The Squatter
Every genome has its uninvited guests. Retrotransposons — "jumping genes" — are among the most prolific. They copy themselves and paste the copies elsewhere in the DNA, spreading not because they benefit their host but because they are very good at spreading. They are parasites, in the strict evolutionary sense: selfish sequences that persist because they can replicate faster than the genome can purge them. In humans, roughly half our DNA descends from transposable elements. For decades, much of it was dismissed as "junk."
Baker's yeast has a different reputation. Saccharomyces cerevisiae is one of the most studied organisms on Earth, and among its peculiarities is an unusually tidy genome. Its centromeres — the structures that latch onto the cell-division machinery and ensure each daughter cell gets the right chromosomes — are remarkably small and precise. Just a few hundred base pairs each, compared to the megabases of repetitive DNA that serve the same role in most other organisms. These "point centromeres" were the first centromeric sequences ever isolated, back in the early 1980s. How they came to be so minimal and exact has been an open question since.
A team led by Andrea Musacchio at the Max Planck Institute and Jef Boeke at NYU has now traced their origin. The answer was hiding in the yeast's closest relatives.
In species of the order Saccharomycodales — the sister group to baker's yeast — the researchers found centromeres that appear to be evolutionary halfway points. These "proto-point" centromeres share key features with the familiar point centromeres: a single specialised nucleosome, orthologous chromosomal positions, conserved protein machinery. But they are structurally looser, more variable, and embedded in DNA with a particular signature.
That signature belongs to LTR retrotransposons.
The centromeric DNA in these related species carries unmistakable fragments of retrotransposon sequences — the long terminal repeats that once flanked genes whose sole purpose was self-copying. In the ancestor of modern yeast, these parasitic elements had inserted themselves at or near centromeric regions. And instead of being expelled, they were co-opted. Their DNA was gradually reshaped — stripped down, refined, made precise — until it became the centromere itself.
The trespasser became the foundation.
This is not the first time biologists have caught evolution repurposing parasitic DNA. Our own immune system's ability to shuffle antibody genes descends from an ancient transposon insertion. But there is something striking about the centromere case. The centromere is not a peripheral feature. It is the mechanism that guarantees faithful inheritance — the structure without which cell division produces monsters. It is as close to sacred as molecular biology gets. And it was built from the remains of a genome's least welcome residents.
The paper's methodology is thorough: Hi-C chromosome-conformation maps, ChIP-seq for the centromeric histone variant Cse4, phylogenetic reconstruction across twelve yeast orders, synteny analysis confirming orthologous centromere positions. The proto-point centromeres are not speculation. They are biochemically validated intermediates — fossils caught in the act of transition.
What makes this work resonate beyond yeast biology is the conceptual inversion. We classify genomic elements by their allegiance: "host" DNA serves the organism, "selfish" DNA serves itself. The centromere story dissolves that boundary. The selfish element did not become altruistic. It was conscripted. Its very structure — the sequences it needed to jump — happened to provide the raw material that chromosome segregation could use. The parasite did not change its nature. The host changed the parasite's context.
This is a pattern that keeps surfacing. What we classified as junk turns out to be an archive of spare parts. What we classified as parasitic turns out to be raw material. The genome does not maintain a tidy distinction between self and invader, useful and useless, essential and expendable. It builds with whatever is at hand. And what is at hand, more often than not, is whatever forced its way in.
There is something almost architectural about it. A city does not demolish every ruin. Sometimes the old wall becomes the foundation of the new building. The squatter's shelter becomes the cornerstone. Not because anyone planned it, but because the material was there, and the material was strong enough.
The centromere — the guarantor of faithful inheritance, the structure on which all of eukaryotic cell division depends — was, in its earliest form, a parasite. It did not earn its place. It was already there, and what was already there was, in the end, enough.
Haase, M.A.B., Lazar-Stefanita, L., Baudry, L., Wudzinska, A., Zhou, X., Rokas, A., Hittinger, C.T., Pfander, B., Musacchio, A. & Boeke, J.D. (2026). "Ancient co-option of LTR retrotransposons as yeast centromeres." Nature. DOI: 10.1038/s41586-025-10092-0