Test Tube Yeast Evolve Multicellularity

[Flatland]

The transition from single-celled to multicellular organisms was one of the most significant developments in the history of life on Earth. Without it, all living things would still be microscopic and simple; there would be no such thing as a plant or a brain or a human. How exactly multicellularity arose is still a mystery, but a new study, published January 16 in Proceedings of the National Academy of Sciences, found that it may have been quicker and easier than many scientists expected.

Test Tube Yeast Evolve Multicellularity

The evolution of multicellular organisms have always been poorly understoond. Recently, scientists managed to demonstrate the evolution of multicellularity in yeast cells through artificial selection.

 

[Ygggdrasil]

Here's a link to the actual paper describing the study: http://dx.doi.org/10.1073/pnas.1115323109 Note that the paper is open access and can be downloaded for free.

While the study is a nice application of experimental evolution to study a very interesting and important question in biology, one major flaw of the study is that yeast evolved from multicellular ancestors. Throughout the evolution of yeast, certain mutations to the genome likely broke some of the machinery required for multicellularity, allowing yeast to become unicellular again. Therefore, some of the changes that the researchers observe may not accurately reflect possible evolutionary pathways that unicellular organisms exploited to evolve multicellularity. Rather, the changes may simply be fixing the changes to the yeast genome that disabled the ancestral multicellularity genes. Performing the experimental evolution on an organism that did not evolve from a multicellular ancestor (perhaps some class of protist) may be more informative.

 

[Flatland]

I did not know that yeast evolved from a multicellular ancestor. What was this ancestor?

 

[Ygggdrasil]

The yeast used in the study, Saccharomyces cerevisiae, is a part of the phylum Ascomycota. Most species in this phylum have the ability to grow in filamentous multicellular structures called hyphae, which can also interconnect to form mycelium. In the order Saccharomycetales, which contains the unicellular yeasts like S. cerevisae, most of the species have lost the ability to organize into multicellular structures although some, like the yeast Candida albicans, can switch between growth in a unicellular or multicellular mode.

The issue of whether these yeasts are merely fixing ancestral genes that were inactivated by evolution probably will be resolved in follow up work, however. Sequencing the DNA of the evolved yeast should allow the researchers to determine the genes that were changed during the evolution experiment. No doubt that these experiments are currently underway.