Fig. 7. Model to explain the abnormal microtubule structures during dd4^S meiosis. Mutations in the TuRC component Dgrip91 may result in either failure of separation of the MTOCs or the collapse of a bipolar spindle shortly after its formation in prophase I (EC, early collapse). Alternatively, a bipolar spindle may be assembled, only to undergo a late collapse (LC) before anaphase I. In the first case monopolar structures, that we term hemi-spindles, form and are able to attach bivalents (blue ellipses), which, because of the lack of biorientation do not congress at metaphase. Some of these chromatin masses, located distal to the astral pole, may stabilize microtubules (green lines) with an opposite polarity relative to those nucleated by the main spindle pole (+ and - represent the putative polarity of microtubules). A structure resembling a central spindle could then form. Only these cases (cones) seem to be able to assemble a contractile ring containing Peanut and F-actin (orange hoop) and undergo cytokinesis. LCs generally result in biconical figures that are probably unable to assemble a central spindle due to a drastic loss of orientation of bivalents relative to a single pole. Consistently, Pav-KLP (yellow squares), which localizes close to the putative plus ends of the microtubules (+) in hemi-spindles, is only seen at the apex of the cones and at the vertices of biconical figures.

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