Mature Drosophila Meiosis I Spindles Comprise Microtubules of Mixed Polarity

New information has been obtained recently regarding microtubule organization in Xenopus extract spindles. These spindles assemble in vitro by chromatin-mediated microtubule nucleation [1] and consist of randomly interspersed long and short microtubules [2] with minus ends distributed throughout the spindle [3]. Fluorescence speckle microscopy has led to the proposal that the Xenopus steady-state spindles contain two overlapping arrays of parallel or antiparallel microtubules with differing poleward-flux velocities [4]. Although some of these features have also been reported for C. elegans female meiotic spindles [5], it is not clear whether they are representative of microtubule organization and dynamics in oocyte meiotic spindles. Here we examine anastral meiosis I spindles of live Drosophila oocytes expressing the microtubule plus end-tracking protein, EB1, fused to GFP, and find fluorescent particles throughout the spindle and movement toward both the poles and the equator. EB1 particle velocities, corresponding to microtubule growth rates, are similar in both directions, but slower than growth from the poles in mitotic spindles of early embryos. Meiosis I spindles yielded data from photo-bleaching analysis showing similar microtubule growth rates and dynamics at the poles and the equator, consistent with spindle microtubules of mixed polarity, differing from early-embryo mitotic spindles.