Lifestyles of the fungal and famous

Yeast is not an organism; it is a lifestyle. Most people are familiar with baker’s yeast, a unicellular species that makes bread rise, ferments alcoholic beverages, and is used as a simple system for understanding cellular biology. But there are actually over 1,500 known species of yeast – a term that simply means single-celled fungi. Baker’s yeast falls into an important and extensively studied group of yeasts known as “budding yeasts” but there is another group of key yeast species called “fission yeasts.” And they are more like us than you might think.

In a Science paper published online last week, researchers from the Broad and beyond described and compared the genomes of the four known species of fission yeast, offering up genomic tools and new avenues of investigation for researchers who use fission yeast to study chromosome abnormalities, cell division, replication, and more.

I wrote a news story about the researchers’ findings (you can check it out here) and got to talk to Nicholas Rhind, a cell biologist who uses fission yeast in his work; Aviv Regev, a core member of the Broad who works with both fission yeast and budding yeast; and Chad Nusbaum, who leads many fungal and microbial genome projects at the Broad. I was surprised to find out just how different fission yeast is from budding yeast, and wanted to share some of what I learned here on the blog.

What’s in a name: Although they share a common name, budding yeast and fission yeast are very distantly related and took different evolutionary paths to become yeast. Budding yeasts are so named because of the way that they reproduce, with a new cell growing out of its parent like a bud branching off from a tree. Fission yeast divides by simply growing longer and longer at both ends and then splitting down the middle to form two cells.

A quick look: Baker’s yeast is about five to ten micrometers in diameter and round in shape. Fission yeast is shaped like a rod – a few micrometers across and up to 14 micrometers in length. At the genomic level, these species look even more dissimilar. For example, fission yeast has only three chromosomes whereas Baker’s yeast has 14.

Distant relations: Researchers estimate that fission yeast and budding yeast diverged somewhere between 300 and 400 million years ago (that’s around the time when amphibians and reptiles first appeared on the planet). This means that fission yeast and budding yeast are about as closely related as a human being and lobe-finned fish.

Beer necessities: Despite their differences, both budding yeast and fission yeast can be used to make alcohol. In fact, Schizosaccharomyces pombe, the most well-known fission yeast species, gets its name from the Swahili word for beer, “pombe” (it was isolated in beer from Eastern Africa). Fission yeast and budding yeast are the only fungi that make ethanol although they evolved this ability separately. Budding yeast can also eat ethanol, but fission yeast cannot.

Fungal fan base: Hundreds of researchers use yeast to study basic but important cellular processes, many of which can go haywire in diseases like cancer. Yeast has been used to study DNA repair pathways, nutrient sensing, cell growth and division, chromosomal segregation, checkpoints for DNA replication, and much more.

Famous and infamous yeasts: In addition to the yeast species already described, there is also a group of yeasts called Candida, which can have more malevolent effects. Candida can cause infections (thrush or yeast infections) in humans and animals. Most of the time, this yeast lives in the mouth, vagina, or intestinal tract of its host without causing any harm, but when it grows unchecked, it can cause illness, especially in infants and people with compromised immune systems.

The pros and cons of simplicity: Baker’s yeast had its genome sequenced in 2002 and is one of the most popular systems for studying cellular biology, but it also has certain limitations. Budding yeasts in general have streamlined their genomes, getting rid of the parts they don’t need, including many pieces that are still important in humans and other animals. Fission yeast has retained more of these groups of genes. The simplicity of budding yeast has certain advantages but studying fission yeast in addition may help researchers develop a more nuanced understanding of biology’s complexities.