Continuous emergence of phototaxis in Dictyostelium discoideum

Abstract

The evolutionary transition from uni- to multicellularity is associated with new properties resulting from collective cell behavior. The social amoeba Dictyostelium discoideum alternating between individual cells and multicellular forms of varying size provides a powerful biological system to characterize such emergent properties. Multicellular forms coined slugs have long been described as chemotactic towards cAMP, and also as phototactic. While chemotaxis is also well-documented at the single-cell level, which explains slug chemotaxis, we asked whether slug phototaxis is an emergent property of multicellularity. For this, we developed an automated microscopy setup to quantify and compare the migration trajectories of single cells and slugs moving in the dark or illuminated with lateral light. We find that single cells, either extracted from phototactic slugs or taken prior to multicellular aggregation, are not phototactic, implying that slug phototaxis results from interactions between cells that lack this property. Further, by analysing slugs composed of a varying number of cells, we find that phototaxis efficiency increases continuously with slug size. Cell-cell interactions combined with self-organization are thus key elements for this property to emerge.

Fig 1. Experimental set-up and slug versus cell size.

(A) Experimental set-up: on the left, cells were placed at 1 cm from the periphery of the Petri dish (gray ring) at the opposite side from the light source. On the right, cells in dark condition were inoculated into the center of the Petri dish. Dashed lines represent region of interest where slugs migrated. (B) Comparison between slug and cell size: fluorescence imaging of a slug composed of 1% fluorescent GFP cells (left) and vegetative cells (right).

Fig 2. Phototaxis is an emergent property of multicellularity.

Trajectories (centered from Eq (1)) and polar representations of start-end directions ${\theta }_{SE}$ (from Eq (2)) of slugs (A) and cells (B and C) when exposed to light (orange) or in the dark (violet). Slug migration showed no preferential direction in the dark but was oriented towards the light source under lateral illumination (A). Vegetative cells (B) and cells from dis-aggregated slugs (C) showed no preferential direction whether in the dark or exposed to lateral light. Directional statistical analysis were computed from Eq (13) for r values and Rayleigh test for uniform angles distribution from Eq (14).

Fig 3. Phototaxis efficiency as a function of slug size:

(A) Schematic representation of parameters: start-end direction estimated from Eq (2) (a) and instantaneous direction from Eq (3) (b). Distribution of start-end directions (B-a) and distribution of instantaneous directions relative to the light source direction (B-b) for 6 intervals of slug sizes (cell number) under lateral illumination (orange) or in the dark (violet). Corresponding phototaxis efficiency (C) estimated from Eq (5). Points in C were fitted with a polynomial function of degree 4. Phototaxis efficiency increases gradually with slug size.