Phagotrophy by a ̄agellate selects for colonial prey: A possible origin of multicellularity

Predation was a powerful selective force promoting increased morphological complexity in a unicellular prey held in constant environmental conditions. The green alga, Chlorella vulgaris, is a well-studied eukaryote, which has retained its normal unicellular form in cultures in our laboratories for thousands of generations. For the experiments reported here, steady-state unicellular C. vulgaris continuous cultures were inoculated with the predator Ochromonas vallescia, a phagotrophic ̄agellated protist (` ̄agellate'). Within less than 100 generations of the prey, a multicellular Chlorella growth form became dominant in the culture (subsequently repeated in other cultures). The prey Chlorella ®rst formed globose clusters of tens to hundreds of cells. After about 10±20 generations in the presence of the phagotroph, eight-celled colonies predominated. These colonies retained the eight-celled form inde®nitely in continuous culture and when plated onto agar. These selfreplicating, stable colonies were virtually immune to predation by the ̄agellate, but small enough that each Chlorella cell was exposed directly to the nutrient medium.