# Continuous emergence of phototaxis in Dictyostelium discoideum

## TL;DR

This study shows that phototaxis in the social amoeba Dictyostelium discoideum emerges as a group behavior when cells come together in multicellular slugs.

## Contribution

The study demonstrates that phototaxis is an emergent property of multicellularity, not present in individual cells.

## Key findings

- Single cells of Dictyostelium discoideum are not phototactic, but slugs are.
- Phototaxis efficiency increases with the number of cells in a slug.
- Cell-cell interactions and self-organization are essential for phototaxis to emerge.

## 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.