Acentrosomal spindles assemble from branching nucleation near chromosomes*

Microtubules are generated at centrosomes, chromosomes, and within mitotic and meiotic spindles during cell division. Whereas microtubule nucleation at the centrosome is well characterized, much remains to be learned about where, when, and how microtubules are nucleated at chromosomes. To address these questions, we reconstituted microtubule nucleation from purified chromosomes in meiotic Xenopus egg extract. We visualized microtubule nucleation at chromosomes using total internal reflection fluorescence microscopy (TIRFM) to find that chromosomes alone can indeed form spindles, and that this occurs predominantly through branching microtubule nucleation. By knocking out motor activity, we show that the organization of the resulting branched networks are consistent with a theoretical model whereby the effectors for branching nucleation are released in a gradient around chromosomes. Moreover, the microtubule nucleation rate in spindles scales with the amount of chromatin. Motors serve to self-organize these chromosomal branched networks into proper spindles, which we rationalize using a model of opposing motors.