The origin of multicellular life is a central scientific question. Prior research has identified multiple benefits that maintain multicellular life, including larger body size and different cell types, but, how single celled life gave rise to complex multicellular organisms remains an open question. The proposed research focuses on observing and experimentally studying the origin of multicellularity as it occurs, using a newly developed research model of baker's yeast. The research has three objectives. First, the environmental conditions promoting the origin of multicellular life will be investigated. Second, the effects of increased mutation rate on the transition to multicellularity will be determined, because mutations can generate conflicts, such as cancer, in multicellular individuals. Third, the evolutionary consequences of multicellularity will be investigated, as multicellularity provides new avenues for adaptation.
The research focuses on a topic of special interest to the general public: the origin of biological complexity. While review of any biological system provides an appreciation of its complexity, the evolution of this complexity frequently remains opaque, even after intensive investigation. A major obstacle has been an inability to examine the origin of complexity as it arises, a limitation which has been overcome with this new system. This model yeast system lends itself to the application of experimental evolution to previously intractable problems in evolutionary biology, with the potential for widespread dissemination of these research techniques. The PIs will release educational materials through links to science education societies, specialist publications such as American Biology Teacher, and extensive outreach activities.
A hallmark of multicellular life is its complexity. While unicellular organisms make up much of Earth’s biomass, the vast majority of the diversity in shapes and sizes of organisms occurs in multicellular organisms. In this study, the origin of diverse and complex forms of multicellular life was directly investigated using experimental evolution. New multicellular forms were generated from unicellular ancestors under laboratory conditions. We experimentally manipulated environmental conditions, propagating only those organisms that could rapidly settle through liquid media. Because settling rate increases with size in liquid media, particularly for microscopic organisms, the experimental regime selected for increased size. We observed the evolution of multicellularity in our experimental populations, using two different unicellular ancestors. In bakers yeast, multicellularity rapidly evolved repeatedly across multiple populations, within 60 days of the initiation of the experiment. Multicellularity arose by persistent attachment of daughter and mother cells following cell replication, instead of the separation of cells which occurs in the unicellular ancestor. Because daughter cells remain attached at the site in which they budded off their mother cell, cellular replication results in the development of branched clusters of yeast cells reminiscent of a snowflake in general shape. Snowflake multicellular yeast have many hallmarks of complex multicellular life, such as separate juvenile and adult life stages, different cell specialization, and the ability to evolve as multicellular organisms. The ability to evolve is characteristic to snowflake multicellular yeast, and is different to other forms of multicellularity that could arise by unrelated cells simply sticking together. We also observed the evolution of multicellular traits in a unicellular algae, using a similar mode of selection. Unlike the study with yeast, multicellularity was much less likely to evolve under the propagation scheme, occurring in only one population. The mechanism was similar, the persistent attachment of daughter and mother cells, but cellular replication of algae is different which greatly changed the shape of the multicellular forms. Like the yeast, the multicellular algae evolved a complex pattern of juvenile and adult stages, in this case evolving different behaviors depending upon cycling of the environment. Over the course of the study, one post-doc was trained who is now an Assistant Professor at Georgia Tech. Four graduate students were partially supported, and who are continuing their research. A large number of undergraduates were involved, one of whom is currently a PhD student and one of whom has completed her MS degree. In addition three high school teachers were trained in carrying out this research, and a new kit was developed for showing the evolution of multicellularlity in grade school, high school, and undergraduate science labs. A new lab class on experimental evolution for all undergraduate biology majors was developed, partly motivated by the multicellularity project. Multiple websites are being maintained to disseminate the research to the public, and the research was broadly disseminated outside academia by newspapers (such as the New York Times) and radio. Multiple presentations were made at meetings involving nonscientists, in part to facilitate cross disciplinary understanding between science and the humanities.
