Symmetry breaking and inter-clonal behavioural variability in a slime mould

Abstract

Cells are dynamic systems capable of switching from isotropic growth to polarized growth even in the absence of any pre-existing external asymmetry. Here, we study this process of symmetry breaking in the acellular slime mould Physarum polycephalum. In these experiments, slime moulds could grow on two identical opposed sources of calcium. We highlighted a positive correlation between growth dynamic, level of symmetry breaking and calcium concentration. We identified three populations of slime moulds within our clonal lineage with similar symmetry breaking behaviours but different motility characteristics. These behavioural differences between slime moulds emerged in the absence of any environmental differences. Such behavioural plasticity could generate cellular diversity, which can be critical for survival.

Keywords: Physarum polycephalum; decision-making; phenotypic plasticity; slime moulds.

Figure 1.

(a) Experimental set-up. (b) The total surface of the slime moulds as a function of time for 0.0005 M (yellow), 0.005 M (black) and 0.05 M (blue) CaHPO₄. Mean (line) ±95% CI (shaded area). (c) Start time, (d) maximum total surface (MTS), (e) speed of the slime moulds on 0.0005 M (yellow), 0.005 M (black) and 0.05 M (blue). Mean ±95% CI. Frequency distribution of the proportion of slime moulds on the left branch at the individual stop time (IST) on 0.0005 M (f), 0.005 M (g) and 0.05 M (h). Blue bars represent the proportion of slime moulds that choose one branch while cream bars indicate a non-choice. The proportion of slime moulds on the winning branch (black) and the losing branch (pink) as a function of time to the IST for 0.0005 M (i), 0.005 M (j) and 0.05 M (k). Red dashed lines represent the threshold to consider that the slime mould made a choice. Mean (line) ±95% CI (shaded area). For panels (b–k), n = 36 per condition. (Online version in colour.)

Figure 2.

Mean values for the (a) start time, (b) MTS and (c) mean speed of the slime moulds for each cluster and for 0.0005 M (yellow), 0.005 M (black) and 0.05 M (blue) CaHPO₄. (d) Bootstrapped (dark grey) and experimental (light grey) proportions of the distribution of nine slime moulds among three clusters. The 12 possible distributions are sorted from the most unpredictable (left) to the most predictable distribution (electronic supplementary material, table S1). For example, 4/3/2 means four slime moulds in one cluster, three in a second one and two in the last cluster, and thus encompasses 4/2/3, 3/4/2, 3/2/4, 2/4/3 and 2/3/4. (e) The surface of the slime moulds on the winning (black) and on the losing branch (pink) as a function of time for the slime moulds from the Petri dishes E and I on the different concentrations of calcium. (Online version in colour.)