportunities for students and faculty to conduct collaborative research that provides powerful, multidimensional, situated‐learning experiences; and repeat photography creates datasets that may benefit future research and teaching.  相似文献   

The role of cultural and creative services in the evaluation of the world urban system     

Roberto Díez-Pisonero  Cándida Gago  Juan Córdoba 《Norsk geografisk tidsskrift. Norwegian journal of geography》2018,72(4):197-216

Traditionally, academic evaluation of the world urban system has been based on economic approaches and focused on primary cities. However, the recent convergence between economic and cultural spheres, known in specialist literature as the ‘new economy’ or ‘cognitive–cultural capitalism’, requires the inclusion of secondary cities and cultural parameters, such as creative and cultural services, to obtain knowledge of the world urban system with greater accuracy. Accordingly, the authors have developed the Synthetic Index of Cultural Components (objective 1). Subsequent statistical treatment through a principal component analysis and a cluster analysis not only shows a representation of the multifunctional character of the world urban system (objective 2) but also identifies the cultural specializations that urban planners use to position cities worldwide (objective 3). The results demonstrate that the reputation of a city derives not only from its productive economic character but also from other cultural creative services that give meaning to its multidimensional perspective. The authors conclude that these services are empirically valid when the cultural dimension is measured, as they are illustrative of the cultural mechanisms that help cities to supply symbolic, experiential, and cultural capital, all of which are vital in the competitive context of globalization.  相似文献   

Impact of lithology and soil properties on abandoned dryland terraces during the early stages of soil erosion by water in south‐east Spain          下载免费PDF全文

Carlos Martínez‐Hernández  Jesús Rodrigo‐Comino  Asunción Romero‐Díaz 《水文研究》2017,31(17):3095-3109

Soil erosion by water in abandoned dry terraces is one of the most important environmental problems in semiarid areas, enhancing biological degradation and reducing possible resources that can be obtained. However, little is known about the effects of the types of lithology and soil properties on the early stages of soil erosion. Therefore, the main aim of this research was to assess the effect of different lithologies (marls, limestones, and metamorphic—phyllites, schists, and greywackes—materials) and soil properties on the early stages of soil erosion by water in abandoned dry terraces, compared with similar terraces still in agricultural use. Soil analyses (texture, aggregate stability, and bulk density) and 22 rainfall simulations were carried out under dry conditions. During the experiments, local inclination, vegetation and stone cover, total organic matter, and antecedent soil moisture were also quantified. The results showed that the highest soil loss (41.41 g/m² in cultivated plots and 17.05 g/m² in the abandoned plots) and runoff (3.79 L/m² in the abandoned plot) occurred on marl substrata. Marls also showed the shallowest infiltration front (9 cm) and lowest infiltration rate (4.3 cm/min). Limestones and, especially, metamorphic areas, showed a lower degree of soil erosion, higher infiltration rates, and deeper infiltration fronts.  相似文献