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991.
The concept of individuality as applied to species, an important advance in the philosophy of evolutionary biology, is nevertheless in need of refinement. Four important subparts of this concept must be recognized: spatial boundaries, temporal boundaries, integration, and cohesion. Not all species necessarily meet all of these. Two very different types of pluralism have been advocated with respect to species, only one of which is satisfactory. An often unrecognized distinction between grouping and ranking components of any species concept is necessary. A phylogenetic species concept is advocated that uses a (monistic) grouping criterion of monophyly in a cladistic sense, and a (pluralistic) ranking criterion based on those causal processes that are most important in producing and maintaining lineages in a particular case. Such causal processes can include actual interbreeding, selective constraints, and developmental canalization. The widespread use of the biological species concept is flawed for two reasons: because of a failure to distinguish grouping from ranking criteria and because of an unwarranted emphasis on the importance of interbreeding as a universal causal factor controlling evolutionary diversification. The potential to interbreed is not in itself a process; it is instead a result of a diversity of processes which result in shared selective environments and common developmental programs. These types of processes act in both sexual and asexual organisms, thus the phylogenetic species concept can reflect an underlying unity that the biological species concept can not. 相似文献
992.
Ovine placental lactogen and the SBU-3 antigen (derived from a trophoblast membrane preparation), two proteins of widely different structure, function and destination, were shown by ultrastructural immunogold techniques to localize in identical structures in the sheep placentome throughout most of pregnancy. Both were restricted to the ultrastructurally similar membrane-bounded granules in the chorionic fetal binucleate cell and the syncytium at the fetomaternal interface. The Golgi body from which the granules form was also doubly labelled but only in the binucleate cell, never the syncytium. This provides further evidence that the binucleate cells migrate and fuse to form the syncytium. The two proteins were homogeneously distributed in the granules and would be released together by exocytosis. Only the lactogen reaches the fetal and maternal circulations so the SBU-3 may have some more local function. In early pregnancy the SBU-3 antigen is found by itself in the granules, indicating that the association with the lactogenic hormone is not obligatory. Neither antigen was found consistently in the otherwise ultrastructurally similar interplacentomal binucleate cell granules, corroborating the presence of two functional populations of binucleate cells. 相似文献
993.
Tissue-specific posttranslational modification of rat apoE. Synthesis of sialated apoE forms by neonatal rat aortic smooth muscle cells 总被引:1,自引:0,他引:1
We have studied the synthesis, modification, and secretion of rat apoE in primary cultures of neonatal aortic smooth muscle cells and adult rat hepatocytes. The cultures were pulsed with [35S]methionine and the intracellular and secreted apoE were immunoprecipitated and analyzed by two-dimensional isoelectric focusing/polyacrylamide gel electrophoresis and autoradiography. A short pulse (10 min) showed the presence of a major unmodified apoE form in both cultures. This form comigrated on two-dimensional gels with the major rat plasma apoE isoprotein. A longer pulse (15-120 min) resulted in the progressive appearance of intracellularly modified apoE isoproteins in both cultures. The apoE secreted by aortic smooth muscle cells consisted exclusively of sialated apoE isoproteins that were sensitive to neuraminidase treatment. In contrast, the apoE secreted by primary cultures of adult rat hepatocytes, organ cultures of neonatal rat liver, as well as rat plasma apoE, contained several minor modified isoproteins. Nascent apoE secreted by the aortic smooth muscle cell cultures floats in the density range of 1.09 to 1.186 g/ml. We conclude that aortic smooth muscle cells can synthesize and secrete sialated apoE isoproteins associated with nascent lipoproteins floating in the high density lipoprotein region. 相似文献
994.
C S Lee M M Ralph K J Gogolin-Ewens M R Brandon 《The journal of histochemistry and cytochemistry》1990,38(5):649-652
We studied the localization of alpha-keratin in the sheep placenta using an alpha-keratin-specific monoclonal antibody (MAb) SBU-1, and examined the feasibility of using this MAb as a marker for determining the purity of isolated uninucleate cells from the placentomal trophoblast. At about 30-50 days of gestation the placentomal and interplacentomal uninucleate cells and some binucleate cells were stained by SBU-1, whereas only the apical region of the syncytial cytoplasm was stained with this MAb. Other cells stained included the uterine and endometrial glandular epithelial cells and fibroblast-like cells in the endometrium and chorionic villi. At about 100-130 days of gestation only the trophoblast uninucleate cells were stained by SBU-1. Approximately 60% of cells isolated from placentomes at 100-130 days of gestation were stained by SBU-1, and they had similar morphological features to the trophoblast uninucleate cells. The number of binucleate cells present was confirmed by their affinity for MAb SBU-3. These results show that MAb SBU-1 is an excellent marker for trophoblast uninucleate cells from placenta of sheep at the later stages of pregnancy. 相似文献
995.
Summary We measured meteorological conditions and estimated the energy costs of thermoregulation for young and adult Adélie Penguins (Pygoscelis adeliae) at a breeding colony near the Antarctic Peninsula. Air temperatures averaged < 5°C and strong winds were frequent. Operative temperatures (Te) for adults ranged from –8 to 28°C, averaging 5–6°C, for the period from courtship to fledging of chicks. The average energy cost of thermoregulation (Cth) for adult penguins was equivalent to 10–16% of basal metabolism. Cth comprised about 15% of the estimated daily energy budget (DEB) of incubating adults, but only about 1% of the DEB of adults feeding chicks. The Te's for chicks older than 14 days ranged from 0 to 31°C, averaging 8.0 C. The Cth for downy chicks ranged from about 31% of minimal metabolic rate (MMR) in 1 kg chicks to about 10% of MMR in 3 kg chicks. Between initial thermal independence (age 12–14 days) and the cessation of parental feeding (age 35–40 days), chicks use about 10–11% of assimilated energy for thermoregulation. Cth is equivalent to about 17% of the MMR of fledglings during their 2–3 week fast. We observed no indication of thermal stress (i.e., conditions in which birds cannot maintain stable Tb) in adults and no indication of cold stress in any age class. However, on clear, calm days when air temperature exceeds 7–10°C for several hours, downy chicks are vulnerable to lethal hyperthermia. 相似文献
996.
Following a series of continuous exposures to 14CO2 for different lengths of time, leaves from Neurachne munroi (C4), N. minor (C3-C4) and N. tenuifolia (C3| were estimated to assimilate 100%, 9% and 2–4%, respectively, of atmospheric CO2 by the C4 pathway. The percentage of 14C-label appearing in malate and aspartate in leaves of N. minor progressively increased with longer exposure times indicating that a significant proportion of its C4 acids are formed as secondary products. In 14CO2/12CO2 pulse/chase experiments, the 14C-label in leaves of N. munroi was rapidly transferred from C4 acids to sugar monophosphates plus sugar diphosphates, and finally to sucrose. In leaves of N. minor, the 14C-label was slowly metabolized from the C-4 carboxyl of malate and asparate (apparent half-time = 250 s), and the formation of C4 acids as secondary products was again evident. 14C-label in serine/glycine accumulated to comparable magnitudes in both N. minor and in N. tenuifolia, but there was an initial lag phase in the accumulation of label in N. minor. C4 photosynthesis is apparently of minimal importance in reducing photorespiration in N. minor, but leaf anatomical specializations and a possible compartmentation of photorespiratory metabolism may be of considerable importance. 相似文献
997.
Significant similarity and dissimilarity in homologous proteins. 总被引:5,自引:0,他引:5
Common practice emphasizes significant sequence similarities between different members of protein families. These similarities presumably reflect on evolutionary conservation of structurally and functionally essential residues. The nonconserved regions, on the other hand, may be either selectively neutral or differentiated. We propose several distributional sequence statistics (e.g., clustering of charged residues, compositional biases, and repetitive patterns) as indicators of differentiation events. These ideas are illustrated with various examples, including comparisons among G protein-coupled receptors, herpesvirus proteins, and GTPase-activating proteins. 相似文献
998.
Disruption of the cystic fibrosis transmembrane conductance regulator gene in embryonic stem cells by gene targeting 总被引:1,自引:0,他引:1
Rosemary Ratcliff Martin J. Evans Joanne Doran Brandon J. Wainwaright Robert Williamson William H. Colledge 《Transgenic research》1992,1(4):177-181
We have successfully disrupted thecftr (cystic fibrosis transmembrane conductance regulator) gene at its endogenous locus in embryonic stem cells by gene targeting.
We are using a double replacement strategy to introduce subtle mutations into exon 10. We report here the first step of creating
a null mutation by insertion of a functionalhprt (hypoxanthine phosphoribosyl transferase) mini-gene into exon 10 of thecftr gene. Targeted embryonic stem cell clones were identified by PCR screening and confirmed by Southern blot analysis. One of
thecftr targeted clones has been injected into recipient blastocysts and shown to contribute to chimaeras. The targeted clones will
now be used as the starting point for a second gene targeting step to remove thehprt gene in exon 10 with the concomitant introduction of the ΔF508 mutation or other mutations. 相似文献
999.
Julia R. Dorin Paul Dickinson Elizabeth Emslie Alan R. Clarke Lorraine Dobbie Martin L. Hooper Stephanie Halford Brandon J. Wainwright David J. Porteous 《Transgenic research》1992,1(2):101-105
We wish to construct a mouse model for the human inherited disease cystic fibrosis. We describe here the successful targeting
in embryonal stem cells of the murine homologue (Cftr) of the cystic fibrosis transmembrane conductance regulator gene, as the first critical step towards this end. The targeting
event precisely disrupts exon 10, the site of the major mutation in patients with cystic fibrosis. The targeted cells are
pluripotent and competent to form chimaeras. 相似文献
1000.
Adrian B. Burd Serita Frey Anna Cabre Takamitsu Ito Naomi M. Levine Christian Lønborg Matthew Long Marguerite Mauritz R. Quinn Thomas Brandon M. Stephens Tom Vanwalleghem Ning Zeng 《Global Change Biology》2016,22(1):121-136
Organic matter (OM) plays a major role in both terrestrial and oceanic biogeochemical cycles. The amount of carbon stored in these systems is far greater than that of carbon dioxide (CO2) in the atmosphere, and annual fluxes of CO2 from these pools to the atmosphere exceed those from fossil fuel combustion. Understanding the processes that determine the fate of detrital material is important for predicting the effects that climate change will have on feedbacks to the global carbon cycle. However, Earth System Models (ESMs) typically utilize very simple formulations of processes affecting the mineralization and storage of detrital OM. Recent changes in our view of the nature of this material and the factors controlling its transformation have yet to find their way into models. In this review, we highlight the current understanding of the role and cycling of detrital OM in terrestrial and marine systems and examine how this pool of material is represented in ESMs. We include a discussion of the different mineralization pathways available as organic matter moves from soils, through inland waters to coastal systems and ultimately into open ocean environments. We argue that there is strong commonality between aspects of OM transformation in both terrestrial and marine systems and that our respective scientific communities would benefit from closer collaboration. 相似文献