ARJUN’s SCIENCE UPDATE

Arjun D, Year 12 writes...

‘Snowflakes’ of yeast uncover secrets of our evolution following an experiment with 600 repeats!

Hundreds of millions of years ago, life on earth transitioned from unicellular (single-celled) organisms to multicellular (many-celled) organisms, and ‘snowflakes’ of yeast hints at how.

Thousands of the unicellular yeast cells (Saccharomyces cerevisiae) have competed for survival every day in an Atlanta lab, eventually becoming multicellular about a decade into the process!

These yeast cells offer insight into what it was like when eukaryotic (containing a nucleus) cells first became multicellular.

There are two processes unicellular organisms can undergo to become multicellular: or

1. Aggregation - Organism develops unicellular and multicellular stages in its life cycle, eg Dictyostelium (species with a nucleus) is found in leaf litter from forests to deserts. They become unicellular when nutrients in leaf litter are high, increasing their surface area to volume ratio. This maximises food absorption. However, when nutrients are low, cells of Dictyostelium become multicellular, turning into a dormant cyst (hollow cavity) to conserve energy.

2. Post division adhesion - Division of single cell by mitosis (cell division producing genetically identical cells) without cell separation because divided cells are strongly attracted to each other, eg Clustered snowflakephenotype (structure) of yeast remain multicellular under both high-and low-nutrient conditions because cells are attracted to each other so post division adhesion is used to make yeast multicellular.

Life cycle of multicellular yeast

The life cycle of multicellular yeast is:

1. Propagule (structure that can be detached to produce new multicelled organism, ie baby yeast) from parent yeast

2. Juvenile phase

3. Adult phase

This keeps repeating and clusters of yeast are genetically identical.

What caused yeast to become multicellular?

Unicellular yeast cells were placed in nutrient rich liquid.

Multicellular yeast sinks in liquid more quickly than unicellular yeast, so they can access more nutrients more quickly as cells settle to bottom of nutrient solution. Therefore, multicellular yeast are more likely to survive than unicellular yeast (they have calculated a 34% advantage in surviving, compared to unicellular cells). This means that the unicellular yeast is encouraged to become multicellular.

Sixty days later it worked: the first snowflake of multicellular yeast appeared because of a mutation (random change in structure of gene) that caused unicellular yeast cells to stay bonded to each other, when they divide, producing multicellular yeast snowflakes!

Key stages in development of multicellular organism:

1. Unicellular DNA forms clusters

- In yeast, a mutation (rare and random change in structure of gene) caused new yeast cells to stay bonded and each cell of multicellular yeast can form a new ‘snowflake’ cluster by cell division.

2. Adaptations of multicelled organism that benefit organism as a whole are much more frequent than adaptations that benefit just individual cells - In yeast, programmed cell death (apoptosis) increases the number of offspring (propagules) even though it decreases size of offspring. This is because cells programmed to die (apoptotic) act as breaking points to produce a greater number of offspring (propagules) from a given number of cells.

3. Cells within a cluster carry out specific tasks which must increase cluster-level fitness (ie the ability of the cluster to survive) - In yeast, some cells are offspring (propagules) attached to parent, while others are involved in maintaining structure of yeast. Both contribute to cluster survival as an organism.

Side effects of multicellularity seen in yeast

1. Larger size, as cell division occurs cells are attracted to each other and so do not separate.

2. New structures form in yeast clusters. For example, branches form in clusters of cells and are clones of multicellular yeast, which eventually separates from parent yeast.

3. Cells specialise into reproductive and nonreproductive tasks, instead of one cell undergoing all these processes. This is shown by evolution of cells in clusters.

What have we learned about multicellularity in yeast (hinting to humans)?

1. Multicellularity can evolve very quickly, under the right conditions, as seen by yeast within 60 days in solution.

2. Unicellular yeast divides into two daughter cells of similar size. However, in multicellular yeast, propagules (offspring) are less than half the size of the parent. So, to produce offspring a cluster has to be a certain size. This is the difference between juvenile and adult life stages.

In general, multicellularity has evolved in many unrelated groups of organisms and, by uncovering the story of yeast, we now have more insight into what humans were like when we first multicelled.

Mysteries still awaiting discovery: Can clumps of ‘snowflake’ yeast develop ways to get nutrients to their innermost members?

In transition from unicellular to multicellular organisms, which cell structures are lost due to extinction?