| Resumo: | <p>The first step in the major evolutionary transition to multicellularity involves single cells forming a group. Why do cells form a group? Studies have shown that colony formation provides a benefit against predation. However, it is not well known whether this response is generic, or specific to certain prey-predator systems. We sought to assess this by measuring colony formation in nine 'algal-putative predator' combinations; the Chlorophyte algae <em>Chlorella vulgaris</em>, <em>Chlorella sorokiniana</em> and <em>Scenedesmus obliquus</em> as prey, and the flagellate <em>Ochromonas spp.</em>, the ciliate <em>Tetrahymena thermophila</em> and the crustacean <em>Daphnia magna</em> as putative predators. We found that colony formation is a generic response of the algae <em>C. sorokiniana</em>, <em>C. vulgaris</em> and <em>S. obliquus</em> to predation by <em>Ochromonas spp.</em>, <em>T. thermophila</em> or <em>D. magna</em>. Notably, the algae formed colonies regardless of whether the putative predator grazed upon the algae, indicating that predation by itself is not always a prerequisite for colony formation, but this response may have evolved in the algae as a defensive mechanism against potential predation. In six combinations, colonies could not be ingested by the predator, suggesting that colonies have a 'size benefit' against predation. These results outline the importance of predation as a generic factor driving colony formation and help enhance our understanding of the benefits and costs involved in colony formation, the first step in the evolution of multicellularity.</p>
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