Chiasmata established by recombination are normally sufficient to ensure accurate chromosome segregation during meiosis by physically interlocking homologs until anaphase I. Drosophila melanogaster female meiosis is unusual in that it is both exceptionally tolerant of nonexchange chromosomes and competent in ensuring their proper segregation. As first noted by Puro and Nokkala [Puro, J., Nokkala, S., 1977. Meiotic segregation of chromosomes in Drosophila melanogaster oocytes. A cytological approach. Chromosoma 63, 273-286], nonexchange chromosomes move precociously towards the poles following formation of a bipolar spindle. Indeed, metaphase arrest has been previously defined as the stage at which nonexchange homologs are symmetrically positioned between the main chromosome mass and the poles of the spindle. Here we use studies of both fixed images and living oocytes to show that the stage in which achiasmate chromosomes are separated from the main mass does not in fact define metaphase arrest, but rather is a component of an extended prometaphase. At the end of prometaphase, the nonexchange chromosomes retract into the main chromosome mass, which is tightly repackaged with properly co-oriented centromeres. This repackaged state is the true metaphase arrest configuration in Drosophila female meiosis.

B chromosomes are small, heterochromatic chromosomes that are transmitted in a non-Mendelian manner. We have identified a stock of Drosophila melanogaster that recently (within the last decade) acquired an average of 10 B chromosomes per fly. These B chromosomes are transmitted by both males and females and can be maintained for multiple generations in a wild-type genetic background despite the fact that they cause high levels of 4th chromosome meiotic nondisjunction in females. Most curiously, these B chromosomes are mitotically unstable, suggesting either the absence of critical chromosomal sites or the inability of the meiotic or mitotic systems to cope with many additional chromosomes. These B chromosomes also contain centromeres and are primarily composed of the heterochromatic AATAT satellite sequence. Although the AATAT sequence comprises the majority of the 4th chromosome heterochromatin, the B chromosomes lack most, if not all, 4th chromosome euchromatin. Presumably as a consequence of their heterochromatic content, these B chromosomes significantly modify position-effect variegation in two separate reporter systems, acting as enhancers of variegation in one case and suppressors in the other. The identification of B chromosomes in a genetically tractable organism like D. melanogaster will facilitate studies of chromosome evolution and the analysis of the mechanisms by which meiotic and mitotic processes cope with additional chromosomes.