Cooperation is a classic solution to hostile environments that limit individual survival. In extreme cases this may lead to the evolution of new types of biological individuals (e.g., eusocial super‐organisms). We examined the potential for interindividual cooperation to evolve via experimental evolution, challenging nascent multicellular “snowflake yeast” with an environment in which solitary multicellular clusters experienced low survival. In response, snowflake yeast evolved to form cooperative groups composed of thousands of multicellular clusters that typically survive selection. Group formation occurred through the creation of protein aggregates, only arising in strains with high (>2%) rates of cell death. Nonetheless, it was adaptive and repeatable, although ultimately evolutionarily unstable. Extracellular protein aggregates act as a common good, as they can be exploited by cheats that do not contribute to aggregate production. These results highlight the importance of group formation as a mechanism for surviving environmental stress, and underscore the remarkable ease with which even simple multicellular entities may evolve—and lose—novel social traits.
Multicellularity-the integration of previously autonomous cells into a new, more complex organism-is one of the major transitions in evolution. Multicellularity changed evolutionary possibilities and facilitated the evolution of increased complexity. Transitions to multicellularity are associated with rapid diversification and increased ecological opportunity but the potential mechanisms are not well understood. In this paper we explore the ecological mechanisms of multicellular diversification during experimental evolution of the brewer's yeast, Saccharomyces cerevisiae. The evolution from single cells into multicellular clusters modifies the structure of the environment, changing the fluid dynamics and creating novel ecological opportunities. This study demonstrates that even in simple conditions, incipient multicellularity readily changes the environment, facilitating the origin and maintenance of diversity.Saccharomyces cerevisiae rapidly evolve multicellular phenotypes in the laboratory under settling selection, 14 providing an experimental model for direct investigation of the effects of this reorganization for the origin and 15 maintenance of diversity [5,6]. Ten replicate populations originated from a single diploid isogenic unicellular 16 strain. Every day a subsample of each culture was taken and centrifuged at very low speed, thereby increasing 17 the representation of large individuals at the bottom of the tube. Following centrifugation, the bottom 100µl 18 were transferred to fresh media and allowed to regrow over 24 hours. After 60 days of selection, all populations 19 evolved multicellular phenotypes [5]. In this system ("snowflake yeast"), post-division adhesion of mother and 20 daughter cells limits the potential for among-cell conflict and confers multicellular heritability, facilitating 21 multicellular adaptation [5,7]. The simplicity of this system allows us to carefully parse the ecological 22 consequences of multicellularity and their effects on the observed diversity. 23Simple evolutionary models, assuming a smooth and unchanging adaptive landscape [8], predict con-24 vergence with respect to size, because size is tightly correlated with fitness during settling selection [5,6]. 25In contrast, high levels of heritable phenotypic variation for snowflake cluster size evolved both within and 26 among replicate populations. Most of the diversity was observed among replicate populations. Nevertheless, 27 after 60 days of selection, nine of the ten populations harbor two or more genotypes that differ in average 28 cluster size, suggesting that the evolution of multicellularity also facilitated morphological diversification 29 within populations. This paper investigates the ecological dynamics of incipient multicellularity in one of 30 these populations and its consequences for rapid diversification [6]. 31 Results 32Phenotypic diversification 33 Focusing on the above mentioned replicate population (C1) after one, four and eight weeks of settling 34 selection, we determined the size distributions ...
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