S. Grava and P. Philippsen Figure 5. Nuclear distribution and dynamics in wild-type and bik1 cells. (A) Radial growth of wild type and bik1 mutant after 4 d of incubation on full medium (AFM) at 30°C. Plates containing DMSO are negative controls for benomyl plates. (B) Observation of nuclear dynamics by time-lapse microscopy in wild-type and bik1 H4-GFP hyphae. Pictures are overlays of DIC (red) and histone 4-GFP signal. The circles mark bypassing events. Numbers indicate time in min. Bars, 5 m. (C) Quantiﬁcation of nuclear oscillation and bypassing in WT and bik1 mutant. Distances between a ﬁxed point set at time t0 and the center of the nuclei were measured at each time point. The ﬁrst ﬁve nuclei shown in B were tracked. nuclei in proximity to the cell cortex. This may facilitate bypassing events of two nuclei of diameter 2–2.5 m in a 4- to 4.5- m large hypha. Bik1 Is Not Essential for Cell Growth and Only Slightly Affects the Control of Nuclear Spacing In A. gossypii, nuclei divide and migrate toward growing hyphal tips and distribute more or less evenly along hyphae with an average distance of 5 m (Gladfelter et al., 2006; Helfer and Gladfelter, 2006; Lang et al., 2010a). Interestingly, even if nuclei actively move back and forth, oscillate, and bypass each other, their overall spacing seems to be regu- lated. Previous studies have shown that the deletion of the MT motor dynein leads to strong nuclear distribution de- fects, i.e., all nuclei cluster at the hyphal tips and severely affects cell growth (Alberti-Segui et al., 2001). Because cMTs 3686 are not essential for cell growth (Alberti-Segui et al., 2001; Kohli et al., 2008), dyn1 phenotypes suggested that cell growth requires a regular nuclear spacing (nuclear clusters could block vesicle transport for example). To know whether the loss of Bik1 not only affects cMT dynamics and interactions with the cortex but also nuclear distribution, we deleted BIK1 in a strain in which nuclei were labeled with histone H4-GFP (considered here as a wild-type strain). As shown in Figure 5A, wild-type and bik1 strains had similar radial growth after 4 d at 30°C on full medium (AFM). Similar to wild type, bik1 spores were able to germinate after 7 h of incubation on AFM at 30°C and displayed a normal cell morphology. The presence of 33 M of the MT-destabilizing drug benomyl did not affect the growth rate of bik1 strains. We observed similar results with bik1 GFP-TUB1 strains (data not shown). To measure Molecular Biology of the Cell