MuLTEERatcliff Lab · Georgia Tech

Outreach

Evolve your own multicellular yeast

We provide free laboratory kits and curricula for high school and college courses. Students can evolve multicellularity de novo using settling selection, or test the hypothesis that small-mouthed predators select for multicellular size. We have sent more than 100 kits worldwide.

Lab 1: Experimental evolution · Lab 2: Predation

Animated microscopy: a Philodina rotifer consuming red-stained unicellular yeast while a blue-stained multicellular snowflake cluster sits untouched nearby.

Predation in action

Predator
Philodina rotifer
Red
Unicellular yeast (Congo red)
Blue
Multicellular yeast (methylene blue)
Lab 1

Evolve multicellular yeast

Duration
3 weeks (daily transfers)
Level
College, advanced high school
Core concept
Settling selection for larger size

Students culture unicellular S. cerevisiae Y55 and a previously evolved snowflake strain, perform daily settling selection for three weeks, and track evolved changes in cluster size and settling speed by microscopy. By selecting for faster or slower settling, students can drive evolution in opposite directions.

Lab 2

Predator-imposed selection for multicellularity

Duration
1–2 class periods (50 min each)
Level
High school and college
Core concept
Size-selective predation

Students feed differentially stained unicellular and multicellular yeast to Philodina rotifers, whose small mouths preferentially consume unicellular cells. Students count prey in the transparent rotifer stomachs and apply a chi-square test. Paramecium aurelia works as an alternative predator.

Free kits

What is in the kit

The full kit: a ziploc bag of stained yeast and spore tubes, two boxes of glass cavity slides, a Falcon tube of YPD+Agar media, and a sterile multi-well culture dish.

The kit

NSF support allows us to provide kits free of charge. Each kit contains the key materials and reagents for the predation lab (Lab 2). Some supplies and equipment must be provided by the instructor.

Kit contents

  • 1 mL fixed, Congo red stained unicellular yeast
  • 1 mL fixed, methylene blue stained multicellular yeast
  • Unicellular yeast spores
  • Multicellular yeast spores
  • 6.5 g of YPD+Agar media (enough for 150 mL of culture media)
  • 10 glass concavity slides
  • 1 sterile multi-well culture dish (for plating yeast spores)

Supplied by the instructor

  • Compound microscopes
  • Transfer pipettes or micropipettes
  • Rotifers (Philodina sp., Carolina Biological Supply #133172) or Paramecium aurelia (#131546)

Request a kit

To order a kit, contact ratcliff@gatech.edu. Please allow about 6 weeks for delivery.

Curricula

Downloads

Lab 1 · Evolution

Student handout

The full student protocol for the 3-week settling-selection evolution lab.

Download PDF

Lab 1 · Evolution

Teacher guide

Preparation notes, scheduling suggestions, and answer keys.

Download PDF

Lab 1 · Evolution

Introductory presentation

Slides introducing multicellularity and the snowflake yeast system.

Download PPTX

Lab 2 · Predation

Student handout

The student protocol for the predation lab (1–2 class periods).

Download PDF

Lab 2 · Predation

Teacher guide

Preparation, rotifer care, and statistical analysis guidance.

Download PDF

Lab 2 · Predation

Introductory presentation

Slides on predation as a selective pressure for multicellularity.

Download PPTX

Both labs

Lab summary

Overview of both labs, materials, and learning objectives.

Download PDF

Assessment

Post-test

Assessment instrument for evaluating student learning outcomes.

Download PDF

All materials are free to use and modify for educational purposes.

In the browser

Grow a cluster in your browser

Students can also grow a snowflake yeast cluster on screen. The lab's interactive 3D simulator starts from a single cell and grows it until a bond snaps and the cluster reproduces. Change the shape of the cells or the strength of their bonds, run the weak, default and strong presets in turn, and watch how the life cycle responds. It runs in any recent browser, with nothing to install.

This section carries an interactive 3D simulator, which needs JavaScript and a recent browser. It grows a cluster from one cell: each cell buds, its daughters stay attached, contacts between cells push the branches apart, and a bond breaks when it is stretched past a set threshold, leaving two clusters that both keep growing.

Cells
1
Clusters
1
Broken bonds
0
Growth steps
0

Loading the simulator…

Cells are coloured by the stretch on their attachments, from blue (slack) to red (at the breaking point). Detached clusters drift apart on screen so you can tell them apart. That drift, and the gentle jiggle, move whole clusters for display only and never feed back into the physics.

Things to try

Ask the model a question

Try changing a variable to investigate its effect on the properties of the cluster.

  • Watch for the first break. Press Run with the default settings. Once a bond snaps, a white outline marks the cluster that still holds the founding cell and coloured outlines mark the pieces that broke away, each of which keeps growing and can break again.
  • Compare bond strengths. Run the three presets in turn. They share a seed and differ only in bond strength, so any difference in when the first branch breaks off, and in how many clusters you end up with, comes from that one setting.
  • Change the shape of the cells. In the MuLTEE's anaerobic lines, cells evolved to be more elongate, lowering packing density and delaying fracture (Populations). Move the shape slider, apply the change, and ask what cell shape alone does to a growing body when nothing else changes.
  • Change the seed. The seed fixes the random sequence of bud positions and budding times, so the same settings can grow a visibly different body, showing how much of the outcome rides on chance events at each bud. Run the same settings with several seeds to see why experiments need replicates.

The model

What it includes, and what it leaves out

This is a deliberately simple model. Cells are rigid ellipsoids that grow, bud and push against their neighbours. Each bond between a mother and daughter stretches like a spring, bends a little (yielding permanently past a modest angle), and breaks for good once it is stretched past the threshold you set. No rule tells a cluster when or where to split; breaks happen because growth crowds the cells. Sizes and times are in arbitrary units and are not matched to real yeast, so the model can show how a branched cluster comes to break apart on its own, but it cannot predict how large or how fast a real cluster grows.

The model also counts every detached piece as a new cluster, without asking whether that piece could actually escape its neighbours. Branch entanglement, which let evolved MuLTEE clusters hold together after many mother-daughter bonds had broken, is therefore outside its scope.

More about the model

A newborn cell starts at about a quarter of mature size (0.23) and grows to size 1; changing the cell shape keeps cell volume the same. Each cell's first daughter grows from its tip, in line with its long axis. Later daughters attach near the same end, at a random position around the cell, and start growing at the later-bud angle you choose. After that, pushing from neighbours can turn them.

A bond can bend by up to 12° and spring back. Bent further, it slowly takes on the new angle permanently, up to 25° in total. A bond breaks only when it is stretched past the breaking point, and a broken bond never reforms. Every group of cells still joined by bonds counts as one cluster, and all clusters keep growing and pushing on each other.

The model waits for the cells to settle after each growth step before growing again, so growth can look uneven, and on rare occasions it pauses or stops. These pauses come from the calculation, not from the biology.

Published curricula

The papers behind the labs

  1. Predator Escape: An Ecologically Realistic Scenario for the Evolutionary Origins of Multicellularity

    Jennifer T. Pentz, Tami Limberg, Nicholas Beermann, William C. Ratcliff

    2015 Evolution: Education and Outreach 8:13 doi.org/10.1186/s12052-015-0041-8

  2. A Novel Laboratory Activity for Teaching about the Evolution of Multicellularity

    William C. Ratcliff, Allison Raney, Sam Westreich, Sehoya Cotner

    2014 The American Biology Teacher 76:81–87 doi.org/10.1525/abt.2014.76.2.3

Acknowledgments

Contributors

These labs were created by Will Ratcliff with Tami Limberg (Great River High School), Nicholas Beermann (MacDowell Montessori High School), and Sehoya Cotner (University of Minnesota). Jennifer Pentz developed the predation lab. We are grateful to Michael Travisano, R. Ford Denison, Mark Borrello, Matthew Herron, Sam Westreich, and Allison Raney for discussions and improvements.

Supported by the National Science Foundation (DEB-1051115) and the Society for the Study of Evolution (2013 Thomas Henry Huxley Award for scientific outreach to Will Ratcliff).