© William C. Ratcliff 2013
Snowflake yeast after 14 days of
The evolution of multicellularity was one of a few events in the history of life that allowed for
increases in biological complexity. All known multicellular organisms evolved from single-celled
ancestors, most notably in the animals, land plants, and fungi. Take a moment to imagine the world
without multicellular organisms. The most vibrant tropical rainforest would be reduced to little more
than a barren open landscape encrusted with a slimy layer of photosynthetic bacteria and algae.
Clearly, the evolution of multicellularity radically changed the structure of life on our planet.
The evolution of multicellularity resulted in radical changes in organismal size and complexity. Single
cells, which for billions of years were organisms in their own right, give up this autonomy and
become parts of new, more complex, higher-level organisms. These evolved cellular differentiation,
allowing the multicellular organism to do things that were never possible before. And perhaps most
remarkably of all, multicellularity has evolved not just once, or twice, but more than 25 times in
All known transitions to multicellularity are ancient. Even the most recent transitions (brown algae,
such as kelp, and the volvocine algae) occurred more than 200 million years ago. Because of their
ancient origin, early multicellular forms have largely been lost to extinction, making it hard for
scientists to study the first steps in the evolution of multicellularity. Until now.
Ratcliff et al. (2012) carried out a novel experiment to evolve simple multicellularity in the lab,
starting with single-celled microbes (see the video abstract). The authors created an environment that
favored strains that evolve to form clusters of cells (the first step in the transition to multicellularity)
by subjecting Baker’s yeast (Saccharomyces cerevisiae) to daily selection for fast settling through
liquid medium. Within just a few weeks, yeast that formed snowflake-shaped clusters of cells (left)
evolved and displaced their single-celled ancestors. “Snowflake” yeast display several hallmarks of
multicellularity, including juvenile and adult life stages, determinate growth, and a rudimentary
cellular division of labor utilizing programmed cell death (left).
In this experiment, students will repeat the experiment of Ratcliff et al. (2012), evolving snowflake
yeast from unicellular ancestors. In addition, they will examine cluster-level adaptation by selecting
for either faster or slower settling in a snowflake yeast.
Please have your students complete these pre and post tests online. This assessment will
provide critical data on this lab’s efficacy, and will help us improve the lab. These quizzes are
quick and anonymous.
Lab 1: Experimental
Evolve your own multicellular yeast
Time: 30 minutes a day for 3 weeks
Snowflake yeast after 60 days of
In the above photos, green cells are
undergoing programmed cell death
(apoptosis), red cells are dead, and
orange cells have recently died from
Time-lapse video of snowflake yeast