Surface structures consisting of chromatin fibers in isolated barley (Hordeum vulgare) chromosomes revealed by helium ion microscopy

Helical metaphase chromatid coiling is conserved

“…Using super-resolution microscopy, we detected thinner, straight chromatin fibers at the monocentromeres and secondary constrictions of barley lacking the helical chromonema organization. This finding is in line with the observations achieved by scanning electron, helium ion, and classical light microscopy in barley (Seto 1972; Iwano et al 1997; Wanner et al 1991; Wanner and Formanek, 2000; Schroeder-Reiter and Wanner 2009; Schroeder-Reiter et al 2012; Sartsanga et al 2021), faba bean (Maruyama 1983; Schroeder-Reiter and Wanner 2009), frog (Seto 1972), Indian muntjac (Rattner 1987), wallaby, and human (Utsumi 1982; Schroeder-Reiter and Wanner 2009). The 200–300 nm chromonema of mouse, consisting of a coiled 100 nm sub-fiber, appears straight in the centromere region (Rattner and Lin 1985, 1987).…”

“…At the monocentromeres and secondary constrictions of barley lacking the helical chromonema organization, we detected via super-resolution microscopy thinner straight chromatin fibers. This finding is in line with the observations achieved by scanning electron, helium ion, and classical light microscopy in barley (Seto, 1972; Iwano et al, 1997; Wanner et al, 1991; Wanner and Formanek, 2000; Schroeder-Reiter and Wanner, 2009; Schroeder-Reiter et al, 2012; Sartsanga et al, 2021), faba bean (Maruyama, 1983; Schroeder-Reiter and Wanner, 2009), animals, such as frog (Seto, 1972), Indian muntiac (Rattner, 1987), wallaby, and also in human (Utsumi, 1982; Schroeder-Reiter and Wanner, 2009). The 200-300 nm chromonema of mouse, consisting of a coiled 100 nm sub-fiber appears straight in the centromere region (Rattner and Lin, 1985, 1987).…”

“…Several studies showed that in vivo chromatin is basically organized in 10 nm fibers and not 30 nm fibers as found after fixation (reviewed in Maeshima et al, 2019). We measured a mean lower-order chromatin fiber diameter of ∼80 nm in DAPI labeled barley metaphase chromosomes while scanning electron microscopy yielded 15-150 nm (Wanner et al, 1991), helium ion microscopy (with improved chromosome preparations preventing artifacts) 11.6 nm (Sartsanga et al, 2021), and human metaphase chromosomes contain reticulate chromatin fibers with a diameter of up to 30 nm (Wako et al, 2020). Due to the visualization of fluorescence signals via a point spread function the signal thickness measurements, such as that from DAPI, may cause, depending on the settings, an overestimation.…”

“…The intermingling of lower-order chromatin fibers between adjacent turns of the chromonema visualized by oligo-FISH in barley explains this observation. Super-resolution microscopy revealed a network of globular (clustered) and fibrous chromatin structures within and at the surface of barley metaphase chromatids, similar to the fibers and 'knobby' structures, possibly corresponding to chromomeres (coiled solenoids) (Serra, 1947;Zatsepina et al, 1983) observed by scanning electron microscopy at the surface of barley (Wanner et al, 1991;Iwano et al, 1997;Wanner and Formanek, 2000;Sartsanga et al, 2021), Chinese hamster (Zatsepina et al, 1983), and human chromosomes (Sumner, 1991). Because our Hi-C contact matrices from barley inter-and metaphase chromosomes, analyzed at 1Mb resolution, do not show TADs or compartments they could not be related to the observed globular structures.…”

“…No reuse allowed without permission. microscopy yielded 15-150 nm (Wanner et al, 1991), helium ion microscopy (with improved chromosome preparations preventing artifacts) 11.6 nm (Sartsanga et al, 2021), and human metaphase chromosomes contain reticulate chromatin fibers with a diameter of up to 30 nm (Wako et al, 2020). Due to the visualization of fluorescence signals via a point spread function the signal thickness measurements, such as that from DAPI, may cause, depending on the settings, an overestimation.…”