Genetics: Combining yeast chromosomes comes without critically compromising cell fitness

New yeast strains have been created that have only 1 or 2 chromosomes - rather than the customary 16 - reports a pair of papers published online this week in Nature.

The genomes of eukaryotes are split into chromosomes, the number of which varies between species. Humans have 23 chromosome pairs, for example, whereas our ape cousins have 24 - and the male jack jumper ant has only one. These differences probably result from accidental fusions or genome duplication events, but it has been unclear what advantages multiple chromosomes might confer, and how tolerant species can be to variations in the total number.

The authors of both papers used the CRISPR-Cas9 technique to edit the genome of the budding yeast Saccharomyces cerevisiae, generating series of yeast strains with progressively fewer chromosomes. Zhongjun Qin and colleagues created a strain with all the genetic information fused into a single chromosome, while Jef Boeke and colleagues independently produced a strain with two chromosomes.

Although fusion considerably alters three-dimensional chromosomal structure, the new strains contain genetic materials that - other than the deletion of a few non-essential genes - are identical to normal S. cerevisiae. The modified yeast cells proved unexpectedly robust, with no major growth defects when cultivated under varied conditions. The fused-chromosome strains, however, did exhibit small fitness limitations and defects in sexual reproduction, such that they could be quickly out-competed by their unmodified counterparts. These findings may begin to explain the advantages of having more chromosomes.