Female meiosis in Drosophila is interesting and exceptional in many respects. First, like all dipterans, Drosophila has a polytrophic type of ovary characterized by the presence of nurse cells, which provide ooplasma and yolk to developing oocytes, whereas meiotic chromosomes are metabolically inactive. Meiosis in this type of ovary is characterized by the incorporation of all chromosomes into a karyosome at early meiotic prophase (1,2). Second, in addition to chiasmatic bivalents, achiasmatic chromosomes also segregate quite regularly from each other; for example, in Drosophila melanogaster the tiny fourth chromosomes are always achiasmatic in oogenesis, but nevertheless segregate regularly from each other. Segregation of achiasmatic chromosomes is regular even when the univalent chromosomes are heterologous. The phenomenon is termed "distributive segregation" and is controlled by the distributive system (for review, see refs. 3-6). To explain achiasmatic segregations, two alternative models have been put forward. One is based on the fact that in female meiosis, conventional diplotene and diakinesis stages are absent and both bivalents and achiasmatic chromosomes are incorporated in a karyosome that persists until prometaphase I. According to the model, physical association of segregating univalents, homologous or heterologous, is established at pachytene while the karyosome is formed and these associations lead to co-orientations of chromosomes on developing spindles (4,7,8). The other model assumes that univalent chromosomes remain randomly arrayed until the spindle is formed and then become organized so that a univalent orients toward the least crowded pole (4-6). Based on in situ localization of centromeric heterochromatin regions within the karyosome, Dernburg et al. (9) have presented evidence that pairing of heterochromatic regions of achiasmatic homologous chromosomes

From: Methods in Molecular Biology, vol. 247: Drosophila Cytogenetics Protocols Edited by: D. S. Henderson © Humana Press Inc., Totowa, NJ

at pachytene determines their segregation, whereas the segregation of achiasmatic heterologs is determined by a mechanism in which each chromosome orients toward the least crowded pole.

In this chapter, we describe chromosome behavior during female meiosis as revealed from fixed material and cytological methods for analyzing female meiosis in detail (7,10).

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