Friday Fungus: Schizosaccharomyces pombe

Picture from Dr. Stephen Kearsey
Fungi, Ascoymcota, Taprinomycotina (Archiascomycetes), Schizosaccharomycetes, Schizosaccharomycetales, Schizosaccharomycetaceae, Schizosaccharomyces, S.pombe

Schizasaccharomyces pombe is, as the picture above states, a fission yeast. This term actually describes a broad range of unicellular fungi that are usually rod shaped and divide by medial fission. At the time its genome was sequenced in 2002, it had the smallest number of protein coding genes (4,824) yet found in a eukaryote, although another well described yeast species, Saccharomyces cerevisiae, has around 6,000 genes for the sake of comparison. However, an ancient whole genome duplication has occurred in S. cerevisiae, but not in S. pombe. Another interesting difference between the two is that while S. cerevisiae is usually a diploid organism that has its genes arranged into 16 chromosomes, S. pombe is usually haploid and has only 3 chromosomes. This suggests, along with many other studies (1,2), that S. pombe diverged from the phylum Ascomycota a long, long time ago...it even predates the divergence of S. cerevisiae from the filamentous fungi, around 1,144 million years ago. Just to clarify, fungi and animals diverged around 965 years ago, meaning that the genetic distance between these two yeasts are about the same as the distance either is to humans (1,2). This is mentioned on Susan Forsburg's lab site, the queen of S. pombe research, along with a ton of other useful information about working with this organism.

Commercially, S.pombe is also used as a fermenter of traditional East African millet beer and, to a lesser extent, wine. In fact, its species epithet is the Swahili word for beer. However, reviews of S. pombe beer have not been stellar, as it appears to hold a strong sulfur taste. S. pombe genetic research on cell division has lead to advances in cancer research, as well as such disease as diabetes and cystic fibrosis. However, just because homologous genes exist between S. pombe and H. sapien, that doesn't mean these genes are being used in the same way. Forsburg warns:

...A final example of similar genes with different functions is provided by the conserved Ras family of GTPases. Ras proteins are found in both yeasts as well as in metazoa. However, while their biochemical activity is the same, their cellular function differs. In S. cerevisiae, RAS genes are part of the nutrient signaling pathway and at least one is required for viability. In S. pombe, a single ras gene functions in the pheromone-response pathway and affects cell morphology, but it is not essential. In mammals, Ras is a well-known transforming protein. These multiple uses of a single biochemical module warn us to be cautious in our extrapolation to larger eukaryotes.

S. pombe is a very ancient yeast species, and an evolutionary analysis of its glycosyl hydrolase family 28 members should be a useful addition to my thesis.