Meta-population structure and the evolutionary transition to multicellularity [NEW RESULTS]

Meta-population structure and the evolutionary transition to multicellularity [NEW RESULTS]: The evolutionary transition to multicellularity occurred on numerous occasions, but transitions to complex life forms were rare. Here we present experiments that contrast two ecological frameworks that differ in the way in which nascent multicellular groups, and their constituent cells, compete. Groups of the bacterium Pseudomonas fluorescens were propagated under a regime requiring reproduction via a life cycle with developmental and dispersal phases. By controlling the extent of mixing during the dispersal phase the relative emphasis on the developmental phase versus the dispersal phase was altered. While all groups possessed paradigmatic features of multicellular individuals including bottlenecks and germ lines, the mode of group interaction substantially impacted both the strength and direction of selection operating at group and cell levels. Constraints on meta-population structure may therefore explain the observation that multicellular aggregates rarely complete the transition to individuality.