Gong Wei J, McKim Kim S, Hawley R Scott
Stowers Institute for Medical Research, Kansas City, Missouri, United States of America.
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.
多重倒位的X染色体平衡体FM7与正常序列同源染色体杂合时，会强烈抑制或消除交叉互换的发生。我们利用LacI-GFP:lacO系统来观察FM7对果蝇卵母细胞减数分裂配对、联会和双链断裂形成的影响。令人惊讶的是，对FM7/X杂合子中三个lacO报告基因对的减数分裂配对和联会分析表明，在70%-80%的卵母细胞中，它们在偶线期/粗线期配对并联会。此外，这些lacO对所定义的区域以正常频率发生双链断裂形成。因此，即使是复杂的畸变杂合子通常也允许果蝇卵母细胞中减数分裂配对、联会和双链断裂形成的频率较高。然而，配对和联会失败的频率仍比具有两条正常序列X染色体的卵母细胞中相应区域观察到的频率高1.5至2倍，并且这种效应在断点附近最为明显。我们提出，断点的杂合性会导致联会复合体结构的局部改变，这种改变会以类似于交叉干扰的方式在二价体的长区域传播，交叉干扰也起到抑制交叉互换的作用。
