Highly read on www.plosgenetics.org 22 Jul–21 Aug

Tracing 150 million years of evolution in the yeast genome has revealed how the microorganism's chromosome numbers have waxed and waned over time.

Genomic upheaval hit about 100 million years ago, when the genome of a yeast ancestor duplicated, doubling the number of chromosomes from 8 to 16. Today, the chromosome count in various species ranges from 6 to 16.

Jonathan Gordon and his colleagues at Trinity College Dublin in Ireland studied several yeast species — some of which arose before the genome duplication event and others after — that have different numbers of chromosomes and last shared a common ancestor about 150 million years ago. The authors focused on two important structures: telomeres, which cap the ends of chromosomes, and centromeres, which help to direct chromosomes into dividing cells. They found that, in most cases, yeast species downsizing their complement of chromosomes did so by fusing the telomeres of two chromosomes and losing one of the centromeres.

PLoS Genet. 7, e1002190 (2011)