Structural evidence for elastic tethers connecting separating chromosomes in crane-fly spermatocytes

Abstract

Different types of anaphase bridges are reported to form between segregating chromosomes during cell division. Previous studies using laser microsurgery suggested that elastic tethers connect the telomeres of separating anaphase chromosomes in many animal meiotic and mitotic cells. However, structural evidence is lacking for their existence. In this study, by correlating live imaging with electron tomography, we examined whether visible structures connect separating telomeres in meiosis I of crane-fly primary spermatocytes. We found structures extending between separating telomeres in all stages of anaphase. The structures consist of two components: one is darkly stained, looking somewhat like chromatin, whereas the other is more lightly stained, appearing filamentous. Although in early anaphase both structures extend between telomeres, in later anaphase, the darker structure extends shorter distances from the telomeres but the lighter structure still extends between the separating telomeres. From these observations, we deduced that these structures represent the “tethers” inferred from the laser-cutting experiments. Because elastic tethers have been detected in a variety of animal cells, they probably are present during anaphase in all animal cells.

Elastic tethers remain functional during anaphase arrest in partially-lysed crane-fly spermatocytes: a possible approach for studying mitotic tethers

“…That tethers are separate structural components connecting telomeres of separating anaphase chromosomes can be seen directly in electron microscope tomograms of crane-fly spermatocytes (Forer and Otsuka, 2023), tethers appearing as two component structures extending between separating partner telomeres.…”

Elastic mitotic tethers remain functional after microtubules are inhibited

“…This study investigates whether inhibiting microtubules during anaphase in crane-fly spermatocytes affects the backward forces on chromosomes produced by elastic mitotic tethers. Mitotic tethers, present in a broad range of animal cells (Forer et al, 2017), physically connect telomeres of each segregating pair of half-bivalents during anaphase in crane-fly primary spermatocytes (Forer and Otsuka, 2023). Each pair is connected in two of the four arms (LaFountain et al ., 2002; Sheykhani et al, 2017).…”

“…Such backward movement is due to elastic tethers and not microtubules because it occurs only when both partner telomeres are intact (LaFountain et al, 2002; Forer et al, 2021); occurs for only the arm fragment piece that has the kinetochore when the arm fragment itself is cut in half (LaFountain et al, 2002); and occurs when taxol stabilizes microtubule activity (Forer et al, 2018). The elastic tethers connecting separating telomeres have been identified electron microscopically (Forer and Otsuka, 2023) and are quite different from microtubules. The backward movement is not caused by ultra-fine DNA bridges because DNA-bridges are not elastic, are found only in a small number of anaphase chromosomes, mostly at the centromeric regions rather than at telomeres, and when present retard or stop anaphase segregation (Chan et al, 2007; Barefield and Karlseder, 2012; Gemble et al ., 2015; Su et al ., 2016).…”

Polymer Modeling Reveals Interplay between Physical Properties of Chromosomal DNA and the Size and Distribution of Condensin-Based Chromatin Loops

“…Genes 2023, 14, 2193 2 of 17 Tethering can be transient and soft, as observed between promoters/enhancers at transcriptional hubs and during temporary relocation and association with transcriptional loci at nuclear periphery [12][13][14][15][16]. Tethering of a chromatin fiber at two ends can put DNA under enough tension to impact loop extrusion by resisting loop-extrusion forces [17,18].…”