Drosophila dd4 mutants reveal that γTURC is required to maintain juxtaposed half spindles in spermatocytes

The weak spindle integrity checkpoint in Drosophila spermatocytes has revealed a novel function of the gamma-tubulin ring complex (ARC) in maintaining spindle bipolarity throughout meiosis. Bipolar and bi-astral spindles could form in Drosophila mutants for dd4, the gene encoding the 91 kDa subunit of gammaTURC. However, these spindles collapsed around metaphase and began to elongate as if attempting anaphase B. The microtubules of the collapsing spindle folded back on themselves, their putative plus ends forming the focused apexes of biconical figures. Cells with such spindles were unable to undergo cytokinesis. A second type of spindle, monopolar hemi-spindles, also formed as a result of either spindle collapse at an earlier stage or failure of centrosome separation. Multiple centrosome-like bodies at the foci of hemi-spindles nucleated robust asters of microtubules in the absence of detectable gamma-tubulin. Timelapse imaging revealed these to be intermediates that developed into cones, structures that also had putative plus ends of microtubules focused at their tips. Unlike biconical figures, however, cones seemed to contain a central spindle-like structure at their apexes and undergo cytokinesis. We conclude that spermatocytes do not need astral microtubules nucleated by opposite poles to intersect in order to form a central spindle and a cleavage furrow.