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11.
ALAN G. KNOX 《Ibis》1990,132(3):454-466
Within the strongly polytypic Red Crossbill L. curvirostra there are many reports of two or more 'subspecies' nesting sympatrically, without interbreeding. This 13-year study examines one such case, in Scotland, where an endemic form is resident and another was thought to occur after irruptions from its main range in continental Europe. Both forms were present in the study area every year; sympatric breeding was proved in 9 years and probably occurred in the other four. There was no suggestion of interbreeding and the Scottish form should be treated as a separate species, L. scotica , the Scottish Crossbill. This is the only endemic species of bird in Britain and one of very few European endemics. The evolution of crossbills in Europe is discussed in the light of recent palynological evidence and the taxonomic status of the Parrot Crossbill L. pytyopsittacus is re-examined. 相似文献
12.
Data on re-mating between and within breeding seasons were collected between 1983 and 1994 as part of a long-term study of the Greater Flamingo Phoenicopterus ruber roseus in the Camargue, southern France. Flamingos have bred in the Rhône delta for centuries and since 1972 have done so annually on an artificial island in the Etang du Fangassier. A sample of chicks has been ringed each year since 1977; many of these birds were later observed back at the breeding site and were used in this study. The mate-switching rate between consecutive breeding seasons was 98.3%. This result is contrary to what has been reported from captive flocks where extensive mate fidelity occurred between consecutive breeding seasons. Mate-switching also was observed in the Camargue within breeding seasons, following breeding failure, and seems to be linked to male breeding strategy in response to the need to synchronize hatching and the limited energy supply of breeding females. The absence of a long-term pair bond in Greater Flamingos contrasts with current views on the evolution of mate fidelity in relation to longevity in birds. 相似文献
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ALAN SHER MARK ST. J. CRANE LOUIS V. KIRCHHOFF 《The Journal of eukaryotic microbiology》1983,30(2):278-283
The differentiation of Trypanosoma cruzi epimastigotes into trypomastigotes was studied in diffusion chambers sub-cutaneously implanted in mice. Using epimastigotes of the Tulahuén strain, transformation was first evident at 16 h after implantation and reached its maximum (92% trypomastigotes) by 24 h. Shortly before their differentiation into trypomastigotes, epimastigotes were found to develop resistance to lysis by the alternative pathway of complement. Furthermore, implantation of stationary-phase (as opposed to log-phase) parasites resulted in the accumulation of large numbers of complement-resistant epimastigotes in the chambers. These observations suggest that epimastigotes pass through a complement-resistant transitional stage before differentiating into trypomastigotes and that transformation may require cell division. In a further series of experiments, epimastigotes recovered 7 h after implantation in mice were found to differentiate into trypomastigotes when cultured in vitro for an additional 17 h at 37°C. This observation indicates that the events which trigger the morphologic transformation of epimastigotes into trypomastigotes can be dissociated operationally from the differentiation process itself. 相似文献
15.
LARRY RUBEN JEAN LAGESON BOB HYZY ALAN B. HOOPER 《The Journal of eukaryotic microbiology》1982,29(2):233-238
Cells of the ciliate Tetrahymena pyriformis GL overproduce and accumulate massive quantities of the heme intermediate, protoporphyrin IX. Protoporphyrin is localized intracellularly in discrete membranous compartments. The amount of porphyrin stored in the cell changes dramatically as cells progress through the growth cycle. Porphyrin overproduction is stimulated by δ-aminolevulinic acid, but only during the mid-stationary phase. Overproduction of protoporphyrin IX apparently results from an increase, late in the growth cycle, of activities subsequent to δ-aminolevulinic acid synthetase. Feedback inhibition in the pathway by accumulated protoporphyrin IX does not occur. The presence of Co2+ completely inhibits accumulation of protoporphyrin IX in a manner reversed by δ-aminolevulinic acid. Sn4+ stimulates protoporphyrin IX accumulation in the culture. 相似文献
16.
MASAMICHI AIKAWA ALAN H. COCHRANE RUTH S. NUSSENZWEIG JOHN RABBEGE 《The Journal of eukaryotic microbiology》1979,26(2):273-279
Plasmodium cynomolgi, Plasmodium knowlesi, and Plasmodium berghei sporozoites, before and after incubation with immune serum, were studied after freeze-fracture by electron microscopy. There were evenly distributed numerous intramembranous particles (IMP) on the P face of the outer membrane. The E face of the plasma membrane had fewer IMP than its P face. The E face of the intermediate membrane had few IMP and also linear arrays of slightly raised ridges running the length of the parasite. The P face of the intermediate membrane had many IMP aligned along the long axis of the sporozoite. On the P face of the inner membrane. IMP were arranged in very distinct rows conforming to the long axis of the parasite; the E face of this membrane had a few randomly distributed IMP. A prominent change in the sporozoite incubated in immune serum was the appearance of a layer of aggregated particles around the parasite. The P face of the plasma membrane had several clear areas devoid of IMP and IMP aggregates. No changes were seen in the other fractured faces of the pellicle. These observations suggest that immune serum acts only on the P face of the plasma membrane. 相似文献
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18.
VALERIE J. DEBUSE ALAN P. N. HOUSE DAVID W. TAYLOR SCOTT A. SWIFT 《Austral ecology》2009,34(6):621-635
Landscape and local‐scale influences are important drivers of plant community structure. However, their relative contribution and the degree to which they interact remain unclear. We quantified the extent to which landscape structure, within‐patch habitat and their confounding effects determine post‐clearing tree densities and composition in agricultural landscapes in eastern subtropical Australia. Landscape structure (incorporating habitat fragmentation and loss) and within‐patch (site) features were quantified for 60 remnant patches of Eucalyptus populnea (Myrtaceae) woodland. Tree density and species for three ecological maturity classes (regeneration, early maturity, late maturity) and local site features were assessed in one 100 × 10 m plot per patch. All but one landscape characteristic was determined within a 1.3‐km radius of plots; Euclidean nearest neighbour distance was measured inside a 5‐km radius. Variation in tree density and composition for each maturity class was partitioned into independent landscape, independent site and joint effects of landscape and site features using redundancy analysis. Independent site effects explained more variation in regeneration density and composition than pure landscape effects; significant predictors were the proportion of early and late maturity trees at a site, rainfall and the associated interaction. Conversely, landscape structure explained greater variation in early and late maturity tree density and composition than site predictors. Area of remnant native vegetation within a landscape and patch characteristics (area, shape, edge contrast) were significant predictors of early maturity tree density. However, 31% of the explained variation in early mature tree differences represented confounding influences of landscape and local variables. We suggest that within‐patch characteristics are important in influencing semi‐arid woodland tree regeneration. However, independent and confounding effects of landscape structure resulting from previous vegetation clearing may have exerted a greater historical influence on older cohorts and should be accounted for when examining woodland dynamics across a broader range of environments. 相似文献
19.
JAMES P. A. NOBLE ALAN LOGAN G. ROBERT WEBB 《Lethaia: An International Journal of Palaeontology and Stratigraphy》1976,9(1):1-18
A distinctive rocky subtidal benthic community (the Terebratulina Community) is described from the Bay of Fundy, Canada. It is shown to consist of three closely intcrspersed sub-communities: a cavity sub-community, characterised by chitons, coelenterates, brachiopods, bryozoans, chordates and annelids; a rock-face subcommunity, similar in composition but enriched in brachiopods and sponges; and an upper-surface subcommunity, dominated by algae, chitons, bivalves and echinoids.
Comparison with recently described Jurassic, Cretaceous and Recent (Mediterranean) hard-ground communities suggests a relative constancy in composition, in terms of higher taxa, since the Mesozoic. Development of these (sub-)communities occurs wherever crevice or cavity systems on hard substrates lead to microenvironments differentiated mainly on the basis of water energy and light.
The composition and trophic structure of the community and the life habits of Terebratulina septentrionalis (Couthouy) are related to aspects of the environment. Marked differences in composition between the living cavity sub-community and the death assemblage in the sediments are shown to be due to differential preservation, fragmentation, population dynamics and limited local transportation. 相似文献
Comparison with recently described Jurassic, Cretaceous and Recent (Mediterranean) hard-ground communities suggests a relative constancy in composition, in terms of higher taxa, since the Mesozoic. Development of these (sub-)communities occurs wherever crevice or cavity systems on hard substrates lead to microenvironments differentiated mainly on the basis of water energy and light.
The composition and trophic structure of the community and the life habits of Terebratulina septentrionalis (Couthouy) are related to aspects of the environment. Marked differences in composition between the living cavity sub-community and the death assemblage in the sediments are shown to be due to differential preservation, fragmentation, population dynamics and limited local transportation. 相似文献
20.