YEAST SHOWS PHYSICS CAN GIVE RISE TO MULTICELLULAR LIFE SANS MUTATIONS

A new study by NCBS scientists offers an 'unconventional view' of how major changes first arise in the course of evolution

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By the time Nishant Narayanasamy joined Shashi Thutupalli's lab at the National Centre for Biological Sciences (NCBS) in Bengaluru in 2019, the lab had a new guest: a yeast colony that had seemingly grown way beyond its expected size.

The snowflake yeast had been shipped from William Ratcliff's laboratory at the Georgia Institute of Technology in the U.S. Regular yeast - the same organism that makes bread fluffy grows as a single cell. When it reaches a certain size, a small bump appears on its surface. The yeast's nucleus, the compartment that holds its genetic material, splits into two and moves into this bud. The bud grows until it reaches a certain size and eventually falls off from the parent, making new yeast.

But a small change in snowflake yeast's genetic composition prevents the bud from falling off. As newer buds appear, the yeast clusters in the shape of a snowflake. And as the cluster grows, in about 12 hours it becomes a large blob visible to the naked eye.

Snowflake yeast has been used to study how lifeforms first became multicellular. Multicellular organisms can grow much larger than those with only one cell as long as they can deal with the other effects of largeness. For example, how does the organism ensure all cells in its body receive the nutrients they need to grow?

This is why most animals and plants evolved specific structures to transport nutrients. Blood and blood vessels do this job in humans.

A simple process

Snowflake yeast has no such facility, however, which means at some point the yeast should stop growing. Any further increase in size would cause at least some of its cells to not get enough nutrients. However, defying expectations, the snowflake yeast in Thutupalli's lab continued to grow that too at an exponential rate. How the yeast attained this feat was a mystery.