Monastral bipolar spindles in meiosis II of male Trichosia pubescens (Sciaridae): early stages of spindle formation and chromosome orientation.
The metaphase spindle of male meiosis II in fungus gnats (Sciaridae) is one example of naturally occurring monastral bipolar spindles. To gain further insights into how the bipolar spindle is formed in the presence of only one polar center, prometaphase of male meiosis II was investigated in the sciarid Trichosia pubescens by means of anti-tubulin immunofluorescence, DAPI chromosome staining, and electron microscopy of ultrathin serial sections. The first step in spindle formation after interkinesis seems to be the organization of an astral half-spindle, probably by MTOC activity of the astral region. With the exception of the non-disjunctional X chromosome, which always lies close to the aster, the chromosomes are found to occupy various positions with respect to the astral region, revealing different orientations of their chromatid kinetochores. It was observed that some of the mal-oriented kinetochores are associated with microtubules that, due to their orientation perpendicular to the spindle axis, are unlikely to originate from the astral region. Therefore, these mal-oriented microtubules are taken as an indication of a dispersed MTOC activity near the chromosomes or at kinetochores. According to recent models of chromosome-induced spindle self-organization [e.g., Merdes et al., 1997: J. Cell Biol. 138:953-956], they could be responsible for the formation of the other (anastral) half-spindle and for amphitelic (bipolar) orientation of the chromosomes.
Fuge
H
H
Department of Biology, University of Kaiserslautern, Kaiserslautern, Federal Republic of Germany. fuge@rhrk.uni-kl.de
D000818:Animals; D002683:Chironomidae / Q000235:genetics*; D008297:Male; D008540:Meiosis; D008870:Microtubules / Q000648:ultrastructure; D008941:Spindle Apparatus / Q000648:ultrastructure*; D014960:X Chromosome / Q000502:physiology / Q000648:ultrastructure*