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981.
Changes in the species composition, photosynthesis, calcification and size-fractionated carbon metabolism by natural phytoplankton assemblages were monitored in three mesocosms under different nutrient conditions during May 1993. In the 3 enclosures, the decline of the diatom-dominated assemblages was followed by the development of a bloom of the coccolithoporid Emiliania huxleyi. Highest growth of E. huxleyi was observed in the mesocosm with a high N : P ratio, suggesting this species is a good competitor at low phosphate concentrations. The transition from diatom- to E. huxleyi-dominated assemblages brought about a sharp reduction of the phytoplankton standing stock and carbon-specific photosynthetic rate. The relative contribution of the smaller size fraction to total photosynthesis increased as the succession progressed. Calcification rate and E. huxleyi cell-specified calcite production were highest during the early stages of development of the E. huxleyi bloom. Distinct changes in the patterns of 14C allocation into biomolecules were noticed during the diatom-E. huxleyi succession. The diatom-dominated assemblage showed high relative 14C incorporation into low molecular weight metabolites (LMWM), whereas proteins and, specially, lipids accounted for the largest proportion of carbon incorporation in the E. huxleyi bloom. The patterns of photoassimilated carbon metabolism proved to be strongly dependent on cellular size, as protein relative synthesis was significantly higher in the smaller than in the larger size fraction, irrespective of the nutrient regime and the successional stage. These results are discussed in relation to the ecological and physiological features of small phytoplankton. 相似文献
982.
Effects of eucalyptus afforestation on leaf litter dynamics and macroinvertebrate community structure of streams in Central Portugal 总被引:3,自引:3,他引:0
To test the hypothesis whether afforestation with Eucalyptus globulus affects litter dynamics in streams and the structure of macroinvertebrate aquatic communities, we compared streams flowing through eucalyptus and deciduous forests, paying attention to: (i) litterfall dynamics, (ii) accumulation of organic matter, (iii) processing rates of two dominant leaf species: eucalyptus and chestnut, and (iv) macroinvertebrate community structure. The amount of allochthonous inputs was similar in both vegetation types, but the seasonality of litter inputs differed between eucalyptus and natural deciduous forests. Eucalyptus forest streams accumulated more organic matter than deciduous forest streams. Decomposition of both eucalyptus and chestnut leaf litter was higher in streams flowing through deciduous forests. The eucalyptus forest soils were highly hydrophobic resulting in strong seasonal fluctuations in discharge. In autumn the communities of benthic macroinvertebrates of the two stream types were significantly different. Deciduous forest streams contained higher numbers of invertebrates and more taxa than eucalyptus forest streams. Mixed forest streams (streams flowing through eucalyptus forests but bordered by deciduous vegetation) were intermediate between the two other vegetation types in all studied characteristics (accumulation of benthic organic matter, density and diversity of aquatic invertebrates). These results suggest that monocultures of eucalyptus affect low order stream communities. However, the impact may be attenuated if riparian corridors of original vegetation are kept in plantation forestry. 相似文献
983.
Antonio Bode Benita Casas Emilio Fernández Emilio Marañón Pablo Serret Manuel Varela 《Hydrobiologia》1996,341(3):225-234
Summary Chlorophyll-a and primary production on the euphotic zone of the N-NW Spanish shelf were studied at 125 stations between 1984 and 1992. Three geographic areas (Cantabrian Sea, Rías Altas and Was Baixas), three bathymetric ranges (20 to 60 m, 60 to 150 m and stations deeper than 200 m), and four oceanographic stages (spring and autumn blooms, summer upwelling, summer stratification and winter mixing) were considered. One of the major sources of variability of chlorophyll and production data was season. Bloom and summer upwelling stages have equivalent mean and maximum values. Average chlorophyll-a concentrations approximately doubled in every step of the increasing productivity sequence: winter mixing — summer stratification — high productivity (upwelling and bloom) stages. Average primary production rates increased only 60% in the described sequence. Mean (± sd) values of chlorophyll-a and primary production rates during the high productivity stages were 59.7 ± 39.5 mg Chl-a m–2 and 86.9 ± 44.0 mg C m–2 h–1, respectively. Significant differences in both chlorophyll and primary production resulted between geographic areas in most stages. Only 27 stations showed the effects of the summer upwelling that affected coastal areas in the Cantabrian Sea and Rías Baixas shelf, but also shelf-break stations in the Rías Altas area. The Rías Baixas area had lower chlorophyll than both the Rías Altas and the Cantabrian Sea areas during spring and autumn blooms, but higher during summer upwelling events. On the contrary, primary production rates were higher in the Rías Baixas area during blooms in spring and autumn. Mid-shelf areas showed the highest chlorophyll concentrations during high productivity stages, probably due to the existence of frontal zones in all geographic areas considered. The estimated phytoplankton growth rates were comparable to those of other coastal upwelling systems, with average values lower than the maximum potential growth rates. Doubling rates for upwelling and stratification stages in the northern and Rías Altas shelf areas were equivalent, despite larger biomass accumulations during upwelling events. Low turnover rates of the existing biomass in the Rías Baixas shelf in upwelling stages suggests that the accumulation of phytoplankton was due mainly to the export from the highly productive rías, while the contribution of in situ production to these accumulations was relatively lower. 相似文献
984.
985.
Begoña Granadino Luiz O. F. Penalva Lucas Sánchez 《Molecular & general genetics : MGG》1996,250(1):89-96
Crosses betweenDrosophila melanogaster females andD. simulans males produce viable hybrid females, while males are lethal. These males are rescued if they carry theD. simulans Lhr gene. This paper reports that females of the wild-typeD. melanogaster population Staket do not produce viable hybrid males when crossed withD. simulans Lhr males, a phenomenon which we designate as the Staket phenotype. The agent responsible for this phenomenon was found to be the StaketX chromosome (X
mel
,Stk). Analysis of the Staket phenotype showed that it is suppressed by extra copies ofD. melanogaster rDNA genes and that theX
mel
,Stk chromosome manifests a weak bobbed phenotype inD. melanogaster X
mel
,Stk/0 males. The numbers of functional rDNA genes inX
mel
,Stk andX
mel
,y w (control) chromosomes were found not to differ significantly. Thus a reduction in rDNA gene number cannot account for the weak bobbedX
mel
,Stk phenotype let alone the Staket phenotype. The rRNA precursor molecules transcribed from theX
mel
,Stk rDNA genes seem to be correctly processed in both intraspecific (melanogaster) and interspecific (melanogaster-simulans) conditions. It is therefore suggested that theX
mel
,Stk rDNA genes are inefficiently transcribed in themelanogaster-simulans hybrids. 相似文献
986.
987.
Claudia Gaspar Iscia Lopes-Cendes Anita L. DeStefano Patrícia Maciel Isabel Silveira Paula Coutinho Patrick MacLeod Jorge Sequeiros Lindsay A. Farrer G. A. Rouleau 《Human genetics》1996,98(5):620-624
Machado-Joseph disease (MJD) is an autosomal dominant spinocerebellar degeneration originally described in families of Portuguese-Azorean
ancestry. The hypothesis that its present world distribution could result from the spread of an original founder mutation
has been raised. To test this possibility we have conducted a linkage disequilibrium study of markers segregating with the
MJD1 locus in a total of 64 unrelated families of different geographical origins. Significant association was detected between
the MJD1 locus and marker alleles at loci D14S280, D14S1050 and D14S81. All affected individuals, except one Chinese family, had allele
3 (237 bp) at D14S280. This finding is consistent with a founder effect in our MJD population. However, distinct haplotypes
were observed in patients originating from the two Azorean islands showing the highest disease prevalence; therefore, the
possible existence of more than one founder mutation can not be excluded with the markers currently available.
Received: 27 February 1996 / Revised: 4 June 1996 相似文献
988.
Lymphocyte subpopulations of regional lymph nodes in human colon and gastric adenocarcinomas 总被引:3,自引:0,他引:3
Beatriz Lores-Vazquez Margarita Pacheco-Carracedo Josefina Oliver-Morales Purificación Parada-Gonzalez F. Gambón-Deza 《Cancer immunology, immunotherapy : CII》1996,42(6):339-342
In order to study the host immune response to tumours, previous knowledge of the cellular composition of regional draining
lymph nodes is necessary. Enlarged regional lymph nodes are a common finding in colon and gastric adenocarcinomas. We have
studied the cellular composition of normal non-reactive and of regional draining lymph nodes of colon and gastric adenocarcinomas.
In normal non-reactive lymph nodes, T lymphocytes (CD2+, CD7+) constituted the largest fraction of the lymphoreticular cells. These lymphocytes were mainly CD4+, and there were more cells expressing the CD45RA isoform of the CD45 antigen than CD45RO. Reactive lymph nodes presented
a decreased proportion of CD4+ CD45RA+ cells and an increased number of B cells. Although most of the T cells in the reactive nodes were CD4+ CD45RO+, their proportion was similar to that found in normal non-reactive nodes. We studied the presence of the molecules CD28 and
CD80 involved in the processes of interaction and activation of T and B lymphocytes. The CD28 molecule was found in all the
T lymphocytes, while the CD80 molecule was weakly expressed on the B lymphocyte membrane.
Received: 4 January 1996 / Accepted: 28 May 1996 相似文献
989.
Geographic differences in the allele frequencies of the human Y-linked tetranucleotide polymorphism DYS19 总被引:13,自引:0,他引:13
Fabrício R. Santos Tudevdagva Gerelsaikhan Bjamba Munkhtuja Tsendsuren Oyunsuren Jörg T. Epplen Sérgio D. J. Pena 《Human genetics》1996,97(3):309-313
We have studied the allele frequency distribution of the microsatellite locus DYS 19 in several populations with different geographical origins worldwide. Three new alleles were found. In addition, remarkable geographic and ethnic differences were observed in the allele frequency profiles and DNA marker (gene) diversity among populations and major ethnic groups. Amerindians showed an overwhelming predominance of the A allele, while in Caucasians the B allele was modal, and in Greater Asians and Africans allele C became predominant. Even within these geographic regions there were significant gradients, as exemplified by the decreasing frequency profile of the B allele from Great Britain over Germany to Slovakia. Thus, DYS 19 emerges as a useful tool for studying the structure and dynamics of human populations. 相似文献
990.
María E. Jiménez-Capdeville Tomás A. Reader Eduardo Molina-Holgado Robert W. Dykes 《Neurochemical research》1996,21(1):1-6
This study examined the effects of a nerve transection on monoamine release from primary somatosensory cortex. The technique
of microdialysis was employed to sample extracellular levels of norepinephrine (NE), 3,4-dihydroxyphenylacetic acid (DOPAC),
5-hydroxyindole-3-acetic acid (5-HIAA) and homovanillic acid (HVA) in the barrel field of freely moving rats following the
surgical transection of the contralateral infraorbital nerve. Microdialysates obtained 3, 4, and 5 days after deafferentation
were analyzed using high-performance liquid chromatography with electrochemical detection. We found a significant increase
in the release of the dopamine metabolites, DOPAC and HVA from the deafferented cortex. Three days after deafferentation the
release of DOPAC was three-fold higher in the deafferented than in the control animals, and remained about 100% higher in
the next two days in this group of animals. The release of HVA showed a gradual increase following the deafferentation procedure,
since a 92% larger value on day 3 increased to a 338% difference on day 5. On the other hand, the release rate of NE and the
levels of the serotonin metabolite 5-HIAA were not significantly affected by the deafferentation procedure. These results
are discussed in the context of the possible participation of dopamine in the reorganization of the deafferented somatosensory
cortex. 相似文献