Two research articles published on August 1 in Nature, a scientific journal, have demonstrated that Baker’s yeast can survive after a total restructuring, not of the genes themselves but of the chromosomes, the thread-like structures that carry our genetic information to make up the genome. The feat was achieved using CRISPR-Cas9 gene-editing technology, which allowed both groups to effectively pack all of the yeast genome, usually spread across 16 separate chromosomes, into just one or two ― ‘minimized’ yeast.
The most remarkable aspect of the study is that drastic restructuring of the chromosomes seems to have no effect on the yeast. Jef Boeke, a geneticist at New York University whose team carried out one of the studies told Nature, “That was the biggest shocker — that you can just get away with this and yeast seem to shrug its shoulders.”
The team led by Boeke managed to create a strain with only two extra long chromosomes by cutting out non-essential telomeres, ends of the chromosome that naturally degrade over time, as well as certain centromeres, sequences in the middle of the genome involved in DNA replication; thereby progressively reducing the number the number of chromosomes (1). However, they were unable to achieve fusion of the final pair. The complementary study, led by Prof Zhongjun Quin, also removed redundant telomeres and centromeres, and then the chromosomes were fused one by one in succession until they were left with only one, thus creating the single-chromosome strain (2).
The only observable difference between the ‘minimized’ yeast strains and the normal strain was in sexual reproduction. Offspring of yeast cells with two genome copies had only one and the ability of the next generation to produce viable spores dropped significantly. Quin’s team noted that single-chromosome strains produced through mating grew more slowly and fewer spores were produced.
Like humans, yeast is made up of eukaryotic cells, which possess a clearly defined nucleus in which the chromosomes are located. Interestingly, the number of chromosomes housed in the nucleus varies quite dramatically between species and seems to have no bearing on the length or complexity of the genome. It’s still unclear why some organisms split their DNA across several chromosomes.
Until now, the focus of genetic research in relation to creating new species has been on producing DNA-sequence changes, but according to Dr Gianni Liti, a geneticist at the University of Cote d’Azur in Nice, France, these studies suggest that natural chromosome fusions could also play an important role in creating new organisms. Moreover, Boake suggests the accompanying reproductive isolation may be useful as it would prevent synthetic yeast released into the environment from mating with wild strains.
(1) Luo, J., Sun, X., Cormack, B. P. & Boeke, J. D. Nature (2018) DOI: 10.1038/s41586-018-0374-x
(2) Shao, Y. et al. Nature (2018). DOI:10.1038/s41586-018-0382-x