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The multiply inverted  X  chromosome balancer  FM7  strongly suppresses, or eliminates, the occurrence of crossing over when heterozygous with a normal sequence homolog. We have utilized the LacI-GFP:  lacO  system to visualize the effects of  FM7  on meiotic pairing, synapsis, and double-strand break formation in  Drosophila  oocytes. Surprisingly, the analysis of meiotic pairing and synapsis for three  lacO  reporter couplets in  FM7/X  heterozygotes revealed they are paired and synapsed during zygotene/pachytene in 70%–80% of oocytes. Moreover, the regions defined by these  lacO  couplets undergo double-strand break formation at normal frequency. Thus, even complex aberration heterozygotes usually allow high frequencies of meiotic pairing, synapsis, and double-strand break formation in  Drosophila  oocytes. However, the frequencies of failed pairing and synapsis were still 1.5- to 2-fold higher than were observed for corresponding regions in oocytes with two normal sequence  X  chromosomes, and this effect was greatest near a breakpoint. We propose that heterozygosity for breakpoints creates a local alteration in synaptonemal complex structure that is propagated across long regions of the bivalent in a fashion analogous to chiasma interference, which also acts to suppress crossing over.

All Paired Up with No Place to Go: Pairing, Synapsis, and DSB Formation in a Balancer Heterozygote