Holocentric chromosomes have evolved independently from X-shaped monocentric chromosomes multiple times in both animals and plants, but the mechanism behind the centromere-type transition is unknown. The international research team assembled the chromosome-scale reference genome and analyzed the holocentromere organization of the lilioid Chionographis japonica.
Remarkably, the holocentromere consists of only 7 to 11 evenly spaced megabase-sized centromere units from telomere to telomere. The size of single centromere units in this plant species is comparable to those in monocentric species and is ~200-fold larger than those of other holocentric plants. “Such a small number of centromere units, but so large, has not yet been demonstrated in any animal or plant organism”, says Dr. Yi-Tzu Kuo, the first author of this study.
The evenly spaced centromere units might be a prerequisite for forming cylindrically-shaped metaphase chromosomes with line-like sister holocentromeres facing opposite poles. During mitotic chromosome condensation, looping and folding of chromatin bring the megabase-sized centromere units along the chromatid close to each other into a line-like holocentromere to function like a single centromere. “This makes the chromosome more stable and robust, because otherwise, it would be torn apart during cell division”, explains Prof. Dr. Andreas Houben, head of IPK`s research group “Chromosome Structure and Function”.
Unlike all the known holocentric genomes possessing uniformly mixed eu- and heterochromatin, in C. japonica , both epigenetically defined chromatin types are organized into distinct domains like in many monocentric species. Gene-active and gene-inactive areas are thus spatially separated from each other.
“The study broadens our knowledge about centromere plasticity and diversity, and also demonstrates the unique value of exploring non-model species for evolutionary comparison to reveal novelties in even well-studied structures like the centromere”, says Dr. Yi-Tzu Kuo.
Nature Communications
Holocentromeres can consist of merely a few megabase-sized satellite arrays
13-Jun-2023