Sunday, 14 April 2013

More On Yeast And Multicellularity

N.B: I would advise reading the post in the link at the end of the article before reading this one, just for clarity. Enjoy!


Yeast cells (Saccharomyces cerevisiae)
A year ago, I reported on two fantastic experiments regarding the origins of multicellular animals. One was conducted at Harvard University, the other at the University of Minnesota's College of Biological Sciences. What they have in common is yeast.

The Minnesota study found that in conditions when food was scarce, yeast cells would form closely packed colonies in order to maximize the capture of nutrients. They hypothesised that this survival behaviour may have been an integral part of the origin of multicellularity.

The Harvard study took this further. They placed yeast cells in a mix of sugar rich and low sugar environments in test tubes, put the test tubes in a centrifuge for a few days and then examined the contents. In the low sugar environments, the yeast cells had formed small, spherical colonies of conjoined cells all descended from a single ancestor. Apart from maximizing nutrient capture, as the cells shared identical genetic material they were able to act with a certain degree of cooperation, with some damaged or defective cells committing a form of biological suicide known as apoptosis, in order to conserve nutrients for healthy, useful cells.

The cooperative colonies from Harvard were even closer to multicellular animals than the Minnesota colonies. This was as far as my article went, a link to which is at the bottom of this post. Now, the University of Minnesota have published results from their original study which takes the story of these cooperative colonies further towards multicellular organisms. The experiment, conducted by William Ratcliff and Michael Traversano, followed a similar line to Harvard's work. They placed yeast cells in a low sugar environment and then agitated the mixture for a few days during which some cells died and others survived in a process mimicking natural selection. The researchers then took the surviving cells and repeated the process. After a few weeks, they had created cooperative colonies.

Unlike the spherical Harvard forms, the Minnesota specimens were similar in form to snowflakes, far more complex. The results did not end there. Some cells within the colony committed apoptosis, lying on vital connecting points within the snowflakes. Their death caused the tips of the structures to break away and form new colonies. Not only could the Minnesota colonies act semi cooperatively, feed more effectively and 'evolve' in a sense, they even had a way of reproducing.

One of the cooperative multicellular yeast colonies
This was over five months ago. Their most recent results are even more remarkable. In order to transfer the more successful inhabitants to fresh test tubes, they allowed the contents of the test tubes to sink to the bottom. The larger, heavier particles sank to the bottom while the lighter particles lay on top. Right at the beginning, all the layers were composed of single cells. When colonies arose, they formed a layer right at the bottom as they were larger, heavier and more successful than the single cells which sat on top in their own layer.

As the months progressed, Ratcliff and Traversano found that the speed at which the colonies sank increased by 45%. Some had simply increased their cell count, with the average changing from 42 to 115 individuals, resulting in a greater overall weight and so sank faster. Others achieved a greater mass by increasing the weight of each cell with the average mass doubling over the five month period. What is interesting is the impact this had in the selective processes which fuelled the 'evolution' of the colonies.

They formed originally in order to maximize the amount of nutrients. However, the larger colonies eventually became so massive that the cells in the centre were unable to feed and so began to die, weakening the overall structure, decreasing their chances of survival into the next test tube generation. The ones with larger cells faced a problem as each member required a far greater amount of nutrients in order to survive, meaning that they had a harder time surviving in the low sugar environment despite their combined efforts to capture food.

A snowflake colony. The green stained cells are about to commit
apoptosis and allow small parts of the structure to break away
to form new colonies in a process mimicking reproduction.
As a result, some colonies found a third option which addressed this cost of living. The cells became larger but less dense, meaning that they required less energy to live, whilst maintaining their weight. The overall colony also changed its shape, moving away from the multi-branched snowflake forms of its ancestors and towards a more spherical shape similar to those from the Harvard experiment. This made them more hydrodynamic and so were able to sink faster than the snowflakes whose branches created drag, hindering  process through the water.

In the context of the natural world, the snowflakes would be easy targets for predators or slower when moving towards food sources. Eventually, they would fall foul of natural selection as the spherical colonies would have become increasingly dominant in their environment. The results of the Minnesota experiment show something incredible: simple cells finding solutions to the problems of living as a multicellular body, creating levels of complexity and diversity in the process.

What is more, as the colonies had arisen through successive 'naturally selected' generations, Ratcliff and Travisano were able to show, through simple genetic sequencing, that the reason for the creation of colonies composed of conjoined cells was not due to the yeast utilizing a set of pre-existing genes (or 'latent multicellular genes' as the author described them) which existed for the purpose colonial survival in nature. Instead they had actually evolved the ability to reproduce via cell division, yet remain attached afterwards, in the lab.

Indeed, once the snowflake forms had evolved, almost none reverted to a single celled state. Many things could have resulted in this, but the researchers hypothesised that reason was a genetic mutation which interrupted the process of mitosis (the form of cell division which creates a new, identical cell), resulting in the conjoined colonies. What is important about this fact is that this could easily have occurred in nature as mutations are more likely to break a biological process rather than fix, enhance or create a completely new one.

While scientists have yet to artificially create multicellular organisms from unicellular ones, the Minnesota study has shown how the leap from one to many cells could have occurred in nature. Various other studies have revealed the environmental conditions around the time when multicellularity is thought to have evolved, conditions which would have created selective pressures similar to those in both Minnesota and Harvard's experiments. While fossil evidence has yet to be found, we can say that there is a clear pathway linking the microscopic world of the single cell with the giants which inhabit our planet today.


I would just like to apologize for the recent lack of posts. Hopefully the length of this one should make up for the gap somewhat. I have been rather busy with work which is only going to increase. I shall try to continue posting, but things are going to be hectic up until the end of July. Nevertheless, keep checking for  new posts. Thank you.