共查询到20条相似文献,搜索用时 15 毫秒
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Grady MM Wright I 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》2006,361(1474):1703-1713
One of the goals of the present Martian exploration is to search for evidence of extinct (or even extant) life. This could be redefined as a search for carbon. The carbon cycle (or, more properly, cycles) on Earth is a complex interaction among three reservoirs: the atmosphere; the hydrosphere; and the lithosphere. Superimposed on this is the biosphere, and its presence influences the fixing and release of carbon in these reservoirs over different time-scales. The overall carbon balance is kept at equilibrium on the surface by a combination of tectonic processes (which bury carbon), volcanism (which releases it) and biology (which mediates it). In contrast to Earth, Mars presently has no active tectonic system; neither does it possess a significant biosphere. However, these observations might not necessarily have held in the past. By looking at how Earth's carbon cycles have changed with time, as both the Earth's tectonic structure and a more sophisticated biology have evolved, and also by constructing a carbon cycle for Mars based on the carbon chemistry of Martian meteorites, we investigate whether or not there is evidence for a Martian biosphere. 相似文献
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Neotyphodium endophytes are assumed to have mutualistic relationship with their grass hosts, mainly resulting from mycotoxin production increasing plant resistance to herbivores by the fungus that subsists on the plant. To study importance of often ignored environmental effects on these associations, we performed a greenhouse experiment to examine the significance of endophyte infection and nutrient availability for bird-cherry aphid (Rhopalosiphum padi) performance on meadow fescue (Lolium pratense). Naturally endophyte-infected (E+), uninfected (E–), or manipulatively endophyte-free (ME–) half-sib families of meadow fescue were grown on two soil nutrient levels. Endophyte infection reduced aphid performance in general. However, to our knowledge, this is the first study to demonstrate experimentally that herbivore performance decreases on E+ host plants with increasing availability of nutrients in soils. Potential improvement in herbivore performance in high nutrient soils and decreased plant performance in low nutrient soils in ME– plants, compared to E– and E+ plants, suggests that loss of endophyte infection after long coevolutionary relationship may be critical to plant fitness. 相似文献
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Clifford N. Matthews 《Origins of life and evolution of the biosphere》1982,12(3):281-283
Since hydrogen cyanide is a component of Titan's hazy atmosphere, HCN polymers might also be present by way of a low energy pathway leading initially to the synthesis of polyaminomalonitrile. Subsequent reactions of HCN with the activated nitrile groups of this HCN homopolymer would then yield heteropolyamidines, readily converted to heteropolypeptides following contact with frozen water on the surface of Titan.Similar HCN polymers in the reducing atmospheres of Jupiter and Saturn could be major contributors to the yellow-brown-orange appearance of these giant planets.Any detection of such HCN chemistry by the Voyager missions or the pending Galileo probe would constitute evidence for the hypothesis that heteropolypeptides on the primitive Earth were synthesized directly from hydrogen cyanide and water without the intervening formation of -amino acids.Paper presented at the 6th College Park Colloquium, October 1981. 相似文献
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W. F. Libby 《Origins of life and evolution of the biosphere》1974,5(3-4):483-486
The possibilities of life on Jupiter are discussed from the point view of life as we know it. That is, we assume that any life on Jupiter would not involve new principles foreign to us. Proteins would be a constituent as would fats and the other building blocks of living organisms on Earth. This leads us to a set of limiting parameters, such as pressure. Studies in the laboratory have shown that proteins and other essential molecules are denatured by pressures of 4000 atm and higher. Thus, we must not expect life in the great depths of the Jovian atmosphere. It could exist only at depths of several hundred kilometers in the atmosphere. Since no solid surface could possibly exist at such altitudes, any organisms present must be small enough to be buoyed up by the turbulent atmospheric currents or must fly or both. Such possibilities, however, seem to be real. The necessary nutrients to preserve life and foster growth could be furnished by the Miller-Urey type reactions of lonizing radiation on the reducing atmosphere undoubtedly present. There can, of course, be no possibility of oxygen on Jupiter, and so the life forms, if they exist, must be anaerobic. Such possibilities are real and have often been cited in connection with the origin of life on Earth. 相似文献
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N. E. Flitters 《International journal of biometeorology》1963,6(2):85-90
Observations in the laboratory under conditions of controlled temperature and relative humidity on the effects of low barometric pressures on insect activity resulting fron hurricane Carla, one of the largest and most destructive hurricanes to affect the middle coast of Texas, revealed that stable flies (STOMOXYS CALCITRANS (L.)), house flies (MUSCA DOMESTICA L.), and Mexican fruit flies (ANASTREPHA LUDENS (Loew)), cockroaches (BLATELLA GERMANICA (L.)), PERIPLANETA AMERICANA (L.), and BLABERUS GIGANTEUS (L.) and mosquitoes (AEDES AEGYPTI (L.), exhibited nervous activity concomitant with the reduction in barometric pressure. Field studies throughout the critical storm period also indicated unusual arthropod activity. Following the hurricane, certain insect species emerged in abnormally large numbers, particularly fall armyworms (LAPHYGMA FRUGIPERDA (J.E. Smith)), asps (MEGALOPYGE OPERCULARIS (J. E. Smith)), and southern green stink bugs (NEZARA VIRIDULA (L.)).
Zusammenfassung WÄhrend des Hurrikans Carla, einer der grössten und verheerendsten Orkane, die je auf die mittlere Küste von Texas eingewirkt haben, wurden im Laboratorium mit kontrollierter Temperatur und Feuchte, die Wirkungen von niedrigem Barometerdruck auf die InsektenaktivitÄt untersucht. Stallfliegen (STOMOXYS CALCITRANS (L.)), Hausfliegen (MUSCA DOMESTICA L.), Mexikanische Fruchtfliegen (ANASTREPHA LUDENS (Loew)), Schaben (BLATELLA GERMANICA L.), PERIPLANETA AMERICANA (L.) und BLABERUS GIGANTEUS (L.) und Mücken (AEDES AEGYPTI (L.)) zeigten wÄhrend der Verminderung des Barometerdruckes eine verstÄrkte AktivitÄt. Bei Studien im Freien wÄhrend der kritischen Orkanperiode wurde ebenfalls eine ungewöhnlich grosse AktivitÄt der Arthopoden beobachtet. Nach dem Orkan tauchten bestimmte Insektenarten in abnorm grosser Zahl auf. besonders (LAPHYGMA FRUGIPERDA (J.E.Schmith)), (MEGALOPYGE OPERCULARIS(J.E.Schmith)) und. (NEZARA VIRIDULA (L.)).
Résumé L'effet, sur l'activité des insectes, des basses pressions barométriques résultant de l'ouragan Carla, un des plus vastes et des plus destructeurs qui aient affecté la cÔte moyenne du Texas, a été. observé au laboratoire dans des conditions de température et d'humidité relative constantes. Les mouches des étables (STOMOXYS CALCITRANS (L.)), les mouches domestiques (MUSCA DOMESTICA L.), les mouches à fruits mexicaines (ANASTREPHA LUDENS (Loew)), les cafards (BLATELLA GERMANICA (L.)), PERIPLANETA AMERICANA (L.) et BLABERUS GIGANTEUS (L.) et les moustiques (AEDES AEGYPTI (L.)) ont manifesté une activité accrue correspondant à la réduction de la pression barométrique. Des études faites en plein air pendant la période d'ouragan ont également permis d'observer chez les arthropodes une activité inaccoutumée. A la suite de l'ouragan sont apparus en quantités anormales certains insectes tels que le(LAPHYGMA FRUGIPERDA (J.E. Smith)), le (MEGALOPYGE OPERCULARIS (J.E.Smith)) et le (NEZARA VIRIDULA (L.)).相似文献
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Bram Kuijper Rebecca B. Hoyle 《Evolution; international journal of organic evolution》2015,69(4):950-968
Existing insight suggests that maternal effects have a substantial impact on evolution, yet these predictions assume that maternal effects themselves are evolutionarily constant. Hence, it is poorly understood how natural selection shapes maternal effects in different ecological circumstances. To overcome this, the current study derives an evolutionary model of maternal effects in a quantitative genetics context. In constant environments, we show that maternal effects evolve to slight negative values that result in a reduction of the phenotypic variance (canalization). By contrast, in populations experiencing abrupt change, maternal effects transiently evolve to positive values for many generations, facilitating the transmission of beneficial maternal phenotypes to offspring. In periodically fluctuating environments, maternal effects evolve according to the autocorrelation between maternal and offspring environments, favoring positive maternal effects when change is slow, and negative maternal effects when change is rapid. Generally, the strongest maternal effects occur for traits that experience very strong selection and for which plasticity is severely constrained. By contrast, for traits experiencing weak selection, phenotypic plasticity enhances the evolutionary scope of maternal effects, although maternal effects attain much smaller values throughout. As weak selection is common, finding substantial maternal influences on offspring phenotypes may be more challenging than anticipated. 相似文献
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All cellular proteins are synthesized by the ribosome, an intricate molecular machine that translates the information of protein coding genes into the amino acid alphabet. The linear polypeptides synthesized by the ribosome must generally fold into specific three-dimensional structures to become biologically active. Folding has long been recognized to begin before synthesis is complete. Recently, biochemical and biophysical studies have shed light onto how the ribosome shapes the folding pathways of nascent proteins. Here, we discuss recent progress that is beginning to define the role of the ribosome in the folding of newly synthesized polypeptides. 相似文献
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《Basic and Applied Ecology》2014,15(8):685-692
Studies on the effects of plant diversity on insect herbivory have produced conflicting results. Plant diversity has been reported to cause positive and negative responses of herbivores. Explanations for these conflicting responses include not only various population-level processes but also changes in plant quality that lead to changes in herbivore performance. In a tree diversity experiment, we investigated the effects of tree diversity on insect herbivory on oak in general and whether the effects of tree diversity on herbivore damage are reflected by the performance (leaf consumption, growth) of the generalist herbivore Lymantria dispar. Our study showed that the feeding damage caused by naturally occurring herbivores on oak trees decreased with increasing diversity of tree stands. The performance of L. dispar on oak leaves was not affected by tree diversity, neither in field nor laboratory experiments. Our results can be explained by the various processes behind the hypothesis of associational resistance. 相似文献
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Cell migration requires the coordination of adhesion site assembly and turnover. Canonical models for nascent adhesion formation postulate that integrin binding to extracellular matrix (ECM) proteins results in the rapid recruitment of cytoskeletal proteins such as talin and paxillin to integrin cytoplasmic domains. It is thought that integrin-talin clusters recruit and activate tyrosine kinases such as focal adhesion kinase (FAK). However, the molecular connections of this linkage remain unresolved. Our recent findings support an alternative model whereby FAK recruits talin to new sites of β1 integrin-mediated adhesion in mouse embryonic fibroblasts and human ovarian carcinoma cells. This is dependent on a direct binding interaction between FAK and talin and occurs independently of direct talin binding to β1 integrin. Herein, we discuss differences between nascent and mature adhesions, interactions between FAK, talin and paxillin, possible mechanisms of FAK activation and how this FAK-talin complex may function to promote cell motility through increased adhesion turnover. 相似文献