Precocious cleavage furrows simultaneously move and ingress when kinetochore microtubules are depolymerized in Mesostoma ehrenbergii spermatocytes.

A “precocious” cleavage furrow develops and ingresses during early prometaphase in Mesostoma ehrenbergii spermatocytes (Forer and Pickett-Heaps, 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 (FerraroGideon et al. 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 MTs are implicated in various furrow positioning models and our experiments studied the responses of the precocious furrows to the absence of spindle microtubules (MTs). We depolymerized spindle MTs during prometaphase using various concentrations of nocodazole (NOC) and colcemid. The expected result is the furrow should regress and chromosomes remain in the midzone of the cell (Cassimeris et al. 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.