Chromatin Remodeling and Male Killing Figure 8. The focus of the cytoplasmic asters contains centrosomal components. Fertilized (A,B) and unfertilized (C–F) eggs obtained by KOS10 females are incubated with antibodies against b-tubulin (green); Cnn and c-tubulin (red) and counterstained with Hoechst 33258 (blue). Cnn and c-tubulin are evident at the spindle poles of normal developing embryos and at the focus of the cytoplasmic asters (arrows, A–D). (E) Detail of an unfertilized oocyte at the end of meiosis with a cluster of centrosomal material within the remnant of the central aster (arrow); Pb, polar bodies. (F) Cnn aggregates are found within the cytoplasm (arrows) and around the meiotic-like spindles (F, arrows), but not at their poles (inset F, arrowheads), despite the presence of microtubule asters and associated bacteria. Bar is 25 mm in C and 4 mm in all other panels. doi:10.1371/journal.pone.0030045.g008 Chromosome condensation and segregation during cell division is mainly dependent on the regulation of chromatin organization [47,48]. Defective chromatin remodeling of the X chromosome might lead to other global effects such as improper replication and tangling of daughter DNA strands, ultimately causing failure of chromatid separation and bridging at anaphase/telophase result- ing in abnormalities of the whole spindle. Accordingly, widespread mitotic defects leading to death of hybrid females produced from Drosophila simulans mothers and Drosophila melanogaster fathers are induced by separation failure of the paternally inherited X chromatids that are improperly packed as heterochromatin in the maternal background [49]. The improper condensation of the paternal chromosomes contributes to the chromatin bridging responsible for embryo death during the cytoplasmic incompat- ibility crosses [3,50–53]. Therefore, the findings of a close relationship between the male-killing phenotype and chromatin remodeling may point to a similar mechanism for male-killing and cytoplasmic incompatibility, suggesting that cytoplasmic incom- patibility and male-killing may involve similar targets or may follow a similar pathway as in the Drosophila system. This is supported by the finding that the transfer of some Wolbachia strains between Drosophila species results in a transition from cytoplasmic incompatibility to male-killing, indicating that these Wolbachia induced phenotypes share common molecular mechanisms [54]. Although our results on Drosophila bifasciata suggest a strong correlation between the male killer Wolbachia and chromatin remodeling in male embryos, a critical issue is how Wolbachia might affect chromatin remodeling. Nucleosomes are pivotal in establishing the architecture of chromosomes. Interactions be- tween nucleosomes result in the formation of fibers that permit further packaging and a more compact chromosomal structure [55,56]. This process is mainly dependent on linker histones that stabilize the interaction between nucleosomes and chromatin fibers [57]. Histone-modifying enzymes or chromatin-remodeling complexes can be targeted to specific promoters by gene-specific or general transcription factors, Wolbachia may interfere with any of the transcription pathways that regulate some of these processes, thus locally altering the structure or positioning of nucleosomes, histone-modifying enzymes or chromatin-remodeling enzymes. Accordingly, it has been demonstrated that Wolbachia-mediated cytoplasmic incompatibility is associated with impaired histone deposition in the male pronucleus [58]. PLoS ONE | www.plosone.org 11 January 2012 | Volume 7 | Issue 1 | e30045