Elastic Tethers Between Separating Anaphase Chromosomes Regulate the Poleward Speeds of the Attached Chromosomes in Crane-Fly Spermatocytes

Abstract

Elastic “tethers” connect separating anaphase chromosomes in most (or all) animal cells. We tested whether tethers are involved in coordinating movements of separating anaphase chromosomes in crane-fly spermatocytes. In these cells the coupled movements of separating chromosomes become uncoupled after the tethers are severed by laser microbeam irradiation of the interzone region between the chromosomes ( Sheykhani et al., 2017 ). While this strongly suggests that tethers are involved with coordinating the poleward chromosome movements, the experiments are open to another interpretation: laser irradiations that cut the tethers also might damage something else in the interzone, and those non-tether components might regulate chromosome movements. In the experiments reported herein we distinguish between those two possibilities by disabling the tethers without cutting the interzone. We cut the arms from individual chromosomes, thereby severing the mechanical connection between separating chromosomes, disconnecting them, without damaging components in the interzone. Disabling tethers in this way uncoupled the movements of the separating chromosomes. We thus conclude that tethers are involved in regulating the speeds of separating anaphase chromosomes in crane-fly spermatocytes.

Blocking Protein Phosphatase 1 [PP1] Prevents Loss of Tether Elasticity in Anaphase Crane-Fly Spermatocytes

“…Cutting arms from chromosomes removes the mechanical connection between separating partners, thereby disabling the tethers without cutting the interzone (Forer and Berns, 2020 ). Cutting arms when chromosomes move backward eliminates the possibility that backwards movements were stopped because of collateral damage to something other than tethers.…”

“…When kinetochore microtubules are severed by laser cuts, both associated chromosomes keep moving at the same speed as prior to the laser cuts. The movements are uncoupled after tethers are severed and are uncoupled when tethers are disabled by cutting arms (Sheykhani et al, 2017 ; Forer et al, 2018 ; Forer and Berns, 2020 ): after severing or disabling tethers and then cutting individual kinetochore fibers, the chromosome associated with the severed microtubules accelerates while the partner moves with unchanged speed. These data show that coordination between separating partner chromosomes requires tethers, the coordination presumably arising because of the tension from the tethers on the arms of the separating chromosomes.…”

“…Using freeware Irfanview , digital images obtained throughout the experiments (recorded every 2–4 s) were trimmed, time stamped using the digital information in each file, and converted into bmp images. The images were compiled into movies using freeware VirtualDub2 , distances were measured using an in-house program (Wong and Forer, 2003 ), and movement graphs were obtained using the program SlideWrite , as described in detail in Ferraro-Gideon et al ( 2013 ) and Forer and Berns ( 2020 ). Portions of the cell were irradiated using a femtosecond laser in a microbeam apparatus described in Shi et al ( 2012 ); Harsono et al ( 2013 ), and Berns ( 2020 ).…”

“…Tethers are probably present in plant cell spindles, too, since stained connections between separating telomeres are also seen in plant cells (Paliulis and Forer, 2018 ), e.g., in cells from Oenothera (Figure 19 in Cleland, 1926 ) and Haemanthus (Figures 8C,D in Bajer and Molè-Bajer, 1986 ). A range of evidence (discussed, for example, in Forer et al, 2017 ; Forer and Berns, 2020 ) shows that the movements of arm fragments are not microtubule-based and require direct physical connections between separating telomeres.…”

Genetic Material Manipulation and Modification by Optical Trapping and Nanosurgery-A Perspective

“…This kind of studies will keep providing critical information on mitosis kinetics, regulation and consequences of chromosomal dysfunctionality (Kong et al, 2017;Milas et al, 2018;Odell et al, 2019). For example, a very recent publication reports on a very fine level of LS disruption of particular mitotic machinery elements in order to better understand mitotic chromosome migration (Forer and Berns, 2020). The LS were employed to first disrupt chromosomal arms, involved in inter-chromosomic tethers connecting oppositely migrating homologous chromosomes, and then damage the kinetochore spindle fibers on individual chromosomes.…”