Heterochromatin Proteins Required for Association of Achiasmate Homologous Chromosomes in Drosophila Oocytes. Christopher C. Giauque, Sharon E. Bickel. Biological Sciences, Dartmouth College, Hanover, NH.

   Physical association of homologous chromosomes throughout meiotic prophase I is essential for their bipolar orientation on the metaphase I spindle and accurate segregation during anaphase I. In most cases, homologous chromosomes undergo recombination and recombinant homologs are held together by arm cohesion along the sister chromatids. However, in Drosophila oocytes, 6-10%; of X chromosomes fail to achieve a crossover and 4th chromosomes never recombine; yet, these achiasmate chromosomes are still able to segregate correctly. Genetic and cytological analyses of achiasmate chromosomes indicate that homology-dependent interactions within their pericentric heterochromatin are required for their proper segregation. However, little is known about the role of heterochromatin proteins in this process. We have used a UAS-GAL4 strategy to knock down heterochromatin proteins in the germ line starting at stage 2 followed by FISH to monitor the pericentric heterochromatin association of achiasmate FM7a/X homologs. Defects in achiasmate homolog association increase significantly when HP1A is knocked down. In addition, reduction of HP1A (in Su(var)205⁵/+; heterozygotes) causes a small but significant increase in FM7a/X missegregation in Drosophila oocytes. We also have utilized two different RNAi hairpins to reduce the methyltransferase Su(var)3-9, which modifies H3K9 and thereby recruits HP1A to heterochromatin. For both hairpins, we observe a similar (and significant) increase in defects. Finally, we have begun to investigate a possible role for the piRNA binding protein Piwi, which is known to physically interact with HP1A and play a role in heterochromatin formation. Our preliminary data indicate that induction of a Piwi RNAi hairpin starting at stage 2 of oogenesis causes defects in FM7a/X pericentric heterochromatin interactions at all subsequent prophase I stages examined. These experiments argue that normal chromatin organization within pericentric heterochromatin is required for maintaining the association of achiasmate homologs during meiotic prophase I in Drosophila oocytes.