For a list of Prof. Ratcliff’s publications, check the “Publications” section, or his Google Scholar profile.
Evolution of multicellularity:
The evolution of multicellular organisms from unicellular ancestors was critical to the evolution of large, complex organisms. While multicellularity has evolved more than 25 times independently during the last 3.5 billion years on Earth, the first steps in this transition remain poorly understood. In our lab, we use experiments and theory to understand:
- Under what conditions does multicellularity evolve?
- How do simple multicellular organisms evolve to be more complex? A key component here is the origin of development.
- What are the biophysical limitations faced by early multicellular organisms, and how are they solved?
- How do early multicellular life cycles arise, and what are their ecological and evolutionary consequences?
- How do cells evolve into mutually-interdependent parts of a multicellular organism? What prevents multicellular organisms from reverting back to unicellularity?
We are currently running a Lenski-inspired Multicellularity Long Term Evolution Experiment (MuLTEE), which we hope will continue for at least the next 25 years. As of fall 2021, we have gone through more than 1,000 rounds of selection (about 5000 generations), evolving snowflake yeast that are ~20,000 times larger than their ancestor. These individual snowflake yeast remain clonal and are visible to the naked eye (larger than fruit flies). They have accomplished this feat through sustained biophysical adaptation, evolving far more elongate cells that ‘entangle’, increasing their mechanical toughness by over 10,000-fold. Individual snowflake yeast evolve from being weaker than gelatin, to as strong and tough as wood. You can read more about this work here. In this system, we are investigating the origin of multicellular development, and the genetic, regulatory, and biophysical bases of this innovation. Below a picture showing how complex these evolved lines have become:
Spatial dynamics of microbial cooperation and conflict:
Multicellularity is just an extreme form of social evolution, which we are also quite interested in. Specifically, we study the role of spatial structure in microbial social evolution. The bacterium Vibrio cholerae forms surface-attached biofilms, which it fiercely defends with a ballistic weapon called the Type 6 Secretory System (T6SS) that kills competitors but not clonemates. We showed that this kind of preferential killing strongly drives a spatial phase transition, separating well mixed competitors into clonal domains. Interestingly, by creating strong clonal assortment, this antagonism is a powerful driver of within-strain cooperation. We also showed that cholera can acquire new T6SS alleles through horizontal gene transfer, which is adaptive when the efficacy of current weapons against future competitors is uncertain.
We are interested in how multicellular life cycles emerge and evolve, leading to the evolution of higher-order heritability. Additionally, we are interested in general aspects of evolutionary theory, in particular bet-hedging and the evolution in unpredictable environments.
We are extremely grateful to the National Science Foundation (DEB and IOS), NASA Exobiology, the Human Frontier Science Program, the National Institutes of Health, the Packard Foundation, the Keck Foundation and the Simons Foundation for supporting our research!