The multicellular, slug stage of the slime mould Dictyostelium discoideum lacks specific sensory cells and organs but can nevertheless respond in a very sensitive manner to external stimuli such as ...temperature and light. Within the migrating slug, the behavior of up to 100,000 individual amoebae is coordinated by cAMP mediated cell-cell signaling and chemotaxis. We report here the striking result that light directly modulates the cAMP cell-cell signaling system. Light-induced secretion of cAMP from the slug tips decreased the period length of optical density waves and speeded up cell movement. A local effect of light on cAMP release within the slug tip could modulate cell movement within the slug and thus control its phototactic turning and orientation toward a light source.
The ESA ERS1-pilot project PP2-D11 was aimed at investigating the capabilities of the ERS-1/2 satellites for continuous land use and deforestation monitoring over a period of 5 years (1993-1997) in a ...tropical rain forest environment. Deploying texture analysis techniques, several land use classes and different forest types could be identified in single ERS-1/2 Synthetic Aperture Radar (SAR) images. The texture features created allowed the discrimination of (1) undisturbed forest of at least two different types, (2) shifting cultivation and agriculture, (3) secondary forest, (4) selectively logged forest, (5) clearings by fire and clearcutting, and (6) settlements and major roads. The results of multitemporal data analysis showed that changes in area of classes (2), (4) and (5) can be readily detected in a multitemporal sequence of ERS SAR images. KFA1000 photographs from the Russian MIR Space Station, Landsat Thematic Mapper (TM) images and ground data acquired during two extensive field excursions served as a reference for the interpretation of the ERS images. All evaluation procedures were performed with standard computer equipment and commercial software packages to ensure operationality and technology transfer towards Indonesia. The results suggest that the European radar satellites ERS-1/2 can be used to monitor and analyse forest conversion and land use patterns in tropical rain forests at scales 1:100000.
We generated Dictyostelium double mutants lacking the two F-actin crosslinking proteins alpha-actinin and gelation factor by inactivating the corresponding genes via homologous recombination. Here we ...investigated the consequences of these deficiencies both at the single cell level and at the multicellular stage. We found that loss of both proteins severely affected growth of the mutant cells in shaking suspension, and led to a reduction of cell size from 12 microns in wild-type cells to 9 microns in mutant cells. Moreover the cells did not exhibit the typical polarized morphology of aggregating Dictyostelium cells but had a more rounded cell shape, and also exhibited an increased sensitivity towards osmotic shock and a reduced rate of phagocytosis. Development was heavily impaired and never resulted in the formation of fruiting bodies. Expression of developmentally regulated genes and the final developmental stages that were reached varied, however, with the substrata on which the cells were deposited. On phosphate buffered agar plates the cells were able to form tight aggregates and mounds and to express prespore and prestalk cell specific genes. Under these conditions the cells could perform chemotactic signalling and cell behavior was normal at the onset of multicellular development as revealed by time-lapse video microscopy. Double mutant cells were motile but speed was reduced by approximately 30% as compared to wild type. These changes were reversed by expressing the gelation factor in the mutant cells. We conclude that the actin assemblies that are formed and/or stabilized by both F-actin crosslinking proteins have a protective function during osmotic stress and are essential for proper cell shape and motility.
Aggregation fields of Dictyostelium amoebae are organized by propagating concentric or spiral waves of cAMP. These waves coordinate cell movement directed toward the aggregation center. We now ...systematically investigated dark-field wave propagation and chemotactic cell movement during late aggregation and mound formation. The period and the signal propagation velocity decreased continuously during aggregation leading to a 15-fold decrease of the chemical wavelength. By analyzing the behavior of single GFP-labeled cells in aggregates and mounds we measured cell movement velocity, changes in cell shape, periodicity of cell movement, and cell trajectories. In early mounds of strain AX-3 dark-field waves propagated frequently as multiarmed (high-frequency) spirals. During the high-frequency waves observed in the early mound stage, cell movement speed is low and cell movement rather undirected. During tip formation the wave period decreased again and the cells started to rotate in the mound at unusually high average speeds of 40 microns/min. The rotation was almost monotonic with no clear periodicity. Since at this time the majority of the cells had already differentiated into prespore cells, this implies that prespore cells moved faster than aggregation stage cells. At 12 hr of development cell movement velocity dropped again and became highly periodic. These measurements show that the relay system is characterized by a specific temporal evolution, which is closely correlated with cellular differentiation. The remarkable changes in cell movement speed and period indicate a qualitative change in signal and movement parameters which might well be caused by the observed switch from high- to low-affinity cAMP receptors during mound formation. This switch might be required to copy with the increase in cell density and most likely plays a crucial role in the process of cell sorting.
Stream formation and spiral wave behaviour during the aggregation of
Dictyostelium discoideum (Dd)are studied in a model based on the Martiel-Goldbeter equations for cAMP relay, combined with ...chemotactic motion of
Ddcells. The results show that stream formation occurs if the turnover rate of intracellular cAMP is increased. This increase in the turnover rate of cAMP
inleads to a dependence of the speed of the cAMP wave on the cell density. We propose that this dependence of wave speed on cell density is the underlying mechanism for stream formation. Besides stream formation, increasing the turnover rate of cAMP
inalso results in a spiral wave period that decreases during aggregation, a phenomenon that is commonly observed
in situ.
Furthermore, the dependence of wave speed on cell density is measured empirically. The speed of the cAMP wave is found to decrease as the wave travels from high to low cell density. This indicates that
in situ, wave speed does depend on cell denisty.
There are cells scattered in the rear, prespore region of the Dictyostelium slug that share many of the properties of the prestalk cells and that are therefore called anterior-like cells (ALCs). By ...placing the gene encoding a cell surface protein under the control of an ALC-specific promoter and immunologically labeling the living cells, we analyze the movement of ALCs within the slug. There is a posterior to anterior cellular flow, and the ALCs change their movement pattern as they enter the prestalk zone. Prestalk cells are periodically shed from the migrating slug. They must be replaced if the correct ratio of prestalk to prespore cells is to be maintained, and we present evidence for the transdifferentiation of prespore into prestalk cells, with ALCs functioning as intermediates in the transition. The slug has, therefore, a surprisingly dynamic structure, both with respect to cellular differentiation and cell movement.
A method is described that allows simultaneous measurement of two spectrally distinguishable green fluorescent protein (GFP) mutants with a confocal microscope. In contrast to previously described ...methods, neither UV excitation nor repetition of scans is required. Therefore the method is well-suited to the long-time observation of living cells in three-dimensional microscopy and time series recording, as demonstrated with GFP-expressing Dictyostelium discoideum cells.
Cellular slime moulds (Dictyosteloids) are characterised by at least two different modes of slug migration. Most species, e.g. Dictyostelium mucoroides, produce a stalk continuously during slug ...migration, while a few species, e.g. Dictyostelium discoideum are characterised by stalk-less slug migration and only produce a stalk upon culmination. Experiments on D. discoideum and theoretical model calculations have shown that D. discoideum slugs are organized by a cAMP scroll wave in the tip which produces planar waves in the back. These waves guide cell movement in slugs: spiralling in the tip and forward movement parallel to the slug axis in the back. Simple changes in model parameters can lead to the formation of a twisted scroll wave which extends throughout the slug. In order to investigate whether such twisted scroll waves occur naturally we have analysed the movement of fluorescently labelled single cells in migrating D. mucoroides slugs. The results show that cells in the prespore zone of D. mucoroides slugs move in a spiral path. Although the velocity of single cells in D. mucoroides is faster than in D. discoideum, the net forward component of their movement is less due to their spiral trajectories. As a result D. mucoroides slugs move more slowly than D. discoideum slugs. The entire D. mucoroides slug also describes a spiralling path leaving corkscrew shaped stalks behind. Based on these observations we propose that cell movement in D. mucoroides slugs is controlled by a propagating twisted scroll wave of cAMP which extends throughout the length of the slug.
PDF
