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 have evolved simple multicellularity in the yeast Saccharomyces cerevisiae. Snowflake yeast, which initially arise from a single mutation, have a multicellular life cycle that includes regular single-celled bottlenecks. As a result, they respond to group-level selection via group-level adaptation, for example evolving higher levels of programmed cell death to facilitate multicellular reproduction, and evolving larger size by evolving to have more elongate cells, which reduces the strain accumulation within the cluster. We are currently running a Lenski-inspired Long Term Evolution Experiment (LTEE), which we hope will continue for at least the next 30 years. As of fall 2019, we have gone through more than 800 rounds of selection (about 4000 generations), and have evolved snowflake yeast that are ~1,000 times larger than their ancestor that possess simple multicellular development. Much of our current research is attempting to understand the genetic, regulatory, and biophysical bases of this innovation.
We are also interested in other model systems, and have evolved simple multicellularity in the green algae Chlamydomonas reinhardtii, under selection for size and predation selection, supporting the widely-held hypothesis that, in nature, predators may be a key early driver of this major transition. In collaboration with Nicole King’s group, we are trying to evolve obligate multicellularity in choanoflagellates.
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 Packard Foundation, the Keck Foundation and the Simons Foundation for supporting our research!