Evolutionary History^{0}

Kingdom: Fungi

Phylum: Ascomycota

Class: Saccharomycetes

Order: Saccharomycetales

Family: Saccharomycetaceae

Genus: Saccharomyces

Species: cerevisiae

Yeast are microscopic eukaryotic organisms belonging to the kingdom Fungi. Most yeast are single celled organisms, though occasionally a group of cells will clump together to form a semi-autonomous multicellular region known as a pseudohyphae. Yeast reproduce asexually either through mitosis or budding, and can vary in size anywhere from 3 micrometers to over 40 micrometers (1 micrometer = 10^-6 meters).[1] Though there are a number of yeast species, for the purposes of this project I will primarily be discussing Sachharomyces cerevisiae. “Saccharomyces means sugar mold or fungus and cerevisiae has its origin in the Gaelic word kerevigia and the old French word cervoise. Both these ancient words for cerevisae mean beer.”[2]

S. cerevisiae is one of the model organisms in eukaryotic biology, and was studied intensely to determine numerous biochemical and microbiological pathways. A number of favorable conditions for growth allowed the yeast to fit into the niche of ideal model organism, including short generation time (1-2 hour doubling time), easy mode of transformation (inserting or altering the genome), and relatively simple genome.[3] Since ancient times S. cerevisiae has been used in the processes of baking, brewing, and winemaking. Primarily through these three processes this microscopic organism has had an untold amount of influence on human society and culture.

Recent studies have found that the DNA of yeast cultured from wine jugs dated to 3150 BC “closely matched the sequence of modern yeast S. cervisiae.”[4] Because ancient bakers realized that some of the dough from one batch of bread could be used “as a starter for a new batch of dough, it is likely that, until recently, the yeast used in baking was evolving, with the participation of man, because starters were transferred sequentially from on baking to the next.”[5] As man traveled throughout the world, they (unknowingly) carried yeast with them, spreading out the organism all over the planet.

In 1857 the French scientist Louis Pasteur (1822 – 1895) showed that yeast were the organism responsible for fermentation, and that fermentation increased in the absence of oxygen (meaning yeast could perform anaerobic functions). In addition, he demonstrated that by using heat it was possible to kill the microorganisms in a broth and therefore stop biochemical processes (such as fermentation) that ruined certain foods. These crucial observations led to the invention of the practice of heat-treating liquids (particularly milk) in order to sterilize them (named pasteurization in his honor).[6] The flood of scientific investigation that came after Pasteur led to a massive amount of knowledge revolving around S. cerevisae, and spurred vast amount of artificial selection of different strains of yeast for numerous industrial and scientific purposes. The total number of unique strains of S. cerevisae is impossible to count, because new strains emerge every day in hundreds of applications. As an example, the National Collection of Yeast Cultures contains an estimated 4-5000 strains from beer brewing alone, with that number rising every day.[7]

http://buchler.phy.duke.edu/wp-content/uploads/2009/11/fungi-evol2.jpg