History of the MuLTEE

Intellectual Foundations

The MuLTEE builds on decades of theoretical and experimental work on major evolutionary transitions:

JT Bonner showed that increased size is the primary driver of early multicellularity, shaping our selective regime.

Andrew Knoll's work on oxygen and the evolution of complex life motivated our metabolic treatments.

Richard Lenski's Long-Term Evolution Experiment with E. coli demonstrated the power of watching evolution unfold in real time across thousands of generations.

Origins: 2010–2018

The Ratcliff Lab spent eight years developing snowflake yeast as a model system for studying the transition to multicellularity. Building on Saccharomyces cerevisiae's genetic tractability, the team evolved and characterized simple multicellular clusters that form through mother-daughter cell adhesion.

Key foundational work by Will Ratcliff and Mike Travisano showed that multicellularity could evolve via mutations in the ACE2 transcription factor, causing daughter cells to remain attached after division. This created branching, snowflake-shaped clusters with an emergent life cycle: groups grow until mechanical strain breaks a cell-cell bond, releasing a clonal propagule.

Critically, they developed gravitational selection as a method to select for larger multicellular size. Larger clusters settle faster through liquid media, allowing efficient screening of millions of clusters daily without requiring predators or other environmental manipulations. This made snowflake yeast an ideal model system for evolving and studying increased size.

Founding the MuLTEE: 2018

While a powerful setup, the original snowflake yeast experiments were constrained to a microscopic scale, with size evolution plateauing after a few hundred generations. In 2018, inspired by Andrew Knoll's ideas and his PhD work growing yeast under different metabolic regimes, Ozan Bozdag developed an evolution experiment to test the role of oxygen concentration on multicellular size evolution. This work revealed that oxygen was indeed a key constraint on size evolution. Growing yeast as petites (unable to respire) removed the oxygen limitation, and supplementing media with sparged air kept O₂ high for aerobic populations. These approaches unlocked the size constraint that had limited earlier experiments.

These results, published in Nature Communications (2021), transformed the initial postdoc project into the MuLTEE. All 15 populations began from an isogenic ancestor with a clean ACE2 deletion, ensuring genetic consistency across replicate lines. Daily gravitational selection has continued ever since.

Timeline