FIGURE 2:

Nuclear movements in Ashbya gossypii. (A) Visualization of long-range nuclear migration and nuclear dynamics in hyphae of A. gossypii. Two merged differential interference contrast and fluorescence images of hyphae with histone H4-GFP–marked nuclei of Supplemental Video S1 were selected to show the efficient polar growth and maintenance of nuclear density. Bar, 10 µm. (B) Qualitative model of nuclear migration control in a hypha growing from left to right, which causes a cytoplasmic flow (arrow) in the growth direction. The indicated nuclei do not necessarily represent the apicalmost six nuclei. Nuclei are autonomous with respect to movements and mitosis (Alberti-Segui et al., 2001; Gladfelter et al., 2006; Lang et al., 2010b). Each SPB (yellow dots) nucleates three cMTs (green lines) based on electron tomography (Gibeaux et al., 2012). Very long cMTs can form because the dynamic cMT parameters are adapted to the very long cells (Grava and Philippsen, 2010; Lang et al., 2010b). As in S. cerevisiae, dynein localizes at the growing end of cMTs (Grava et al., 2011) and Num1 patches form at the cortex (Schmitz and Landmann, unpublished data). Therefore dynein can be captured at the cortex by Num1, which initiates pulling on the cMT, thereby moving the nucleus, essentially as described for pulling of the anaphase spindle in S. cerevisiae by the dynein off-loading model (Lee et al., 2003, 2005; Sheeman et al., 2003; Markus and Lee, 2011; Lammers and Markus, 2015). The cMT continues to grow during the pulling (Grava and Philippsen, 2010) until it eventually starts depolymerizing if its plus end undergoes a catastrophe. Nucleus 1 is pulled backward and nucleus 2 forward. The distances pulled are sufficiently long to cause nuclear bypassing. Next nucleus 1 stops moving due to balance of backward and forward pulling forces, whereas nucleus 2 continues to move forward. Nucleus 3 migrates with the cytoplasmic flow. No cortical contact of cMTs. Nucleus 4 first moves forward and then backward as the mitotic nucleus is pulled. Up to a certain length, elongating spindles can assume any angle with respect to the growth axis (Alberti-Segui et al., 2001; Grava et al., 2011). Nucleus 5 is immobile first and then moves backward because the backward-oriented cMT elongated and is pulled. Nucleus 6 moves forward and switches to backward movement because the forward-oriented cMT shrinks and the backward-oriented cMT grows, changing the balance of forces.