Spermatogenesis in Sciara coprophila. II. Precocious chromosome orientation in meiosis II.

In the second meiotic division of spermatogenesis in Sciara coprophila the X dyad undergoes a directed nondisjunction appearing precociously at one pole. All other chromosomes behave in a normal fashion aligning on the metaphase plate and dividing. Crouse has determined that this directed nondisjunction is governed by a region of the X centromere heterochromatin known to contain the rDNA (Crouse et al., 1977; Crouse, 1979). In order to further characterize this system we have utilized DAPI c-banding and rDNA in situ hybridization to demonstrate that the precocious chromosome (X or translocation chromosome) orients at metaphase II with its centromere end juxtaposed to the pole. Even when the controlling region is not near the centromere as in the case of translocations T1 and T32, the precocious chromosome orients with the centromere end rather than the controlling region end adjacent to the pole. These data may argue that precocious positioning is established at telophase I and maintained throughout the second meiotic division. - We have examined the hypothesis that the controlling region is transcriptionally active at metaphase II and can find no evidence for this speculation. This argues that if an RNA product is related to precocious positioning it must be synthesized earlier in spermatogenesis. - An analysis of naturally occurring tetraploid spermatocytes demonstrates that the two independent precocious chromosomes of such cells are always associated with the same pole. This datum in conjunction with the observation that tetraploid primary spermatocytes have only one monopolar spindle and not two, further supports the notion that a precocious chromosome-pole interaction may be established in meiosis I and maintained throughout meiosis II.