All paired up with no place to go: pairing, synapsis, and DSB formation in a balancer heterozygote

PLoS Genet. 2005 Nov;1(5):e67. doi: 10.1371/journal.pgen.0010067. Epub 2005 Nov 18.


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.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Chromosome Pairing*
  • Crossing Over, Genetic
  • Drosophila
  • Genes, Reporter
  • Genetic Techniques
  • Heterozygote
  • Meiosis
  • Models, Genetic
  • Mutation
  • Oocytes / metabolism
  • Rad51 Recombinase / metabolism
  • X Chromosome*


  • Rad51 Recombinase