Topoiosomerase II is required for the proper separation of heterochromatic regions during female meiosis. Stacie E. Hughes¹, R. Scott Hawley^1,2. 1) Stowers Inst Med Res, Kansas City, MO; 2) Department of Molecular and Integrative Physiology, Kansas University Medical Center, Kansas City, Kansas.

   Heterochromatic regions are essential and sufficient for the segregation of achiasmate chromosomes during meiosis I in Drosophila melanogaster females. Heterochromatic threads connecting achiasmate chromosomes have been observed during prometaphase I in oocytes and may be part of the mechanism by which heterochromatin ensures proper achiasmate chromosome segregation. How these heterochromatic threads are established and resolved and the mechanism by which heterochromatin properly segregates achiasmate chromosomes are unknown. Decreasing the levels of topoisomerase II (top2) by RNAi in the later stages of female meiosis results in a defect in the separation of heterochromatic regions after spindle assembly. In many late-stage oocytes only a single large focus could be observed for fluorescent in situ hybridization probes to heterochromatic regions of all four chromosomes. In other oocytes, the heterochromatic regions were stretched into long and abnormal projections. Despite these aberrant heterochromatic configurations we could observe spindles in top2 RNAi oocytes, though some lacked tapered poles or were elongated to accommodate the DNA projections. Based on CID localization, centromeres appear to be located at the tip of these DNA projections. Finally, achiasmate chromosomes exhibit a near complete failure to move precociously towards the spindle poles during prometaphase I. These data suggests that Topoisomerase II is involved in the resolution of DNA entanglements in the heterochromatin during meiosis I. These entanglements may be part of the mechanism ensuring proper alignment and segregation of the achiasmate chromosomes and likely give rise to the heterochromatic threads observed in prometaphase I. The studies indicate that Topoisomerase II plays important roles in meiosis other than resolving replication intermediates during DNA replication, such as properly separating and orienting chromosomes during meiosis I.