Centromeres serve the same basic purpose in almost all living organisms. These regions of DNA ensure that chromosomes are properly separated during cell division. Despite this common role, the structure of centromeres varies widely. Some organisms have long stretches of repetitive DNA, but yeast use much smaller and simpler versions known as “point” centromeres. This amazing diversity, combined with the fact that centromeres evolve rapidly, has puzzled scientists for decades.
A research team led by Andrea Musacchio, director of the Max Planck Institute for Molecular Physiology in Dortmund, along with Jeff Boeke of New York University’s Grossmann School of Medicine, has uncovered the origin and evolutionary history of yeast centromeres. Scientists have identified what they call “protopoint” centromeres. This is an intermediate form that links today’s small yeast centromeres with their more complex ancestors. These earlier versions contained pieces of parasitic DNA. This discovery highlights one of the most dramatic examples of evolutionary change at the DNA level.
centromere paradox
Centromeres are specific sites on chromosomes to which cellular machinery attaches during cell division. This machine separates each chromosome so that the two new daughter cells receive the correct genetic material. Because of this role, centromeres are essential for accurate chromosome segregation in all dividing cells, from yeast to humans.
Although the cellular machinery responsible for chromosome segregation has remained highly conserved throughout evolution, the DNA at centromeres changes surprisingly quickly. Scientists call this puzzling pattern the “centromere paradox.” Yeast provides one of the most striking examples of this phenomenon, as its centromeres are unusually small and precisely defined. In a new study, researchers at the Max Planck Institute and New York University have uncovered the first mechanistic explanation of how these distinctive yeast centromeres evolved and identified their genetic origins.
Important discoveries in yeast evolution
Lead author Max Haase details the new findings in the following interview.
What discovery did you make?
Our paper explains how centromeres, a very important chromosomal feature in brewer’s yeast, arose. In yeast they are very small and precise. A surprising oddity in the tree of life has puzzled chromosome biologists for decades. This study shows a likely intermediate stage in their evolution and traces where the DNA of these special centromeres originally came from.
Why so much excitement?
We discovered previously unknown centromeres in related yeast species. This centromere appears to be intermediate between the large repeat-rich centromeres and the small centromeres of brewer’s yeast. The DNA in these centromeres is associated with a type of “jump gene” (a mobile piece of DNA) called a retrotransposon, suggesting that these elements provided the raw material for evolution to reshape modern yeast centromeres. This provides a genetically specific explanation for how yeast acquired this unusual centromere type.
Why is your discovery important to the scientific community?
Yeast centromeres were the first centromeres to have functional DNA sequences isolated and characterized in detail, starting with the work of Clarke and Carbon in the early 1980s, but how such small, precisely defined centromeres evolved remained a mystery. By showing how one type of centromere is reassembled from another, our research addresses this long-standing question and shows how “selfish” or parasitic pieces of DNA can be tamed and turned into the DNA that cells now rely on to make up their chromosomes. This provides a concrete example of how the central parts of chromosomes can be completely rebuilt during evolution by reusing DNA that once appeared to be the “junk” of the genome.
What’s the next step?
Next, we want to understand how kinetochores, the protein machinery that recognizes centromeres, are able to accommodate such dramatic changes in centromeric DNA over the course of evolution. As part of this, we are addressing the open question of how centromeres assemble kinetochores. We are also looking for additional cases where transposons are being reused to build chromosome structures like centromeres to see how common this type of genomic innovation is.
