Helical chromonema coiling is conserved in eukaryotes

Abstract

SUMMARYEfficient chromatin condensation is required to transport chromosomes during mitosis and meiosis, forming daughter cells. While it is well accepted that these processes follow fundamental rules, there has been a controversial debate for more than 140 years on whether the higher‐order chromatin organization in chromosomes is evolutionarily conserved. Here, we summarize historical and recent investigations based on classical and modern methods. In particular, classical light microscopy observations based on living, fixed, and treated chromosomes covering a wide range of plant and animal species, and even in single‐cell eukaryotes suggest that the chromatids of large chromosomes are formed by a coiled chromatin thread, named the chromonema. More recently, these findings were confirmed by electron and super‐resolution microscopy, oligo‐FISH, molecular interaction data, and polymer simulation. Altogether, we describe common and divergent features of coiled chromonemata in different species. We hypothesize that chromonema coiling in large chromosomes is a fundamental feature established early during the evolution of eukaryotes to handle increasing genome sizes.

Hypothesis: The opposing pulling forces exerted by spindle microtubules can cause sliding of chromatin layers and facilitate sister chromatid resolution

“…A more recent version of the radial-loop model is based on results obtained using genomewide chromosome conformation capture (Hi-C) techniques combined with polymer simulations of chromatin fibers (Lieberman-Aiden et al, 2009), which led to the proposal that mitotic chromatin forms nested loops of ~0.5 Mb (consisting of ~400-kb outer loops and ~80-kb inner loops) mediated by condensin (Gibcus et al, 2018). Furthermore, according to early microscopic observations (Ohnuki, 1965;Rattner and Lin, 1985;Boy de la Tour and Laemmli, 1988;Câmara et al, 2023), these authors suggested that the loops are organized forming a helical array. Other recent reports based on Hi-C analysis of different species (Schloissnig et al, 2021;Kuvalová et al, 2023) also consider that chromatin in chromosomes is helically folded.…”

Advances in plant chromatin

“…Amanda S. Camara, Ivona Kubalova and Veit Schubert bring a comprehensive overview of chromatin organization, focusing on how coiled chromatin thread, called chromonema, assembles into chromatids during mitosis (Câmara et al., 2023). Starting with the lower‐order structure, they stress that a 30‐nm fibre is unlikely to form a structural unit, as revealed by electron microscopy.…”