Indecisive Behavior of Amoeba Crossing an Environmental Barrier

Abstract

We report here a new kind of behavior that seems to be 'indecisive' in an amoeboid organism, the Physarum plasmodium of true slime mold. The plasmodium migrating in a narrow lane stops moving for a period of time (several hours but the duration differs for each plasmodium) when it encounters the presence of a chemical repellent, quinine. After stopping period, the organism suddenly begins to move again in one of three different ways as the concentration of repellent increases: going through the repulsive place (penetration), splitting into two fronts of going throught it and turning (splitting) and turning back (rebound). In relation to the physiological mechanism for tip migration in the plasmodium, we found that the frontal tip is capable of moving further although the tip is divided from a main body of organism. This means that a motive force of front locomotion is produced by a local process at the tip. Based on this finding, a mathematical model for front locomotion is considered in order to understand the dynamics for both the long period of stopping and three kinds of behavior. A model based on reaction-diffusion equations succeeds to reproduce the experimental observation. The origin of long-time stopping and three different outputs may be reduced to the hidden instabilities of internal dynamics of the pulse, which may be a skeleton structure extracted from much more complex dynamics imbedded in the Physarum plasmodium.

Allometry in Physarum plasmodium during free locomotion: size versus shape, speed and rhythm

“…The exponential decay of the thickness indicates that almost all protoplasm is concentrated in the frontal part. Takagi et al (2007) have reported that a frontal tip amputated from a plasmodium during one-dimensional locomotion can migrate for some time at the same speed as before without the regular thickness oscillation. This selfmigration of the amputated frontal tip is evidence that migration ability can be localised at the frontal tip.…”

“…Using the frontal part, various sizes of plasmodia (N=54) were prepared as described previously by Ishigami et al (1987) for smaller sizes [volume<0.1 mm 3 (N=18)] and Takagi et al (2007) for larger sizes [volume>0.1 mm 3 (N=36)]. The smaller plasmodia were placed on plain agar gel in Petri dishes (9 cm in diameter) and were allowed to move freely.…”

“…These plasmodia were also allowed to move freely and after a few hours they exhibited stable, almost one-dimensional locomotion with shapes distributed in the longitudinal direction of the lane and a sheet-like leading edge ( Fig. 2E) (Takagi et al, 2007). The experiments were conducted at room temperature (24±1°C) and the humidity inside the apparatus was maintained at around 90% relative humidity (RH) using a plastic lid that was lifted a little.…”

“…The plasmodia were illuminated from below using an infrared beam (λ∼950 nm) that was passed through light diffuser plates and viewed from above (Takagi et al, 2007). The transmitted light images were magnified using either a lens (NIKKOR-H Auto 28 mm f/3.5; Nikon or 55 mm telecentric f/2.8; Computar) or a microscope (C-DSD115; Nikon) and captured using a CCD camera (XC-ST70; Sony) or a cooled CCD camera for the smaller plasmodia and 0.072-0.363 mm pixel −1 for the larger plasmodia.…”

“…This selfmigration of the amputated frontal tip is evidence that migration ability can be localised at the frontal tip. Furthermore, the fact that the migration lasts for only a short time (Takagi et al, 2007) suggests that the engine in the frontal tip is a self-organised local structure that is dynamically and stably maintained in the plasmodium system. It has also been reported that the polarised intracellular chemical pattern corresponds to the polarity of the plasmodium; for example, the ATP concentration is higher at the locomotion front and becomes lower towards the tail (Mori et al, 1986;Ueda et al, 1987).…”