Yeast study yields insights into origin of multicellular life
Researchers in the BioTechnology Institute (BTI), lead by faculty member Michael Travisano, have demonstrated how multicellular organisms can evolve from single-celled individuals. This important transition has long been a mystery to scientists. The researchers showed that the ability to cluster and the ability to live together cooperatively in groups were key steps. This research not only yields new insights on multicellularity, but also may improve understanding about the processes of aging and the development of cancer.
The Travisano research group has been looking at yeast clusters in the model system Saccharomyces cerevisiae, more commonly known as baker’s yeast. Through careful harvesting of yeast cells that settle at the bottom of a test tube after mild centrifuge, they have been able to select for multicellular strains that form snowflake-shaped clusters.
Multicellular yeast display primitive division of labor, one of the requirements of multicellularity. To reproduce itself, a small portion of the snowflake breaks off at “breakpoints” created by apoptotic cells (highlighted above in green) and then grows. In this regard, there is a division of labor – some cells serve as the progenitors of another cluster and some cells die to allow other cells to separate. (photo courtesy Michael Travisano and www.micropop.org)
In a study recently published in the Proceedings of the National Academy of Sciences (PNAS), the researchers demonstrated that clustering of the yeast cells was adaptive and that beyond simply clustering, cells within these colonies, though physiologically similar, evolved different functions and traits. In particular, improvements in colony reproduction were achieved by a process called apoptosis.
“Arguably one of the most important traits in existing multicellular organisms is apoptosis,” explained postdoctoral research associate Will Ratcliff, first author on the paper.
Apoptosis is the genetically programmed death of a cell that is critical for multicellular development and maintenance. Unlike traumatic cell death caused by injury, apoptosis occurs as a response to biochemical changes in the cell. Through microscopic observation of the multicellular snowflake-shaped yeast clusters, the team found that dead and dying cells formed weak links that provided break points for separation of a daughter cluster. This genetically programmed death thus promoted multicellular reproduction.
“It’s a structural interaction,” observed Travisano, “caused by cell death that aids reproduction.”
Though reproduction of individual yeast cells can be either sexual (through the production of spores) or asexual (by means of cell division), the multicellular reproduction of the newly evolved yeast colonies was asexual. The structural breaking off of new smaller colonies, though not characteristic of multicellular animals, is a strategy found in plants and was facilitated by increased apoptosis. This asexual reproduction of multicellular yeast also presents researchers with some insights into the aging process (progressive cell damage and death) and into the unregulated development of cancer cells.
“It’s a model of a cancer tumor,” concluded Ratcliff. “There are a lot of similarities to how we think cancer might occur and evolve.”