Analysis of optical density wave propagation and cell movement in the cellular slime mould Dictyostelium discoideum

We have studied optical density wave propagation during aggregation of the cellular slime mould Dictyostelium discoideum in a quantitative manner by digital image analysis. The waves are mostly single ended spiral waves starting from an aggregation center. We can measure a variety of parameters such as oscillation frequency, wave propagation velocity and wave shape. This allows the construction of dispersion curves under a variety of experimental conditions. During later development where the optical density waves are no longer visible we have started to measure movement of fluorescently labelled cells. Our main conclusions from these measurements are that the cells continue to move chemotactically to periodic signlas both in aggregates and in slugs. There is a dramatic difference in the movement pattern of prestalk and prespore cells: Prestalk cells move perpendicular to the long axis of the slug, they are most likely organized by a scroll wave. Prespore cells seem to move almost perpendicular to the prestalk cells, in the direction of the tip. This behaviour is explained on the basis of different relay properties of prespore and prestalk cells

Digital image processing of optical density wave propagation in Dictyostelium discoideum and analysis of the effects of caffeine and ammonia

Waves of chemotactic movement during the early phase of aggregation in Dictyostelium discoideum were analyzed by digital image processing in a manner that immediately shows the following parameters: wave propagation velocity, period length, wave amplitude und wave shape. We have characterized the aggregation of AX-2 and the streamer F mutant NP 377 in terms of these parameters and investigated the influence of caffeine and ammonia. It was found that during normal aggregation oscillation frequency increases while at the same time wave propagation velocity decreases. Caffeine, a known inhibitor of cyclic AMP relay, reduces oscillation frequency and wave propagation velocity in a dose-dependent manner but most notably leads to the appearance of bimodal (harmonic) oscillations. These bimodal waves are also found in streamer F mutants without caffeine during early aggregation. The effect of caffeine is interpreted as an increase in the average chemotactic deadaptation time due to elevated cyclic GMP levels after a cyclic AMP stimulus. This increased deadaptation time results in some cells responding to every chemotactic signal, while others respond only to every second signal, leading to mixed population behavior and hence biphasic optical density waves. Ammonia has no significant influence on oscillation frequency and wave propagation velocity but shows a clear increase in the amplitude of the optical density waves. This may indicate a more vigorous chemotactic response by individual cells or a better synchronization of the responding cell populations due to shortened chemotactic deadaptation times

Twisted scroll waves organize Dictyostelium mucoroides slugs

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

Multiarmed Spirals in Excitable Media

Numerical studies of the properties of multiarmed spirals show that they can form spontaneously in low excitability media. The maximum number of arms in a multiarmed spiral is proportional to the ratio of the single spiral period to the refractoriness of the medium. Multiarmed spirals are formed due to attraction of single spirals if these spirals rotate in the same direction and their tips are less than one wavelength apart, i.e., a spiral broken not far from its tip can evolve into a 2-armed spiral. We propose this mechanism to be responsible for the formation of multiarmed spirals in mounds of the slime mold Dictyostelium discoideum