Centromeres serve the identical basic purpose in just about all types of life. These regions of DNA make sure that chromosomes separate properly when cells divide. Despite this shared role, centromeres vary dramatically of their structure. Some organisms have large stretches of repetitive DNA, while yeast use much smaller and simpler versions called “pointed” centromeres. This amazing diversity, combined with the proven fact that centromeres evolve rapidly, has puzzled scientists for many years.
A research team led by Andrea Moschio, director of the Max Planck Institute of Molecular Physiology in Dortmund, together with Jeff Boecke of the NYU Grossman School of Medicine, has now unraveled the origin and evolutionary history of yeast centromeres. The scientists identified what they describe because the “proto-point” centromere, an intermediate form that connects today's tiny yeast centromeres to their more complex ancestors. These older versions contained fragments of parasitic DNA. This discovery highlights one of the dramatic examples of evolutionary change on the DNA level.
The centromere paradox
Centromeres are specific locations on chromosomes where the cellular machinery attaches during cell division. This machinery separates each chromosome in order that the 2 recent daughter cells receive the right genetic material. Because of this role, centromeres are essential for proper chromosome segregation in all dividing cells, from yeast to humans.
Although the cellular machinery liable for chromosome segregation has been highly conserved throughout evolution, the DNA found at centromeres changes surprisingly rapidly. Scientists call this puzzling pattern the “centromere paradox.” Yeast provides one of the striking examples of this phenomenon because its centromeres are unusually small and thoroughly defined. In the brand new study, researchers from the Max Planck Institute and NYU have uncovered the primary mechanistic explanation for a way these specific yeast centromeres evolved and pinpointed their genetic origins.
A key discovery in yeast evolution
First creator Max Haas explains the brand new findings intimately in the next interview.
What have you ever discovered?
Our paper explains how a vital feature of the chromosome – the centromere – got here into being in brewer's yeast. In yeast they're extremely small and precise—a striking oddity within the tree of life that has puzzled chromosome biologists for many years. In this work, we show a possible intermediate step of their evolution and trace where the DNA for these specialized centromeres originally got here from.
Why is it so exciting?
We found previously unknown centromeres in related yeast species that look like halfway between the big, repeat-rich centromeres and the small ones in brewer's yeast. The DNA in these centromeres belongs to a category of “jumping genes” (mobile pieces of DNA) called retrotransposons, suggesting that these elements provided the raw material that evolved into modern yeast centromeres. This provides a solid genetic explanation for a way yeast ended up with this unusual centromere type.
Why are your findings necessary to the scientific community?
Yeast centromeres were the primary centromeres whose functional DNA sequences were isolated and characterised, starting with the work of Clark and Carbone within the early Eighties, yet it stays a mystery how such small, well-defined centromeres could have evolved. By showing how one form of centromere will be remodeled from one other, our work solves this long-standing query and shows how bits of “selfish” or parasitic DNA will be tamed and became the DNA that cells now depend on to prepare their chromosomes. This provides a concrete example of how the core of a chromosome will be completely reshaped over evolution by recombining DNA that when looked like genomic “junk”.
What are your next steps?
Next, we wanted to grasp how the kinetochore—the protein machinery that recognizes centromeres—could accommodate such dramatic changes in centromere DNA over evolutionary time. As a part of this, we're tackling the open query of how centromeres assemble the kinetochore. We are also searching for additional cases where transposons have been reused to construct chromosome structures resembling centromeres, to see how common such a genome innovation is.