We describe the general occurrence in animal cells of elastic components ("tethers") that connect individual chromosomes moving to opposite poles during anaphase. Tethers, originally described in crane-fly spermatocytes, exert force on chromosome arms opposite to the direction the anaphase chromosomes move. We show that they exist in a broad range of animal cells. Thus tethers are previously unrecognised components of general mitotic mechanisms that exert force on chromosomes and they need to be accounted for in general models of mitosis in terms of forces on chromosomes and in terms of what their roles might be.

In a typical cell division, chromosomes align at the metaphase plate before anaphase commences. This is not the case in Mesostoma spermatocytes. Throughout prometaphase, the three bivalents persistently oscillate towards and away from either pole, at average speeds of 5-6 μm/min, without ever aligning at a metaphase plate. In our experiments, nocodazole (NOC) was added to prometaphase spermatocytes to depolymerize the microtubules. Traditional theories state that microtubules are the producers of force in the spindle, either by tubulin depolymerizing at the kinetochore (PacMan) or at the pole (Flux). Accordingly, if microtubules are quickly depolymerized, the chromosomes should arrest at the metaphase plate and not move. However, in 57/59 cells, at least one chromosome moved to a pole after NOC treatment, and in 52 of these cells, all three bivalents moved to the same pole. Thus, the movements are not random to one pole or other. After treatment with NOC, chromosome movement followed a consistent pattern. Bivalents stretched out towards both poles, paused, detached at one pole, and then the detached kinetochores quickly moved towards the other pole, reaching initial speeds up to more than 200 μm/min, much greater than anything previously recorded in this cell. As the NOC concentration increased, the average speeds increased and the microtubules disappeared faster. As the kinetochores approached the pole, they slowed down and eventually stopped. Similar results were obtained with colcemid treatment. Confocal immunofluorescence microscopy confirms that microtubules are not associated with moving chromosomes. Thus, these rapid chromosome movements may be due to non-microtubule spindle components such as actin-myosin or the spindle matrix.

A "precocious" cleavage furrow develops and ingresses during early prometaphase in Mesostoma ehrenbergii spermatocytes (Forer and Pickett-Heaps Eur J Cell Biol 89:607-618, 2010). In response to chromosome movements which regularly occur during prometaphase and that alter the balance of chromosomes in the two half-spindles, the precocious furrow shifts its position along the cell, moving 2-3 μm towards the half cell with fewer chromosomes (Ferraro-Gideon et al. Cell Biol Int 37:892-898, 2013). This process continues until proper segregation is achieved and the cell enters anaphase with the cleavage furrow again in the middle of the cell. At anaphase, the furrow recommences ingression. Spindle microtubules (MTs) are implicated in various furrow positioning models, and our experiments studied the responses of the precocious furrows to the absence of spindle MTs. We depolymerized spindle MTs during prometaphase using various concentrations of nocodazole (NOC) and colcemid. The expected result is that the furrow should regress and chromosomes remain in the midzone of the cell (Cassimeris et al. J Cell Sci 96:9-15, 1990). Instead, the furrows commenced ingression and all three bivalent chromosomes moved to one pole while the univalent chromosomes, that usually reside at the two poles, either remained at their poles or moved to the opposite pole along with the bivalents, as described elsewhere (Fegaras and Forer 2018). The microtubules were completely depolymerized by the drugs, as indicated by immunofluorescence staining of treated cells (Fegaras and Forer 2018), and in the absence of microtubules, the furrows often ingressed (in 33/61 cells) at a rate similar to normal anaphase ingression (~ 1 μm/min), while often simultaneously moving toward one pole. Thus, these results indicate that in the absence of anaphase and of spindle microtubules, cleavage furrows resume ingression.

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