Adaptation and divergence during experimental evolution of multicellular Saccharomyces cerevisiae

The evolution of multicellularity was one of the key innovations in the history of life on Earth. Virtually all morphological and ecological diversity in macro-organisms builds upon the evolutionary potential associated with multicellularity. We examined the potential for ecological diversity to rapidly arise following transitions to multicellularity. Replicate microcosms containing the yeast Saccharomyces cerevisiae were maintained under serial transfer. Prior to transfer to fresh media each day, S. cerevisiae underwent settling selection via mild centrifugation. Those individuals reaching the bottom of the centrifuge tube were transferred to fresh media. After sixty days, all microcosms contained multicellular individuals that develop via mother-daughter adhesion. In nine of the ten microcosms, at least two distinctive morphological genotypes were evident at sixty days, and in eight of them, the variants were multicellular. We observed substantial morphological variation across replicates, with relatively little parallelism in the size of multicellular individuals or in the size variation within microcosms. These results suggest surprising amounts of contingency in the evolution of ecological diversity, and that "replaying life's tape" would lead to divergent outcomes.