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91.
A basic problem in community ecology is determining whether a community of interacting species will survive in the long term.
A criterion ensuring this is that of permanence (or uniform persistence), which is based on the idea that species densities
for large time are above minimum non-zero levels. There are various mathematical techniques for investigating permanence,
but they do not yield an estimate for the minimum levels, and these may lie below minimum viability levels in the biological
sense of ‘Practical Persistence’. Here we study a technique for obtaining explicit expressions for the minimum levels when
one species is ‘slow’. This is illustrated for a predator–prey problem governed by difference equations, and we note that
the technique is applicable even when the dynamics is chaotic..
Received: 7 April 1997 / Revised version: 25 March 1998 相似文献
92.
Twenty-eight site-directed mutations were introduced into the fission yeast gene (pcn1
+) that encodes proliferating cell nuclear antigen (PCNA) and their in vivo effects analyzed in a strain with a null pcn1 background. Mutants defective in enhancing processivity of DNA polymerase δ have previously been identified. In this study, we assessed all of the mutants for their sensitivities to temperature, hydroxyurea,
UV irradiation and methyl methanesulfonate (MMS), and specific mutants were also tested for sensitivity to γ irradiation.
One cold-sensitive allele, pcn1-3, was characterized in detail. This mutant had a recessive cold-sensitive cdc phenotype and showed sensitivity to hydroxyurea,
UV, and γ irradiation. At the non-permissive temperature pcn1-3 protein was able to form homotrimers in solution and showed increased stimulation of both synthetic activity and processivity
of DNA polymerase δ relative to the wild-type Pcn1+ protein. Epistasis analyses indicated that pcn1-3 is defective in the repair pathway involving rad2
+
but not defective in the classical nucleotide excision repair pathway involving rad13
+
. Furthermore, pcn1-3 is either synthetically or conditionally lethal in null checkpoint rad backgrounds and displays a mitotic catastrophe phenotype in these backgrounds. A model for how pcn1-3 defects may affect DNA repair and replication is presented.
Received: 5 July 1997 / Accepted: 10 October 1997 相似文献
93.
Louise Shaxson 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》2009,364(1526):2141-2151
How can we strengthen the science–policy interface for plastics, the environment and human health? In a complex policy area with multiple stakeholders, it is important to clarify the nature of the particular plastics-related issue before trying to understand how to reconcile the supply and demand for evidence in policy. This article proposes a simple problem typology to assess the fundamental characteristics of a policy issue and thus identify appropriate processes for science–policy interactions. This is illustrated with two case studies from one UK Government Department, showing how policy and science meet over the environmental problems of plastics waste in the marine environment and on land. A problem-structuring methodology helps us understand why some policy issues can be addressed through relatively linear flows of science from experts to policymakers but why others demand a more reflexive approach to brokering the knowledge between science and policy. Suggestions are given at the end of the article for practical actions that can be taken on both sides. 相似文献
94.
95.
96.
Eun Jin Park Joon Young Song Min Ju Choi Ji Ho Jeon Jah-yeon Choi Tae Un Yang Kyung Wook Hong Ji Yun Noh Hee Jin Cheong Woo Joo Kim 《The Korean journal of parasitology》2014,52(4):425-428
A 45-year-old-male who had underlying ulcerative colitis and presented with fever and dry cough. Initially, the patient was considered to have invasive aspergillosis due to a positive galactomannan assay. He was treated with amphotericin B followed by voriconazole. Nevertheless, the patient deteriorated clinically and radiographically. The lung biopsy revealed eosinophilic pneumonia, and ELISA for Toxocara antigen was positive, leading to a diagnosis of pulmonary toxocariasis. After a 10-day treatment course with albendazole and adjunctive steroids, the patient recovered completely without any sequelae. Pulmonary toxocariasis may be considered in patients with subacute or chronic pneumonia unresponsive to antibiotic agents, particularly in cases with eosinophilia. 相似文献
97.
Abhishek Chatterjee Celia Caballero-Franco Dannika Bakker Stephanie Totten Armando Jardim 《The Journal of biological chemistry》2015,290(42):25579-25594
Enterohemorrhagic Escherichia coli is a causative agent of gastrointestinal and diarrheal diseases. Pathogenesis associated with enterohemorrhagic E. coli involves direct delivery of virulence factors from the bacteria into epithelial cell cytosol via a syringe-like organelle known as the type III secretion system. The type III secretion system protein EspD is a critical factor required for formation of a translocation pore on the host cell membrane. Here, we show that recombinant EspD spontaneously integrates into large unilamellar vesicle (LUV) lipid bilayers; however, pore formation required incorporation of anionic phospholipids such as phosphatidylserine and an acidic pH. Leakage assays performed with fluorescent dextrans confirmed that EspD formed a structure with an inner diameter of ∼2.5 nm. Protease mapping indicated that the two transmembrane helical hairpin of EspD penetrated the lipid layer positioning the N- and C-terminal domains on the extralumenal surface of LUVs. Finally, a combination of glutaraldehyde cross-linking and rate zonal centrifugation suggested that EspD in LUV membranes forms an ∼280–320-kDa oligomeric structure consisting of ∼6–7 subunits. 相似文献
98.
The cell cycle of donor cells as a major factor that affects cloning efficiency remains debatable. G2/M phase cells as a donor can successfully produce cloned animals, but a minimal amount is known regarding nuclear remodeling events. In this study, porcine fetal fibroblasts (PFFs) were carefully synchronized at G1 or M phase as donor cells. Most of the cloned embryos reconstructed from PFFs at G1 (G1-embryos) or M (M-embryos) phase formed a pronucleus-like nucleus (PN) within 6-h post fusion (hpf), but the M-embryos formed PN earlier than the G1-embryos did. Moreover, 77.4% of the M-embryos formed two PNs, whereas the G1-embryos formed a single PN. The rate of extrusion of polar body-like structures by the M-embryos was significantly lower than that extruded by the G1-embryos (26.3% vs. 37.1%, P?0.05), and DNA synthesis in most embryos in both groups was initiated at 9–12 hpf. Most of the M-embryos were octoploid before the first cleavage. Furthermore, 81.25% of the blastomeres of blastocysts developed from the M-embryos showed abnormal ploidy compared with those developed from the G1-embryos (22.55%). However, some of the blastomeres remained diploid in all the M-embryos tested. A portion of the blastomeres restored normal diploidy in some of the M-embryos at the blastocyst stage. This finding provides an explanation for M-embryos developing to term. 相似文献
99.
100.