Chromosomes selectively detach at one pole and quickly move towards the opposite pole when kinetochore microtubules are depolymerized in Mesostoma ehrenbergii spermatocytes

Sheykhani

2018

Front. Cell Dev. Biol.

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Various experiments have indicated that anaphase chromosomes continue to move after their kinetochore microtubules are severed. The chromosomes move poleward at an accelerated rate after the microtubules are cut but they slow down 1–3 min later and move poleward at near the original speed. There are two published interpretations of chromosome movements with severed kinetochore microtubules. One interpretation is that dynein relocates to the severed microtubule ends and propels them poleward by pushing against non-kinetochore microtubules. The other interpretation is that components of a putative “spindle matrix” normally push kinetochore microtubules poleward and continue to do so after the microtubules are severed from the pole. In this study we distinguish between these interpretations by treating cells with taxol. Taxol eliminates microtubule dynamics, alters spindle microtubule arrangements, and inhibits dynein motor activity in vivo. If the dynein interpretation is correct, taxol should interfere with chromosome movements after kinetochore microtubules are severed because it alters the arrangements of spindle microtubules and because it blocks dynein activity. If the “spindle matrix” interpretation is correct, on the other hand, taxol should not interfere with the accelerated movements. Our results support the spindle matrix interpretation: anaphase chromosomes in taxol-treated crane-fly spermatocytes accelerated after their kinetochore microtubules were severed.

Section: Discussionsupporting

confidence: 85%

Anaphase Chromosomes in Crane-Fly Spermatocytes Treated With Taxol (Paclitaxel) Accelerate When Their Kinetochore Microtubules Are Cut: Evidence for Spindle Matrix Involvement With Spindle Forces

Sheykhani

2018

Front. Cell Dev. Biol.

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“…This model is silent on why dynein relocates to the tips of severed kinetochore microtubules, but it predicts that the movement slows down when the stub encounters an obstacle ( Spurck et al, 1997 ). The different interpretations remain unresolved, but since chromosomes in Mesostoma spermatocytes move poleward very rapidly (up to 200 μm/min) in the absence of kinetochore microtubules ( Fegaras and Forer, 2018 ) and since chromosomes in crane-fly spermatocytes with severed kinetochore microtubules (and severed tethers) accelerate even in the presence of taxol ( Forer et al, 2018 ), it seems likely that forces external to spindle microtubules produce forces for movement. Regardless of which models give correct representations of the poleward forces on chromosomes, models of mitotic spindle forces as a whole must include forces from tethers, forces that act on chromosome arms in the opposite direction to those that propel anaphase chromosomes poleward.…”

Section: Discussionmentioning

confidence: 99%

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

2020

Front. Mol. Biosci.

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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.

“…However, since the chromosomes stretch out before detaching, there is tension in the chromosomes from the stretch. Thus the movements of detached kinetochores conceivably might be due to recoil from the tension, though we argued against this possibility ( Fegaras and Forer, 2018a ). In this article we test whether recoil causes the movements.…”

mentioning

confidence: 87%

“…When microtubules are depolymerized in Mesostoma spermatocytes, the bivalents stop mid-oscillation and the sister kinetochores of each half-bivalent move toward their respective poles ( Fegaras and Forer, 2018a ). This causes the bivalents to stretch, possibly due to non-microtubules associated force production via the spindle matrix and/or actin-myosin.…”

Section: Introductionmentioning

confidence: 99%

“…This causes the bivalents to stretch, possibly due to non-microtubules associated force production via the spindle matrix and/or actin-myosin. After a few minutes being stretched, all the kinetochores oriented to one pole detach and move rapidly toward the opposite pole, at speeds averaging 38 μm/min and with instantaneous speeds up to 100 μm/min ( Fegaras and Forer, 2018a , b ). One reason that all kinetochores move to the same pole might be because this is related to non-random segregation, because all male partners segregate to one pole and all females to the other.…”

Section: Introductionmentioning

confidence: 99%

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Evidence of Non-microtubule Spindle Forces in Mesostoma ehrenbergii Spermatocytes

Fegaras-Arch

2020

Front. Mol. Biosci.

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We tested conclusions reached in previous experiments in which Mesostoma spermatocyte chromosomes moved rapidly to a pole in the absence of microtubules: after 10 µM nocodazole (NOC) depolymerized metaphase spindle microtubules, kinetochores from each of the 3 bivalents detached from the same pole and rapidly moved to the other pole, at speeds averaging 37.7 µm/min. with some as high as 100 µm/min. We concluded that these very fast movements were due to nonmicrotubule forces arising from a spindle matrix. However, since the chromosomes stretch out before detaching, there is tension in the chromosomes from the stretch. Thus the movements of detached kinetochores conceivably might be due to recoil from the tension, though we argued against this possibility (Fegaras and Forer, 2018a). In this article we test whether recoil causes the movements. We cut bivalents into 2 pieces, using a femtosecond laser, before addition of NOC. When 1 bivalent was severed, all kinetochores moved to one pole in 12/15 cells; when 2 bivalents were severed, all kinetochores moved to one pole in 4/6 cells; and when all 3 bivalents were severed all kinetochores moved to one pole in 3/9 cells. The bivalent "halves" moved rapidly, with average speeds of 47 µm/min, velocities that are not significantly different from those in cells without any laser-cut bivalents (p > 0.05). Since kinetochores move at the same speeds whether they are part of bivalents or not, NOC-induced chromosome movements are not due to recoil from tension along the full-length bivalent, strongly supporting the idea that non-microtubule forces move chromosomes in Mesostoma spermatocytes.