What makes snowflake yeast larger and stronger than their ancestors? New study decodes

Entanglement is a widespread and important multicellular trait

The researchers had to investigate the physical interactions between the cells within the yeast clusters in order to determine the specific biophysical mechanisms that permitted growth to macroscopic size. The researchers utilized a scanning electron microscope to photograph thousands of incredibly thin slices of the yeast, which revealed their interior structure. Since normal light microscopes could not reach the large, densely packed groups, this method was necessary. They found that the groups could expand to such enormous sizes thanks to a brand-new physical mechanism. 

The yeast's branches had intertwined; the cluster cells had developed vine-like behavior, wrapping themselves around one another and fortifying the entire structure. In this subset of multicellular life, entanglement is a common and significant multicellular characteristic.

Why snowflake yeast was chosen as the model organism

The MuLTEE was started in 2018 with single-celled snowflake yeast by Ozan Bozdag, a research scientist and former postdoctoral researcher in Ratcliff's group who is also the first author of the publication. 

Because all multicellular lineages began as small and basic organisms, many of them evolved over time to become larger and more robust, the team made its choice based on creature size. It is believed that the capacity to develop huge, robust bodies, which necessitates novel biophysical breakthroughs, contributes to complexity growth.

This theory, however, had never been put to the test in a scientific setting. The team's study of the evolution of simple cell groups into organisms, with specialization, coordinated growth, emergent multicellular behaviors, and life cycles—the features that set an organism capable of sustained evolution apart from a pile of pond scum—was made possible by the snowflake yeast.

The long-term objective of the researchers is to comprehend the process by which multicellular life emerges. It might be conceivable to create totally new species for specialized uses in disciplines like biotechnology if multicellular life could be engineered from scratch.

Study Abstract: 

While early multicellular lineages necessarily started out as relatively simple groups of cells, little is known about how they became Darwinian entities capable of sustained multicellular evolution. Here we investigate this with a multicellularity long-term evolution experiment, selecting for larger group size in the snowflake yeast (Saccharomyces cerevisiae) model system. Given the historical importance of oxygen limitation, our ongoing experiment consists of three metabolic treatments—anaerobic, obligately aerobic and mixotrophic yeast. After 600 rounds of selection, snowflake yeast in the anaerobic treatment group evolved to be macroscopic, becoming around 2 × 104 times larger (approximately mm scale) and about 104-fold more biophysically tough, while retaining a clonal multicellular life cycle. This occurred through biophysical adaptation—evolution of increasingly elongate cells that initially reduced the strain of cellular packing and then facilitated branch entanglements that enabled groups of cells to stay together even after many cellular bonds fracture. By contrast, snowflake yeast competing for low oxygen remained microscopic, evolving to be only around sixfold larger, underscoring the critical role of oxygen levels in the evolution of multicellular size. Together, this research provides unique insights into an ongoing evolutionary transition in individuality, showing how simple groups of cells overcome fundamental biophysical limitations through gradual, yet sustained, multicellular evolution.