The sympatric breeding of Common and Scottish Crossbills Loxia curvirostra and L. scotica and the evolution of crossbills

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.     

Re-mating between and within breeding seasons in the Greater Flamingo Phoenicopterus ruber roseus

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.     

Incubation in Mice Provides a Signal for the Differentiation of Trypanosoma cruzi Epimastigotes to Trypomastigotes1

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.     

Growth Cycle-Dependent Overproduction and Accumulation of Protoporphyrin IX in Tetrahymena: Effect of Heavy Metals1

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.     

Freeze-Fracture Study of Malaria Sporozoites: Antibody-Induced Changes of the Pellicular Membrane*

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.     

Shrub encroachment in North American grasslands: shifts in growth form dominance rapidly alters control of ecosystem carbon inputs

Shrub encroachment into grass-dominated biomes is occurring globally due to a variety of anthropogenic activities, but the consequences for carbon (C) inputs, storage and cycling remain unclear. We studied eight North American graminoid-dominated ecosystems invaded by shrubs, from arctic tundra to Atlantic coastal dunes, to quantify patterns and controls of C inputs via aboveground net primary production (ANPP). Across a fourfold range in mean annual precipitation (MAP), a key regulator of ecosystem C input at the continental scale, shrub invasion decreased ANPP in xeric sites, but dramatically increased ANPP (>1000 g m⁻²) at high MAP, where shrub patches maintained extraordinarily high leaf area. Concurrently, the relationship between MAP and ANPP shifted from being nonlinear in grasslands to linear in shrublands. Thus, relatively abrupt (<50 years) shifts in growth form dominance, without changes in resource quantity, can fundamentally alter continental-scale pattern of C inputs and their control by MAP in ways that exceed the direct effects of climate change alone.