Phagocytosis in Dictyostelium: Nibbling, Eating and Cannibalism

ABSTRACT. Phagocytosis is a highly conserved biological process that serves numerous functions in a wide variety of organisms. Over the past few decades Dictyostelium has proven to be an excellent organism for investigations in cell biology and this is certainly no less the case for a study of phagocytosis. This review examines three distinct phagocytic activities which have been characterized in Dictyostelium. The first, "vegetative phagocytosis," represents the classical eukaryotic microbial uptake of food particles (bacteria). The second, a predatory form of phagocytosis, arises when one species such as Dictyostelium caveatum attacks another species of slime mold, engulfing small pieces of the target prey. This has been termed "cell nibbling." The third phagocytic process is "sexual cannibalistic phagocytosis." In this situation a zygote giant cell, having arisen from the fusion of gametic amoebae, attracts unfused nonzygotic amoebae of the same species and engulfs them as a food source. While cell nibbling has not been actively studied, vegetative and sexual cannibalistic phagocytosis have received varying amounts of attention leading to the idea that some of the elements (e.g., glycoprotein receptors and a G_αs subunit) involved in certain of these phagocytic events may be the same. On the other hand, some unique events (e.g., filopodial induction in prey by D. caveatum ) are also worthy of further investigation. Among other things, the presence of self-nonself recognition, the existence of opsonin-like substances and the presence of signal transduction elements (e.g., an A2-like receptor that negatively modulates sexual phagocytosis) once considered to be extant only in higher organisms suggest that much can be learned about phagocytosis in general by further studies in the classic, eukaryotic microbe Dictyostelium discoideum and related species.     

Growth and provisioning strategies of Northern Fulmars Fulmarus glacialis

Growth and foraging strategies in procellariiforms show a great deal of variation, but the fulmarine petrels are notable in that chicks are fed frequently and develop unusually rapidly. This study examined age-related and daily variation in provisioning of the Northern Fulmar Fulmarus glacialis throughout the chick-rearing period at Fair Isle in 1997. In common with many other petrels, meal mass showed an initial rise with age, probably because of a gradual increase in chick gut capacity, but then levelled off. By comparison, feeding frequency showed little age-specific variation until chicks reached the oldest age-class, when the number of meals declined to less than a third of the previous level as chicks underwent mass recession prior to fledging. Compared with the limited day-to-day variation in mean provisioning rates for the whole sample, food delivery to particular chicks was much more variable, suggesting that differences in feeding rates were determined by stochastic factors influencing the feeding success of individual parents. The caloric density of feeds and their size in relation to adult mass were lower in Northern Fulmars than in most other Procellariiformes. This implies that adults are not heavily dependent on stomach oil formation to raise the energy content of the payload, but rely on a high feeding frequency to maintain adequate rates of energy transfer to chicks.     

Temporal response of stream benthic macroinvertebrate communities to the synergistic effects of anthropogenic acidification and natural drought events

1. We evaluated whether the surprisingly weak biological recovery associated with declines in acid deposition were related to drought‐induced re‐acidification of streams. We used test site analysis (TSA) to characterise temporal changes (1995–2003) in the degree and nature of impairments to stream benthic macroinvertebrate (BMI) communities influenced by acid deposition and drought. 2. The BMI communities in four historically impacted test streams were compared with communities in 30 minimally impacted reference streams. Six multivariate (e.g. ordination axes scores) and four traditional (e.g. % Diptera) summary metrics were used to describe BMI communities. Using all metrics simultaneously (i.e. Mahalanobis or generalised distance), the TSA provided a single probability that a test community was impaired. If a test community was significantly impaired, a further analysis was done to identify the metric(s) important in distinguishing the test community from reference condition. 3. Results of the TSAs indicated that the generalised distances between test communities and the reference condition were inversely related to stream water pH (n = 36). The TSAs also indicated ordination metrics based on BMI abundance were important in distinguishing significantly impaired communities from reference conditions. Temporal trends indicated that there has been short‐term recovery of these BMI communities, but that overall improvements have been hampered by acid or metal toxicity associated with drought‐induced re‐acidification of the streams. 4. Our use of a variety of summary metrics to obtain a single statistical test of significance within the context of the reference‐condition approach provided a simple and unambiguous framework for evaluating the biological condition of test sites.     

Cluster root development in Grevillea robusta (Proteaceae). I. Xylem, pericycle, cortex, and epidermis development in a determinate root

The cluster roots of Grevillea robusta A. Cunn. ex R. Br. are composed of determinate rootlets that stop growing, but remain physiologically active for several months. Their apical organization, both before and after maturation, was studied by light and transmission electron microscopy. Each cell layer forms a dome, with an initial cell at its end. Xylem elements form a complicated triarch array at the base of the rootlet, passing along the rootlet as two files, and then joining at the tip to form a single file, surrounded by six pericycle cells. At the base of the rootlet, shorter xylem cells and thick-walled support cells are visible. A root cap, present in rootlets grown in vermiculite, was eventually displaced by root hair growth. Rootlets grown in Hoagland's solution lacked root caps and were significantly shorter than those grown in vermiculite. Cell fate was analysed in terms of cell position and is discussed in terms of pattern and development.     

Bacterial dynamics in two high-arctic lakes

The heterotrophic planktonic bacteria in two high-arctic lakes were studied by direct microscope count and the enzymatic uptake of ¹⁴C labelled glucose which generally conformed to Michaelis-Menten kinetics. Bacterial numbers and activity in oligo-trophic Char Lake ranged from 0.1 to 2.0×10^?3 bacteria/l and a maximum uptake velocity (V_max) of 1.8 × 10^?3μg glucose l^? h^?1. Nearby Meretta Lake received waste water from the Department of Transport Base at Resolute and this eutrophication was reflected in higher bacterial numbers of 2-80 × 10⁸/1 and Kmax of 0.1 × 10^?1-7.5 × 10^?1 fig glucose l^?1 h^?1 The Kmax per cell in Char Lake was 3 × 10^?11μg glucose l^?1 h^?1 and changed little between the period of solid ice cover in May and ice-free conditions in August. Bacterial cycles could not be related to phytoplankton cycles in either lake. Comparison of kinetic data from several lakes suggests a relationship between the bacterial uptake rate of glucose and phytoplankton production. Both bacterial numbers and activity in Char Lake may be very close to the minima to be expected in undisturbed freshwater environments.     

Developing methods for quantifying the apparent abundance of fiddler crabs (Ocypodidae: Uca) in mangrove habitats

Counting the number of individuals emerging from burrows is the most practical method for estimating the apparent abundance of Australian Uca species living in mangrove habitats. Experiments were conducted to investigate the effect on counts of quadrat design, distance of observer, quadrat size, recovery time and observational technique. Significant differences in the apparent abundance of one species were found when the subjects were within 2 m of the observer, and when a conspicuous quadrat was used. The largest quadrat tested provided the least variability in counts but an intermediate size (0.56 m²) was more practical. Most Uca active within a 30-min period emerged during the first 10 min regardless of site, species, sex or season. There was a linear correlation between scanning and continuous observation indicating that the former method could be useful when sampling time was limited. Temporal changes in the apparent abundance of Uca suggest that long-term sampling and more detailed studies will be worthwhile.     

Methyl bromide emissions to the atmosphere from temperate woodland ecosystems

The environmental importance of methyl bromide (CH₃Br) arises from its contribution to stratospheric ozone loss processes and, as a consequence, its emissions from anthropogenic sources are subject to the Montreal Protocol. A better understanding of the natural budget of CH₃Br is required for assessing the benefit of anthropogenic emission reductions and for understanding any potential effects of environmental change on global CH₃Br concentrations. Measurements of CH₃Br flux in temperate woodland ecosystems, in particular, are very sparse, yet these cover a large fraction of terrestrial land surface. Results presented here from 18 months of field measurements of CH₃Br fluxes in four static flux chambers in a woodland in Scotland and from enclosures of rotting wood and deciduous and coniferous leaf litter suggest net emissions from temperate woodlands. Net CH₃Br fluxes in the woodland varied between the chambers, fluctuating between net uptake and net emissions (?73 to 279 ng m^?2 h^?1 across 161 individual measurements), and with no strong seasonality, but with time‐averaged net emission overall [27±57 (1 SD)] ng m^?2 h^?1]. This work demonstrates that scale‐up needs to be based on sufficient individual measurements to provide a reasonably constrained estimate of the long‐term mean. Mean (±1 SD) net CH₃Br emissions from deciduous and coniferous leaf litter were 43 (±33) ng kg^?1 (dry weight) h^?1 and 80 (±37) ng kg^?1 (dry weight) h^?1, respectively, and ～1–2 ng kg^?1 (fresh weight) h^?1 from rotting woody litter. Despite the intrinsic variability, data obtained here consistently point to the conclusion that the temperate forest soil/litter ecosystem is a net source of CH₃Br to the atmosphere.     

Systematic assessment of terrestrial biogeochemistry in coupled climate–carbon models

With representation of the global carbon cycle becoming increasingly complex in climate models, it is important to develop ways to quantitatively evaluate model performance against in situ and remote sensing observations. Here we present a systematic framework, the Carbon‐LAnd Model Intercomparison Project (C‐LAMP), for assessing terrestrial biogeochemistry models coupled to climate models using observations that span a wide range of temporal and spatial scales. As an example of the value of such comparisons, we used this framework to evaluate two biogeochemistry models that are integrated within the Community Climate System Model (CCSM) – Carnegie‐Ames‐Stanford Approach′ (CASA′) and carbon–nitrogen (CN). Both models underestimated the magnitude of net carbon uptake during the growing season in temperate and boreal forest ecosystems, based on comparison with atmospheric CO₂ measurements and eddy covariance measurements of net ecosystem exchange. Comparison with MODerate Resolution Imaging Spectroradiometer (MODIS) measurements show that this low bias in model fluxes was caused, at least in part, by 1–3 month delays in the timing of maximum leaf area. In the tropics, the models overestimated carbon storage in woody biomass based on comparison with datasets from the Amazon. Reducing this model bias will probably weaken the sensitivity of terrestrial carbon fluxes to both atmospheric CO₂ and climate. Global carbon sinks during the 1990s differed by a factor of two (2.4 Pg C yr^?1 for CASA′ vs. 1.2 Pg C yr^?1 for CN), with fluxes from both models compatible with the atmospheric budget given uncertainties in other terms. The models captured some of the timing of interannual global terrestrial carbon exchange during 1988–2004 based on comparison with atmospheric inversion results from TRANSCOM (r=0.66 for CASA′ and r=0.73 for CN). Adding (CASA′) or improving (CN) the representation of deforestation fires may further increase agreement with the atmospheric record. Information from C‐LAMP has enhanced model performance within CCSM and serves as a benchmark for future development. We propose that an open source, community‐wide platform for model‐data intercomparison is needed to speed model development and to strengthen ties between modeling and measurement communities. Important next steps include the design and analysis of land use change simulations (in both uncoupled and coupled modes), and the entrainment of additional ecological and earth system observations. Model results from C‐LAMP are publicly available on the Earth System Grid.     

Deficiency in African plant distribution data – missing pieces of the puzzle

Biodiversity is spatially unevenly distributed and so is the information on its spatial patterns. This uneven distribution of information on species occurrences is an important impediment to the conservation of biodiversity. Based on 185 427 collection records of 5873 plant species in sub-Saharan Africa, we analyse the availability of distribution data suitable for the GIS-based mapping of plant diversity patterns at a one-degree resolution. Using the bioclimatic model GARP, distribution ranges for each species were modelled. In order to identify data-deficient areas, the documented and modelled diversity patterns were compared. Only for a few, well-known centres of plant diversity are there comparatively many data collection records available. For several of the areas with very few collection records, such as the Guinean montane forests, the north-western Congolian lowland forests, and the southern Albertine Rift montane forests, the model predicts a species richness much higher than currently documented. In many of the data deficient areas, difficult conditions for scientific research appear to have limited collection activities for decades. Only strategic field collections can fill these gaps. Another cause of data deficiency is that data collected and digitized do not match the quality requirements for GIS-based work at the super-regional scale. In particular, regional databases documenting partial ranges of species are rarely connected. One challenge for the Global Strategy for Plant Conservation is therefore to establish international collaborative structures and technical standards that will allow analysis of biogeographical patterns across political boundaries.  © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 150 , 355–368.     

Variation in local abundance and species richness of stream fishes in relation to dispersal barriers: implications for management and conservation

1. Barriers to immigration, all else being equal, should in principle depress local abundance and reduce local species richness. These issues are particularly relevant to stream‐dwelling species when improperly designed road crossings act as barriers to migration with potential impacts on the viability of upstream populations. However, because abundance and richness are highly spatially and temporally heterogeneous and the relative importance of immigration on demography is uncertain, population‐ and community‐level effects can be difficult to detect. 2. In this study, we tested the effects of potential barriers to upstream movements on the local abundance and species richness of a diverse assemblage of resident stream fishes in the Monongahela National Forest, West Virginia, U.S.A. Fishes were sampled using simple standard techniques above‐ and below road crossings that were either likely or unlikely to be barriers to upstream fish movements (based on physical dimensions of the crossing). We predicted that abundance of resident fishes would be lower in the upstream sections of streams with predicted impassable barriers, that the strength of the effect would vary among species and that variable effects on abundance would translate into lower species richness. 3. Supporting these predictions, the statistical model that best accounted for variation in abundance and species richness included a significant interaction between location (upstream or downstream of crossing) and type (passable or impassable crossing). Stream sections located above predicated impassable culverts had fewer than half the number of species and less than half the total fish abundance, while stream sections above and below passable culverts had essentially equivalent richness and abundance. 4. Our results are consistent with the importance of immigration and population connectivity to local abundance and species richness of stream fishes. In turn, these results suggest that when measured at appropriate scales (multiple streams within catchments), with simple protocols amenable to use by management agencies, differences in local abundance and species richness may serve as indicators of the extent to which road crossings are barriers to fish movement and help determine whether road‐crossing improvements have restored connectivity to stream fish populations and communities.