Scientists find evolutionary origin of key cellular structure in yeast

Scientists discover the evolutionary origin of a key cellular structure

Researchers have identified the evolutionary missing link that explains how a fundamental cellular structure, the centromere, transformed from a large, complex form into a compact, genetic one. The discovery, made by studying yeast, resolves a long-standing biological puzzle about the origins of so-called "point" centromeres.

Centromeres are essential for accurate chromosome segregation during cell division. Yet, their DNA architecture varies wildly across different organisms, making their evolutionary history difficult to trace.

The puzzle of the brewer's yeast

The common brewer's yeast, Saccharomyces cerevisiae, is a prime example of this mystery. Its centromeres are compact, genetically defined points, a stark shift from the large, repeat-rich, and epigenetically controlled centromeres of its ancestors.

How this dramatic transition occurred has been unresolved for decades. The new research, published in Nature, provides the answer by identifying intermediate "proto-point" centromeres in related yeast species.

Proto-point centromeres bridge the evolutionary gap

These proto-point centromeres contain a single centromeric nucleosome positioned over an AT-rich DNA core. They show a relaxed organization and greater sequence variability in their flanking regulatory elements compared to established point centromeres.

Critically, in two species, these proto-point centromeres are located within clusters of retrotransposon-derived repeats. This directly links ancestral repeat-rich centromeres to modern genetically encoded ones.

The key characteristics of the discovered proto-point centromeres include:

• A single centromeric nucleosome over an AT-rich core sequence.
• Relaxed organization of flanking cis-regulatory elements.
• Location within retrotransposon-derived repeat clusters in some species.

Selfish genetic elements co-opted for essential function

Comparative and phylogenetic analyses indicate that both proto-point and point centromeres evolved from an ancestor with retrotransposon-rich centromeres. The study pinpoints a specific family of retrotransposons, Ty5 sequences, as the genetic substrate for this evolution.

This provides a clear mechanistic route for how an epigenetic centromere—one defined by protein markers—can become a genetically specified one, hardwired into the DNA sequence.

More broadly, the findings demonstrate how "selfish" genetic elements, like retrotransposons, can be co-opted by the genome to perform essential chromosomal functions. The research resolves a fundamental question in chromosome biology and reveals a surprising evolutionary pathway for a critical cellular structure.