Centromeres are regions of DNA where the cell's machinery grabs each chromosome and pulls it into the two daughter cells. Centromeres are essential for accurate chromosome segregation in every dividing cell, from yeast to humans. Although the machinery that governs chromosome segregation is deeply conserved, centromeric DNA evolves rapidly – a phenomenon known as the "centromere paradox". A particular striking example of this rapid evolution is the "point" centromere in yeast. In their new study, the teams from the MPI and NYU provide the first mechanistic route explaining the transition of the yeast centromere and identified its genetic origin.
First author Max Haase explains the new findings in detail in the following interview.
What is the discovery you made?
Our paper explains how a very important chromosome feature - the centromere - in brewer's yeast came to be. In yeast they are extremely small and precise - a striking oddity in the tree of life that has puzzled chromosome biologists for decades. In this work, we show a likely intermediate stage in their evolution and trace where the DNA for these special centromeres originally came from.
Why is it so exciting?
We found previously unknown centromeres in related yeast species that look like halfway stages between large, repeat-rich centromeres and the tiny ones in brewer's yeast. The DNA at these centromeres is related to a class of "jumping genes" (mobile pieces of DNA) called retrotransposons, suggesting that these elements provided the raw material that evolution reshaped into modern yeast centromeres. This gives a concrete genetic explanation for how yeast ended up with this unusual centromere type.
Why are your findings important for the scientific community?
Yeast centromeres were the first centromeres whose functional DNA sequence was isolated and worked out in detail, beginning with work by Clarke and Carbon in the early 1980s, yet it has remained a mystery how such tiny, precisely defined centromeres could have evolved. By showing how one kind of centromere can be rebuilt from another, our work addresses this long-standing question and shows how bits of "selfish" or parasitic DNA can be tamed and turned into DNA that cells now rely on to organize their chromosomes. This provides a concrete example of how a core part of the chromosome can be completely restructured over evolution by repurposing DNA that once looked like genomic "junk".
What are the next steps you will take?
Next, we want to understand how the kinetochore—the protein machinery that recognizes centromeres—can accommodate such dramatic changes in centromere DNA over evolutionary time. As part of this, we are tackling the open question of how centromeres assemble the kinetochore. We are also looking for additional cases where transposons have been re-used to build chromosome structures like centromeres, to see how common this kind of genome innovation is.
