Adhesion and Interiorization of Trypanosoma cruzi in Mammalian Cells1

A quantitative method for experimentally separating the adhesion and interiorization phases of the interaction of Trypanosoma cruzi with mammalian cells was developed. Incubation of confluent monolayers of mammalian cells with epimastigotes or trypomastigotes at 4°C allowed the evaluation of the number of adhered parasites that do not become interiorized at this temperature. Quantification of interiorized parasites at 34°C was achieved by employing hypotonic lysis to eliminate the extracellularly adhered trypomastigotes. Both adhesion and interiorization were found to be proportional to the time of exposure of cells to parasites and to the multiplicity of infection. These phenomena occur normally for trypomastigotes in the absence of serum with LLC-MK₂ cells, HeLa cells, and 3T3 fibroblasts. Moreover, it was possible to obtain trypomastigotes that presented the same infectivity to LLC-MK₂ cells as did parasites obtained in the presence of 2% fetal calf serum after 10 serial passages in a medium devoid of serum. Inhibition of adhesion (of epimastigotes and trypomastigotes) and of interiorization (of trypomastigotes) was obtained with inactivated normal serum from several sources, a saturation effect being observed at a final concentration of 20%. Bovine serum albumin, at the concentrations present in the sera, had no inhibitory effect. Trypomastigotes that have been pre-incubated with 40% FCS (45 min at 4°C) showed decreased adhesion and interiorization indices, effects that can be reversed by trypsinization of the parasites prior to exposure of the cells. A progressive internalization of previously attached trypomastigotes was observed on raising the temperature from 4°C to 34°C; no spontaneous detachment of parasites was detected up to 120 min. Approximately 75% of the adhered parasites were found inside the cells after 45 min at 34°C. The presence of normal inactivated calf serum during incubation at 34°C resulted in a certain degree of detachment and in a lower interiorization index.     

Can genetic manipulation of plant nitrogen assimilation enzymes result in increased crop yield and greater N-use efficiency? An assessment

The literature on the relations between plant nitrogen (N) assimilation enzymes and plant/crop N assimilation, growth and yield is reviewed to assess if genetic manipulation of the activities of N assimilation enzymes can result in increased yield and/or increased N use efficiency. The available data indicate that (I) levels of N assimilation enzymes do not limit primary N assimilation and hence yield; (II) root or shoot nitrate assimilation can have advantages under specific environmental conditions; (III) for cereals, cytosolic glutamine synthetase (GS1) is a key enzyme in the mobilisation of N from senescing leaves and its activity in senescing leaves is positively related to yield; and (TV) for rice (Oryza sativd), NADH-glutamate synthase (NADH-GOGAT) is important in the utilisation of N in grain filling and its activity in developing grains is positively related to yield. In our opinion, selection of plants, from either a genetically manipulated population or genetic resources, with expression of nitrate reductase/nitrite reductase primarily in the root or shoot should increase plant/crop growth and hence yield under specific environmental conditions. In addition for cereals the selection of plants with high GS1 in senescing leaves and in some cases high NADH-GOGAT in developing grains could help maximise the retrieval of plant N in seeds.     

The evolution of oligotrophy: implications for the breeding of crop plants for low input agricultural systems

Oligotrophy, the obligate or facultative capacity to live in low‐nutrient habitats, has played a major role in the evolution of photosynthetic organisms.

• ? Energy/carbon deficiency: evolution of photosynthesis about 3.5 Gyr (billion years) ago, then use of H₂O as electron donor, and accumulation of O₂ from about 2.3 Gyr ago.
• ? Deficiency in combined N: evolution of biological N₂ fixation about 2.0‐2.3 Gyr ago.
• ? Deficiency in soluble relative to particulate organic C: evolution of phagotrophy in eukaryotes, opening the way to endosymbiotic origin of photosynthesis in eukaryotes.
• ? Deficiency of P and Fe resulting from oxygenation: evolution of mechanisms increasing access to P and Fe.
• ? Deficiency of H₂O for land plants gaining C from the atmosphere: evolution of homoiohydry following origin of significant land flora from 0.5 Gyr ago.
• ? Deficiency of CO₂ resulting from increased weathering by land plants: evolution of large leaves.
• ? Increased competition for resources among land plants: evolution of mechanisms economizing in use of soil‐derived resources, and increasing ability to acquire resources.

Economising on resource use in photosynthetic organisms is subject to a number of constraints. There are very limited possibilities for reducing the use of N in proteins with a given catalytic function, but greater possibilities using substitution of an analogous protein with that function. The same applies to Fe. Possibilities for economising on the use of P are very limited if the growth rate is to be maintained: the marine cyanobacterium Prochlorococcus is a good example of restricted P requirement. H₂O use can be constrained by C₄ and, especially, CAM photosynthesis. A possible role of the study of oligotrophy in the context of sustainable, low‐input agriculture includes modified agricultural practice to minimise losses of resources. Information on oligotrophy and its evolution can also be used to inform the alteration of crop plants by genetic modification related to resource acquisition (e.g. associative, or nodule‐based, symbiotic diazotrophy) and the economy of resource use (e.g. partial or complete conversion of a C₃ crop to a C₄ crop which could economise in the use of N and/or H₂O). The attempts to convert C₃ to C₄ plants have not thus far been fully successful, and the advantages of conversion to C₄ are being increasingly offset by the effect of increasing atmospheric CO₂ on C₃ plants. However, more success has been achieved with selection of the most appropriate diazotrophic symbionts for crop plants in particular environments.     

The Partitioning of Nitrate Assimilation Between Root and Shoot of a Range of Temperate Cereals and Pasture Grasses

Nitrate reductase activity (NRA, in vivo assay) and nitrate(NO^-₃) content of root and shoot and NO^-₃ and reduced nitrogencontent of xylem sap were measured in five temperate cerealssupplied with a range of NO^-₃ concentrations (0·1–20mol m^–3) and three temperate pasture grasses suppliedwith 0·5 or 5 0 mol m^–3 NO^-₃ For one cereal (Hordeumvulgare L ), in vitro NRA was also determined The effect ofexternal NO^-₃ concentration on the partitioning of NO^-₃ assimilationbetween root and shoot was assessed All measurements indicatedthat the root was the major site of NO3 assimilation in Avenasatwa L, Hordeum vulgare L, Secale cereale L, Tnticum aestivumL and x Triticosecale Wittm supplied with 0·1 to 1·0mol m^–3 NO^-₃ and that for all cereals, shoot assimilationincreased in importance as applied NO^-₃ concentration increasedfrom 1.0 to 20 mol m^–3 At 5.0–20 mol m^–3 NO3,the data indicated that the shoot played an important if notmajor role in NO^-₃ assimilation in all cereals studied Measurementson Lolium multiflorum Lam and L perenne L indicated that theroot was the main site of NO^-₃ assimilation at 0.5 mol m^–3NO^-₃ but shoot assimilation was predominant at 5.0 mol m^–3NO^-₃ Both NRA distribution data and xylem sap analysis indicatedthat shoot assimilation was predominant in Dactylis glomerataL supplied with 0.5 or 5.0 mol m^–3 NO^-₃ Avena sativa L., oats, Hordeum vulgare L., barley, Secale cereale L., rye, x Triticosecale Wittm., triticale, Triticum aestivum L., wheat, Dactylis glomerata L., cocksfoot, Lolium multiflorum Lam., Italian ryegrass, Lolium perenne L., perennial ryegrass, nitrate, nitrate assimilation, nitrate reductase activity, xylem sap     

History of Shiny Cowbird Molothrus bonariensis brood parasitism in Trinidad and Tobago

In attempting to evaluate the evolutionary stability of the reproductive strategies of an avian brood parasite, it is important to know whether or not the observed levels of parasitism on host species have changed through time and whether or not new host species are being used in the region under study. In this regard, the islands of Trinidad and Tobago provide an excellent opportunity because they are ornithologically well known and the expansion of the parasitic Shiny Cowbird Molothrus bonariensis into the West Indian region is thought to have originated from Trinidad and Tobago. On the whole, host choice has remained remarkably stable over the past 50–60 years. The three hosts (House Wren Troglodytes aedon, Red-breasted Blackbird Leistes militaris and Yellow-hooded Blackbird Agelaius icterocephalus) known to be heavily parasitized historically remain heavily parasitized today. Only one instance of parasitism of a new host (Tropical Kingbird Tyrannus melancholicus) is recorded. In the case of one intensively studied host, the Yellow-hooded Blackbird, cowbird parasitism had a minimal negative effect on the host's reproductive success. Reproductive equilibrium in this study seemed to be maintained by behavioural adaptations of host and parasite to each other. The stable pattern of host choice by Shiny Cowbirds in Trinidad and Tobago may not develop elsewhere in the West Indian region. Before the cowbird's expansion in the West Indies, potential hosts had no prior experience of brood parasites. Consequently, the island bird populations are unlikely to have evolved defensive strategies and thus may be more vulnerable to reproductive failure as a result of cowbird parasitism, precluding the development of a stable system at this point in time.     

Emergence of temperate pasture grases from different sowing depths: importance of seed weight, coleoptiele plus mesocotyl length and shoot strength

Inter-specific relationships between mean seed weight, coleoptile + mesocotyl (sub-coleoptile internode) length and width, shoot strength and emergence from different sowing depths were examined for timothy (Phleum pratense L., mean seed weight 0.33-0.48 mg depending on seed line), cocksfoot (Dactylis glomerata L., 0.65-0.78 mg), perennial ryegrass (Lolium perenne L., 1.71-2.19 mg), tall fescue (Festuca arundinucea Schreb., 1.73–2.60 mg), annual ryegrass (Lolium multiflorum Lam., 5.10-5.20 mg) and prairie grass (Bromus willdenowü Kunth., 10.5–12.2 mg). Across species at 10,30 and 60 mm sowing depths in the field and 10, 15 and 30 mm sowing depths under controlled environment conditions, there was a significant (P < 0.05) positive correlation between emergence % and mean seed weight. Across species at 10–30 mm sowing depth under controlled environment conditions, emergence % was not significantly correlated with coleoptile + mesocotyl length but there were significant positive correlations between emergence % and coleoptile and mesocotyl width: shoot strength increased with increased coleoptile width across species. For seed lines of timothy of different mean seed weight (0.21-0.81 mg), emergence %, coleoptile + mesocotyl length and coleoptile and mesocotyl width increased with increased seed weight at 10 and 15 mm sowing depth. Shoot strength increased with increased coleoptile width for timothy. For emerged and non-emerged cocksfoot and timothy seedlings regardless of seed weight, mean coleoptile + mesocotyl length was > 10 mm at 10 mm sowing depth. It is concluded that at 10–30 mm sowing depth, increased emergence % with increased seed weight across species is not due to increased coleoptile + mesocotyl length. It is proposed that increased emergence % with increased seed weight across species at 10–30 mm sowing depths and across seed lines of timothy at 10 mm sowing depth is primarily due to increased coleoptile and mesocotyl width resulting in increased shoot strength and hence an increased ability to penetrate the substrate.     

Influence of pregnancy on the thermal biology of the lizard, Sceloporus jarrovi: why do pregnant females exhibit low body temperatures?

1. Selected body temperatures of female lizards, Sceloporus jarrovi , were measured on a photothermal gradient during late pregnancy and again when postpartum, and pregnant females were subjected to one of three fluctuating temperature regimes that simulated body temperatures of (1) pregnant females, (2) postpartum females or (3) allowed normal thermoregulation.
2. Overall, females selected lower body temperatures when pregnant (mean = 32·0°C) than when postpartum (mean = 33·5°C).
3. Females regulated body temperature more precisely when pregnant than when postpartum as judged by their smaller variances in body temperature throughout the day.
4. When pregnant, females selected a lower mean maximum body temperature (mean: pregnant = 32·8°C; postpartum = 34·5°C) than when postpartum, but selected mean minimum body temperatures did not differ.
5. None of the experimental temperature treatments was detrimental to pregnant females. Female body length increased during pregnancy but the rate of increase did not differ among treatments. Moreover, length-adjusted body mass of postpartum females did not differ among treatments.
6. Pregnant females that experienced postpartum body temperatures produced neonates that were smaller in body mass and length than pregnant females that experienced pregnant body temperatures and females that were allowed to thermoregulate.
7. For neonates resulting from the postpartum body temperature treatment, the disparity in the body length, but not mass, was still observed at 9 days of age, although survival and growth of neonates was high and did not differ among treatments.
8. The results demonstrate that pregnant females could maintain higher postpartum body temperatures without compromising their physical condition, but select relatively low body temperatures, presumably to avoid decrements in offspring fitness.