Alternating selection for dispersal and multicellularity favors regulated life cycles

Abstract

The evolution of complex multicellularity opened paths to increased morphological diversity and organizational novelty. This transition involved three processes: cells remained attached to one another to form groups, cells within these groups differentiated to perform different tasks, and the groups evolved new reproductive strategies. Recent experiments identified selective pressures and mutations that can drive the emergence of simple multicellularity and cell differentiation but the evolution of life cycles, in particular, how simple multicellular forms reproduce has been understudied. The selective pressure and mechanisms that produced a regular alternation between single cells and multicellular collectives are still unclear. To probe the factors regulating simple multicellular life cycles, we examined a collection of wild isolates of the budding yeast, S. cerevisiae. We found that all these strains can exist as multicellular clusters, a phenotype that is controlled by the mating type locus and strongly influenced by the nutritional environment. Inspired by this variation, we engineered inducible dispersal in a multicellular laboratory strain and demonstrated that a regulated life cycle has an advantage over constitutively single-celled or constitutively multicellular life cycles when the environment alternates between favoring intercellular cooperation (a low sucrose concentration) and dispersal (a patchy environment generated by emulsion). Our results suggest that simple multicellularity in wild isolates could be under selection and is regulated by their genetic composition and the environments they encounter and that alternating patterns of resource availability may have played a role in the evolution of life cycles.

Clonal-aggregative multicellularity entrained by salinity in one of the closest relative of animals

“…Unicellular-to-multicellular switches are controlled by fluctuating environmental parameters in other species, such as salinity for cyanobacteria in brackish environments (89) and periodic flooding for cave bacteria attached to surfaces (90). A selective advantage for regulated life cycles in fluctuating environments has also been supported by laboratory experiments in yeast (91) and by theoretical models (5,92,93). The phenotypic plasticity of C. flexa might thus enable it to reap both the benefits of multicellularity (for feeding and collective behavior) under permissive conditions, and of differentiation into solitary cyst-like cells for individual survival under harsh conditions.…”