Peter Conlin

The entrenchment of multicellularity: group-specific adaptation limits evolutionary reversion to unicellularity

During the transition to multicellularity, cells evolve from free-living individuals into parts of a new higher-level organism. The first step of this transition, the evolution of simple undifferentiated multicellular groups, occurs readily in diverse unicellular species. However, nascent multicellular organisms are also expected to be genetically labile – prone to reversion to unicellularity due to typically high fitness costs of multicellularity in the absence of selection favoring large group size. Successful transitions therefore depend on subsequent mutations that epistatically constrain the possible routes to reversion, promoting the entrenchment of the higher-level unit and allowing its persistence through time. Here I will report on our ongoing efforts to study the entrenchment of multicellularity in digital evolution experiments with the AVIDA artificial life platform and laboratory evolution experiments with the snowflake yeast model system. We find that changes in both the fitness effect of reversion mutations and the availability of reversion mutations contribute to the entrenchment of multicellularity and can occur over a relatively short period of time. Entrenchment is a general outcome of the evolutionary process, but I will attempt to provide rationale for why it may be both particularly likely and particularly relevant for evolutionary transitions in individuality which involve large-scale reorganizations of the fitness landscape and the opening up of previously inaccesible evolutionary paths and new phenotypic niches.