Giauque Christopher C, Bickel Sharon E
Department of Biological Sciences, Dartmouth College, Hanover, New Hampshire 03755.
Department of Biological Sciences, Dartmouth College, Hanover, New Hampshire 03755
Genetics. 2016 May;203(1):173-89. doi: 10.1534/genetics.115.186460. Epub 2016 Mar 16.
Accurate segregation of homologous chromosomes during meiosis depends on their ability to remain physically connected throughout prophase I. For homologs that achieve a crossover, sister chromatid cohesion distal to the chiasma keeps them attached until anaphase I. However, in Drosophila melanogaster wild-type oocytes, chromosome 4 never recombines, and the X chromosome fails to cross over in 6-10% of oocytes. Proper segregation of these achiasmate homologs relies on their pericentric heterochromatin-mediated association, but the mechanism(s) underlying this attachment remains poorly understood. Using an inducible RNA interference (RNAi) strategy combined with fluorescence in situ hybridization (FISH) to monitor centromere proximal association of the achiasmate FM7a/X homolog pair, we asked whether specific heterochromatin-associated proteins are required for the association and proper segregation of achiasmate homologs in Drosophila oocytes. When we knock down HP1a, H3K9 methytransferases, or the HP1a binding partner Piwi during mid-prophase, we observe significant disruption of pericentric heterochromatin-mediated association of FM7a/X homologs. Furthermore, for both HP1a and Piwi knockdown oocytes, transgenic coexpression of the corresponding wild-type protein is able to rescue RNAi-induced defects, but expression of a mutant protein with a single amino acid change that disrupts the HP1a-Piwi interaction is unable to do so. We show that Piwi is stably bound to numerous sites along the meiotic chromosomes, including centromere proximal regions. In addition, reduction of HP1a or Piwi during meiotic prophase induces a significant increase in FM7a/X segregation errors. We present a speculative model outlining how HP1a and Piwi could collaborate to keep achiasmate chromosomes associated in a homology-dependent manner.
减数分裂过程中同源染色体的准确分离取决于它们在整个前期I保持物理连接的能力。对于发生交叉的同源染色体，交叉点远端的姐妹染色单体黏连使它们保持连接直至后期I。然而，在野生型黑腹果蝇卵母细胞中，4号染色体从不发生重组，并且X染色体在6 - 10%的卵母细胞中未能发生交叉。这些无交叉同源染色体的正确分离依赖于它们着丝粒周围异染色质介导的关联，但这种连接的潜在机制仍知之甚少。我们采用诱导性RNA干扰（RNAi）策略并结合荧光原位杂交（FISH）来监测无交叉的FM7a/X同源染色体对着丝粒近端的关联，以探究果蝇卵母细胞中无交叉同源染色体的关联和正确分离是否需要特定的异染色质相关蛋白。当我们在前期中期敲低HP1a、H3K9甲基转移酶或HP1a结合伴侣Piwi时，我们观察到着丝粒周围异染色质介导的FM7a/X同源染色体关联出现显著破坏。此外，对于HP1a和Piwi敲低的卵母细胞，相应野生型蛋白的转基因共表达能够挽救RNAi诱导的缺陷，但表达单个氨基酸改变而破坏HP1a - Piwi相互作用的突变蛋白则无法做到。我们发现Piwi稳定地结合在减数分裂染色体的许多位点上，包括着丝粒近端区域。此外，减数分裂前期HP1a或Piwi的减少会导致FM7a/X分离错误显著增加。我们提出了一个推测性模型，概述了HP1a和Piwi如何协同作用以同源依赖性方式使无交叉染色体保持关联。
