Abstract
The Drosophila gene ald encodes the fly ortholog of mps1, a conserved kinetochore-associated protein kinase required for the meiotic and mitotic spindle assembly checkpoints. Using live imaging, we demonstrate that oocytes lacking Ald/Mps1 (hereafter referred to as Ald) protein enter anaphase I immediately upon completing spindle formation, in a fashion that does not allow sufficient time for nonexchange homologs to complete their normal partitioning to opposite half spindles. This observation can explain the heightened sensitivity of nonexchange chromosomes to the meiotic effects of hypomorphic ald alleles. In one of the first studies of the female meiotic kinetochore, we show that Ald localizes to the outer edge of meiotic kinetochores after germinal vesicle breakdown, where it is often observed to be extended well away from the chromosomes. Ald also localizes to numerous filaments throughout the oocyte. These filaments, which are not observed in mitotic cells, also contain the outer kinetochore protein kinase Polo, but not the inner kinetochore proteins Incenp or Aurora-B. These filaments polymerize during early germinal vesicle breakdown, perhaps as a means of storing excess outer kinetochore kinases during early embryonic development.
Highlights
Female meiosis in Drosophila has proved to be a useful model system for studying the mechanisms of chromosome segregation and the regulation of the meiotic cell cycle
This presence of physically associated X chromosomes on the same side of the spindle during prometaphase was first observed in fixed images from nod oocytes [4]. It was documented in ald oocytes [5] and in FM7/X wild-type oocytes. This back-and-forth movement of achiasmate bivalents is usually completed within the first several hours after germinal vesicle breakdown (GVBD) in these oocytes, after which the nonexchange chromosomes achieve a stable balance on opposite sides of the spindle
Using live imaging of female meiosis, we find that mps1 mutants do not delay the cell cycle as is normally observed in wild-type flies
Summary
Female meiosis in Drosophila has proved to be a useful model system for studying the mechanisms of chromosome segregation and the regulation of the meiotic cell cycle. While the formation of chiasmata through homologous recombination is normally necessary and sufficient to maintain pairing, Drosophila can accurately segregate achiasmate chromosomes through the distributive segregation system, and a number of genes are required for segregating nonexchange chromosomes [2] In this system, paired blocks of homologous heterochromatin take the place of chiasmata to maintain chromosomal associations until prometaphase [3]. It was documented in ald oocytes [5] and in FM7/X wild-type oocytes This back-and-forth movement of achiasmate bivalents is usually completed within the first several hours after GVBD in these oocytes, after which the nonexchange chromosomes achieve a stable balance on opposite sides of the spindle
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