Maternally transmitted Wolbachia often cause cytoplasmic incompatibility (CI), a sperm modification that kills host embryos lacking the endosymbiont. CI produces defects in paternal chromosome replication, condensation, and segregation during the first zygotic cell cycle, but a significant fraction of embryos progress normally through this and subsequent cycles and only exhibit defects at later developmental stages. These results, together with documented CI-induced epigenetic chromatin modifications, suggest heritable chromatin modifications are responsible for the developmentally delayed defects. Here, we conducted a Position-Effect Variegation (PEV) screen in Drosophila melanogaster using In(1)wm4 to test for persistent effects on heterochromatin-mediated silencing in adults that survived CI. We show that Wolbachia acts as a variegation suppressor, or Su(var), increasing eye pigment when present in CI-inducing fathers, a reproducible effect observed across several maternal genotypes that differed in CI strength. That is, passage of the In(1)wm4 through Wolbachia-infected males limits the spread of heterochromatin into the neighboring euchromatin in the progeny. This effect is consistent with disruption of heterochromatin establishment at the mid-blastula transition, when stochastic spreading of heterochromatin determines whether the displaced white gene is silenced. Surprisingly, maternal Wolbachia did not revert the PEV modification, and in one genotype, Wolbachia increased suppression. Together, our results demonstrate that Wolbachia-mediated chromatin effects persist to adulthood, are not corrected by CifA-dependent rescue, and can be compounded by maternal Wolbachia. These findings establish that rescue is incomplete at the level of heterochromatin-mediated silencing and suggest that CI-specific and constitutive Wolbachia chromatin effects may operate through at least partially independent pathways.
Hill, H. J., Sullivan, W., Cooper, B. S.
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