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排序方式: 共有700条查询结果,搜索用时 31 毫秒
121.
Gideon A. Ngwa Terence T. Wankah Mary Y. Fomboh-Nforba Calsitus N. Ngonghala Miranda I. Teboh-Ewungkem 《Bulletin of mathematical biology》2014,76(10):2476-2516
A reproductive stage-structured deterministic differential equation model for the population dynamics of the human malaria vector is derived and analysed. The model captures the gonotrophic and behavioural life characteristics of the female Anopheles sp. mosquito and takes into consideration the fact that for the purposes of reproduction, the female Anopheles sp. mosquito must visit and bite humans (or animals) to harvest necessary proteins from blood that it needs for the development of its eggs. Focusing on mosquitoes that feed exclusively on humans, our results indicate the existence of a threshold parameter, the vectorial reproduction number, whose size increases with increasing number of gonotrophic cycles, and is also affected by the female mosquito’s birth rate, its attraction and visitation rate to human residences, and its contact rate with humans. A stability analysis of the model indicates that the mosquito can establish itself in the environment if and only if the value of the vectorial reproduction number exceeds unity and that mosquito eradication is possible if the vectorial reproduction number is less than unity, since, then, the trivial steady state which always exist is unique and is globally and asymptotically stable. When a persistent vector population steady state exists, it is locally and asymptotically stable for a range of reproduction numbers, but can also be driven to instability via a Hopf bifurcation as the reproduction number increases further away from unity. The model derivation identifies and characterizes control parameters relating to activities such as human-mosquito contact and the mosquito’s survival chances between blood meals and egg laying. Our results show that the total mosquito population size increases with increasing number of gonotrophic cycles. Therefore understanding the fundamental aspects of the mosquito’s behaviour provides a pathway for the study of human-mosquito contact and mosquito population control. Control of the mosquito population densities would ultimately lead to malaria control. 相似文献
122.
Siirola E Grischek B Clay D Frank A Grogan G Kroutil W 《Biotechnology and bioengineering》2011,108(12):2815-2822
Crotonase superfamily enzymes catalyze a wide variety of reactions, including hydrolytic C–C bond cleavage in symmetrical β‐diketones by 6‐oxo camphor hydrolase (OCH) from Rhodococcus sp. The organic solvent tolerance and temperature stability of OCH and its structurally related ortholog Anabaena β‐diketone hydrolase have been investigated. Both enzymes showed excellent tolerance toward organic solvents; for instance, even in the presence of 80% (v/v) THF or dioxane, OCH was still active. In most solvent mixtures, except methanol, the stereospecificity was conserved (>99% e.e. of product), hence neither the type of solvent nor its concentration appeared to have an effect on the stereoselectivity of the enzyme. Attempts to correlate the observed activities with log P, functional solvent group or denaturing capacity (DC) of the solvent were only successful in the case of DC for water miscible solvents. This study represents the first investigation of organic solvent stability for members of the crotonase superfamily. Biotechnol. Bioeng. 2011;108: 2815–2822. © 2011 Wiley Periodicals, Inc. 相似文献
123.
Chen N Hanly L Rieder M Yeger H Koren G 《Canadian journal of physiology and pharmacology》2011,89(5):335-343
Ifosfamide-induced nephrotoxicity is a serious adverse effect in children undergoing chemotherapy. Our previous cell and rodent models have shown that the antioxidant N-acetylcysteine (NAC), used extensively as an antidote for acetaminophen poisoning, protects renal tubular cells from ifosfamide-induced nephrotoxicity at a clinically relevant concentration. For the use of NAC to be clinically relevant in preventing ifosfamide nephrotoxicity, we must ensure there is no effect of NAC on the antitumor activity of ifosfamide. Common pediatric tumors that are sensitive to ifosfamide, human neuroblastoma SK-N-BE(2) and rhabdomyosarcoma RD114-B cells, received either no pretreatment or pretreatment with 400 μmol/L of NAC, followed by concurrent treatment with NAC and either ifosfamide or the active agent ifosfamide mustard. Ifosfamide mustard significantly decreased the growth of both cancer cell lines in a dose-dependent manner (p < 0.001). The different combined treatments of NAC alone, sodium 2-mercaptoethanesulfonate alone, or NAC plus sodium 2-mercaptoethanesulfonate did not significantly interfere with the tumor cytotoxic effect of ifosfamide mustard. These observations suggest that NAC may improve the risk/benefit ratio of ifosfamide by decreasing ifosfamide-induced nephrotoxicity without interfering with its antitumor effect in cancer cells clinically treated with ifosfamide. 相似文献
124.
Hartman G 《Journal of human evolution》2011,60(4):452-463
The Golan Heights borders the Upper Jordan Valley on its eastern side and likely served as a prime foraging area for hominin groups that inhabited the Upper Jordan Valley during the Mid-Pleistocene. This study tests the hypothesis that Mid-Pleistocene climate in the Golan region was similar to that of the present day. Carbon isotope composition of present day plant communities and soil organic carbon from the Golan were compared to those of paleosols from Nahal Orvim to reconstruct Mid-Pleistocene paleoclimatic conditions. After correcting the paleosol values for recent changes in atmospheric carbon isotope values and potential biodegradation, the isotopic results show a strong similarity to those of present day local plants and soils. These results indicate that during the Mid-Pleistocene, the Golan was dominated by C3 vegetation, shared similar climatic conditions with the present day, and displayed long-term environmental stability. The span of time of paleosol formation is unknown and might cover multiple climatic episodes; thus, although short climatic fluctuations may have occurred, their impact was not substantial enough to be detected in the Nahal Orvim paleosols. This study concludes that the Golan slopes provided hominins and large grazers with a reliable and highly nutritious foraging area that complemented the Jordan Valley riparian ecosystem. 相似文献
125.
Crouch NR Smith GF 《Botanical journal of the Linnean Society. Linnean Society of London》2011,166(3):301-309
South Africa, as a megadiverse country (±21 700 vascular plants, 4800 vertebrates and 68 900 invertebrates described), is presently engaged with an extended, modified Global Strategy for Plant Conservation (GSPC). The country is fortunate in having a strong tradition of systematics research and, inter alia, houses several million preserved plant specimens (±1 million databased and georeferenced), allowing taxonomists and conservationists to track both the occurrence and distribution of indigenous and naturalized plant species. These rich local resources have been extensively drawn upon to deliver, with varying degrees of success, the 16 outcome-oriented GSPC 2010 Targets. The National Environmental Management: Biodiversity Act (NEMBA, 2004), the National Biodiversity Strategy and Action Plan (NBSAP) and the National Biodiversity Framework (NBF) have provided a robust legislative, enabling and policy framework for making operational and advancing GSPC-related efforts. However, within an emerging economy, the conservation of biodiversity has competed for government resources with housing, sanitation, primary education, basic health care and crime prevention, delivery of which translates to the currency of politicians: votes. A key challenge identified by local (and global) biodiversity scientists for the current GSPC phase is broad-scale advocacy, communicating the changing state of nature, and the inter-relatedness of biodiversity and human well-being. The nature of meeting this challenge is explored. 相似文献
126.
Isolation by environment 总被引:1,自引:0,他引:1
The interactions between organisms and their environments can shape distributions of spatial genetic variation, resulting in patterns of isolation by environment (IBE) in which genetic and environmental distances are positively correlated, independent of geographic distance. IBE represents one of the most important patterns that results from the ways in which landscape heterogeneity influences gene flow and population connectivity, but it has only recently been examined in studies of ecological and landscape genetics. Nevertheless, the study of IBE presents valuable opportunities to investigate how spatial heterogeneity in ecological processes, agents of selection and environmental variables contributes to genetic divergence in nature. New and increasingly sophisticated studies of IBE in natural systems are poised to make significant contributions to our understanding of the role of ecology in genetic divergence and of modes of differentiation both within and between species. Here, we describe the underlying ecological processes that can generate patterns of IBE, examine its implications for a wide variety of disciplines and outline several areas of future research that can answer pressing questions about the ecological basis of genetic diversity. 相似文献
127.
The desert plant Zygophyllum dumosum displays unique epigenetic constraints, not found in other perennial desert plants, namely, it possesses mono- but not di- and tri-methylated histone H3 at lysine 9 (H3K9). We investigated the proposal that lack of dimethylated H3K9 (H3K9me2) is not restricted to Z. dumosum, but a feature uniquely evolved in the Zygophyllaceae. To this end, we analyzed the state of H3K9me2 in various species including Z. simplex (annual), Peganum harmala (hemicryptophyte), Nitraria retusa (shrub) and Balanites aegyptiaca (tree) from the Negev Desert (Israel) and Larrea tridentata (creosote bush), a prominent species in the Mojave, Sonoran, and Chihuahuan Deserts of western North America. All but one of these plants showed dimethylation of H3 at lysine 4 (H3K4me2), but no detectable levels of H3K9me2. The exception was Nitraria retusa, recently separated from the Zygophyllaceae family, which possesses H3K9me2, further supporting its partition into a distinct family (Nitrariaceae). Interestingly, the analysis of Krameria cistoidea (Krameriaceae), which is listed under the Zygophyllales, showed the presence of H3K9me2. It appears that lack of H3K9me2 has uniquely evolved in the Zygophyllaceae (sensu stricto), suggesting that this phenomenon has a strong genetic background. Thus, epigenetic information revealed for Zygophyllaceae can be useful to phylogenetic approaches. 相似文献
128.
Siméon Tchakonté Gideon A. Ajeagah Dramane Diomandé Adama Idrissa Camara Pierre Ngassam 《Aquatic Ecology》2014,48(4):379-395
Urban land use has increased dramatically over the past few decades, resulting in high variability in nutrients loading which is likely to alter the biological component of urban streams. Freshwater snails and environmental variables that might structure their diversity and distribution were studied from September 2012 to September 2013 in three contiguous watersheds in Douala. Twelve stations were monitored monthly, two of these are located in a suburban forest area, and the rest situated in urbanized and industrialized zones. Snails were collected using a long-handled net (30 × 30 cm side, 400-µm mesh). Meanwhile, measurements of the environmental variables were taken. Ten species were recorded which are as follows: Melanoides tuberculata (Thiaridae); Gabbiella africana (Bythiniidae); Physa acuta, Aplexa sp. (Physidae); Lymnaea natalensis, Lymnaea stagnalis, Lymnaea columella (Lymnaeidae); Biomphalaria pfeifferi, Bulinus forskalii (Planorbidae) and one undetermined taxon of Bythiniidae. All these snails were identified at nine of the ten urban stations; no species being found in suburban stations. These urban streams have very poor health status with highly polluted waters. Among the species found P. acuta (76.95 %), L. natalensis (19.46 %) and M. tuberculata (2.79 %) were the most abundant. Multiple stepwise regression analysis, Spearman correlation test and redundancy analysis showed that snail occurrences and abundances were probably influenced by water temperature, conductivity, suspended solids, alkalinity, nitrites, nitrates, ammonium, phosphates, oxydability, biochemical oxygen demand, rainfall, encumbrance rate of the riverbed and water width. Moreover, snail dynamics showed a seasonal pattern with peak population abundances and recruitment of young generations during rainy season. This malacological survey spotlighted the impacts of anthropogenic activities on snail’s diversity and distribution, with the proliferation of the invasive pulmonate P. acuta in Douala urban streams. 相似文献
129.
130.
Gideon Y. Stein Gabriel Herscovici Roman Korenfeld Shlomi Matetzky Shmuel Gottlieb Danny Alon Natalie Gevrielov-Yusim Zaza Iakobishvili Shmuel Fuchs 《PloS one》2014,9(1)