Wednesday, 18 January 2012

Another Step Taken Down The Evolutionary Path To Multicellularity

A colony of yeast cells (Saccharomyces cerevisiae) under a microscope
Yeast (Saccharomyces cerevisiae) is one of the quiet agents of the world. Its anaerobic respiration produces carbon dioxide and ethanol from sugars, and as a result, it is used in a variety of processes, from making bread rise to making beer alcoholic. Yet, its use is not just based around food industries. Recently, studies of yeast have given scientists insight into the evolution of multicellularity. Some have shown the advantages to becoming multicellular, while others have shown how such a process may have occurred.

Many bacterial fossils exist as large colonies held together by sediment and slime. These colonies can grow to the size of a football. They are composed of multiple cells and are macroscopic. Yet they are not considered to be multicellular because the cells act as individuals rather than as a single functioning body. The first true multicellular organisms appear around 800 million years ago. So what triggered the change from colony to a single being?

This is where yeast comes in. Scientists at the University of Minnesota's College of Biological Sciences, led by Dr Will Ratcliff. Ratcliff and his associates thought that bridging the famous multicellular gap would be 'just about the coolest thing we could do.' Using a model based around yeast. Ratcliff and his co-worker, Professor Michael Traversano, took samples of brewer's yeast and placed them in a nutrient rich environment.

The Francevillian Group Fossils are considered the oldest
remains of multicellular animals of Earth at 2.1 billion years old.
After a few days, they placed the contents in a centrifuge to separate the heavier contents of the samples from the lighter ones. When they removed the contents, they found that the cells had grouped together into little clusters.

Yeast cells break down complex sugars such as starch by excreting enzymes. The complex sugars are broken down into simpler ones which are absorbed through the cell membrane.

Researchers at Harvard University found that yeast cells, when placed in a sugar low environment, grouped together to accelerate the rate of decomposition and the rate of absorption due to a higher concentration of the simple molecules. They concluded that a similar process occured billions of years ago which led to the rise of multicellularity.

This new study showed something else. These were not random balls of individual cells but were related to each other and had remained attached during cell division. Their genetic similarity meant that they were able to live attached with some degree of cooperation. When the cluster reached a critical size, old cells committed a form of biological suicide known as apoptosis, allowing younger, healthier cells to absorb enough sugar to stay alive, leading to a strong and healthy community that is, at the very least, partially multicellular.

Evolutionary biologists believe that multicellularity evolved at least 25 different times in nature. Becoming multicellular requires the presence of free oxygen in the atmosphere. This allows cells to manufacture collagen molecules, which bind them together into one body. Free oxygen appeared in the atmosphere at least 2.4 billion years ago. This would have given small single celled organisms similar to yeast 1.6 billion years to become a cooperative community and eventually a single organism. 'Our multicellular yeast are a valuable resource for investigating a wide variety of medically and biologically important topics' Traversano said.

'Cancer was recently described as a fossil from the origin of multicellularity, which can be directly investigated with the yeast system. Similarly the origins of aging, development, and the evolution of complex morphologies are open to direct experimental investigation that would otherwise be difficult or impossible.' Ratcliff and Traversano have stated that their next research project will look at the role of multicellularity in cancer and other areas of biology.