Chinese scientists used CRISPR to create a new species of yeast, according to recent research published in Nature. This marks the first time the controversial gene editing technology has been used to successfully create an artificial species with a single genome and demonstrates the technology’s potential for “large-scale genome engineering.”
Yeast are found in a number of foods you encounter on a day-to-day basis, including beer, bread, and yogurt. “Yeast” is a catch-all term for an incredibly diverse group of single-celled species that belong to one of two phyla, the Ascomycota and Basidiomycota. The yeast you would encounter in foods belong to the former phylum. For the last 20 million years or so, these yeast cells have all had 16 chromosomes.
The reason why some species have more chromosomes than another is a biological mystery. Humans, for example, have their genes arranged on 46 different chromosomes, whereas one species of fern has 1260 chromosomes. The only known eukaryotic organism (i.e., those with cells that have a nucleus enclosed in a membrane) that naturally has a single chromosome are male “jack jumper” ants.
In China, a team of researchers led by Zhogjun Qin, a microbiologist at the Key Laboratory of Synthetic Biology in Shanghai, wanted to see if it was possible to artificially reduce the number of yeast chromosomes to just one.
“The advantages to a eukaryotic cell of multiple chromosomes instead of a single chromosome is not clear,” the researchers wrote in their report. “In this study, we have reorganized the genome of Saccharomyces cerevisiae [the same yeast species used to brew beer] into one giant chromosome, in order to explore whether a yeast cell with an artificially fused single chromosome can survive and complete a sexual cycle.”
CRISPR uses an enzyme called Cas9 to excise small portions of DNA and introduce genetic changes at that location. The core element of the CRISPR system is a small piece of RNA that binds to a specific DNA sequence in a genome and the Cas9 enzyme. Once the RNA is bound to the DNA sequence, Cas9 cuts the DNA at the targeted location and the cell’s natural DNA repair mechanisms work to repair the DNA sequence.
To create a new species of yeast, Qin and his colleagues used CRISPR to splice the special bit of DNA that holds a chromosome together, known as a centromere, from the yeast chromosomes. Then the chromosomes were linked together randomly to form one long chain using the yeast’s natural genetic recombination mechanisms.
“Our pilot experiments showed that eight pairs of randomly selected chromosomes could all be successfully fused, and the resulting strains grew as robustly as the wild type strains, indicating that the yeast cells could tolerate random fusion of two chromosomes,” the researchers reported.
Although the artificially engineered yeast cells are able to reproduce fine with one another, their reproductive success is limited when paired with wild species. The researchers see their results as contributing to a better understanding of how evolution affects chromosome structure and function.