Microtubules are unidirectional from the last nucleus to the hyphal tip. (A) Cartoon depicting the development of a uninucleate A. nidulans spore into a mature multinucleate (in red) hypha, containing unidirectional microtubule arrays (in light and dark blue) from the last nucleus to the growing tip. Microtubule polarity is indicated by plus and minus symbols. (B) Microtubule plus ends are polarized toward the hyphal tip in wild-type hyphae. Microtubules are visualized with GFP–α-tubulin (top), microtubule plus ends with EB1-mCherry, and nuclei with histone H1–mCherry (middle). The bottom image is a merged image of the top and middle images. Dashed lines indicate the outline of the hyphae. Bar, 5 µm. (C) Bar graph showing the percentage of microtubules (±SEM) with plus ends oriented toward the hyphal tip. Only microtubules between the hyphal tip and the most proximal nucleus were analyzed. For each strain, >60 independent hyphae and >700 EB1 comets were analyzed.