Data from: Multicellular group formation in response to predators in the alga Chlorella vulgaris

A key step in the evolution of multicellular organisms is the formation of
cooperative multicellular groups. It has been suggested that predation
pressure may promote multicellular group formation in some algae and
bacteria, with cells forming groups to lower their chance of being eaten.
We use the green alga Chlorella vulgaris and the protist Tetrahymena
thermophila to test whether predation pressure can initiate the formation
of colonies. We found that: (1) either predators or just predator
exoproducts promote colony formation; (2) higher predator densities cause
more colonies to form; and (3) colony formation in this system is
facultative, with populations returning to being unicellular when the
predation pressure is removed. These results provide empirical support for
the hypothesis that predation pressure promotes multicellular group
formation. The speed of the reversion of populations to unicellularity
suggests that this response is due to phenotypic plasticity and not
evolutionary change.