A gradient green-beard gene in fission yeast

The social behaviors of microbes provide unique opportunities for testing social evolution theories. How can altruistic behaviors arise by natural selection is a central challenge in biology. Green-beard effect has been proposed as a basic mechanism for the evolution of altruistic behaviors. Yet, green-beard genes are generally thought to be rare. Here, we find that the Schizosaccharomyces pombe gsf2 gene mediates flocculation-like aggregation, and flocculation is triggered by acid stresses. gsf2-expressing cells preferentially adhere to each other. The expression of gsf2 is costly, but gsf2-expressing cells preferentially adhere to each other and protect each other from external stress. Gsf2 is highly variable in natural populations, likely contributing to different flocculation intensity. These findings suggest that gsf2 is a gradient green-beard gene that drives the altruism among gsf2 carriers. Moreover, we find that gsf2 is a new gene that originated very recently. Our results provide insights into the origin and evolution of green-beard genes.