Light microscope descriptions of meiosis I in males of the fungus gnat Sciara coprophila suggested the presence of a monopolar spindle in which maternal and limited chromosomes move poleward while paternal chromosomes "back away" from the pole . The ultrastructural analysis reported here, based upon serial sections of cells in different stages of meiosis I, shows that the spindle is indeed monopolar with a distinctive differentiation, the polar complex, at one pole . This complex is the focus of a conical radiation of spindle microtubules . Kinetochores of paternal chromosomes face the complex and microtubules associated with these kinetochores run toward the complex. No kinetochore microtubules were discovered on maternal or limited chromosomes. When the position of paternal, maternal, and limited chromosomes is compared at various stages, it is found that limited chromosomes always remain near the polar complex, paternal chromosomes remain far from it and only maternal chromosomes move closer to the pole . Apparently, chromosome segregation does not depend on paternal chromosomes "backing away" from the pole, and the required movement of maternal chromosomes take place in the absence of kinetochore microtubules . In the prophase nucleus, limited and maternal chromosomes are already spatially separate from paternal chromosomes before the spindle forms. Thus, the monopolar spindle functions only to increase the distance between already segregated sets of chromosomes. An extensive system of microtubule-associated membranes outlines the spindle; the possibility that maternal chromosome movement is somehow related to the presence of this membrane is discussed .The precise chromosome behavior characteristic of mitosis and meiosis is understood, at least in purely mechanical terms, as resulting from the interaction of chromosomes with a spindle of bipolar symmetry (e .g . see reference 12) . In typical meiosis I, for example, homologous maternal and paternal chromosomes pair during prophase and these pairs congress midway between the two spindle poles . At this stage, metaphase, each pair of chromosomes is linked with both spindle poles via chromosomal spindle fibers, the fiber associated with one chromosome running toward one pole, the fiber of the other running toward the opposite pole (bipolar orientation) . In anaphase, the two chromosomes of each pair separate, and each moves toward the pole to which it is linked by a spindle fiber . In other words, the twofold symmetry of the spindle and the symmetrical manner in which chromosomes engage on the spindle is fundamental for the separation of equivalent sets of chromosomes .Descriptions of monopolar spindles in several insects (for references, see 25, p . 22) challenge the generalization that the spindle is "essentially always a bipolar structure" (12) . Perhaps
Meiosis in male Sciara is unique with a single centrosome. A monopolar spindle forms in meiosis I, but a bipolar spindle forms in meiosis II. The imprinted paternal chromosomes are eliminated in meiosis I; there is non-disjunction of the X in meiosis II. Despite differences in spindle construction and chromosome behavior, both meiotic divisions are asymmetric, producing a cell and a small bud. Observations of live spermatocytes made with the LC-PolScope, differential interference contrast optics and fluorescence revealed maternal and paternal chromosome sets on the monopolar spindle in meiosis I and formation of an asymmetric monastral bipolar spindle in meiosis II where all chromosomes except the X congress to the metaphase plate. The X remains near the centrosome after meiosis I and stays with it as the spindle forms in meiosis II. Electron microscopy revealed amorphous material between the X and the centrosome. Immunofluorescence with an antibody against the checkpoint protein Mad2 stains the centromeres of the maternal X dyad in late meiosis I and in meiosis II where it fails to congress to the metaphase plate. Mad2 is also present throughout the paternal chromosomes destined for elimination in meiosis I, suggesting a possible role in chromosome imprinting. If Mad2 on the X dyad mediates a spindle checkpoint in meiosis II, it may delay metaphase to facilitate formation of the second half spindle through a non-centrosomal mechanism.
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