Toxic Y-chromosome in D. miranda

Large portions of eukaryotic genomes consist of transposable elements (TEs), and the establishment of transcription-repressing heterochromatin during early development safeguards genome integrity. Repeat-rich Y chromosomes can act as reservoirs for TEs (toxic Y effect), and TEs may exploit vulnerabilities associated with incomplete genomic defenses during early development. Here, we contrast the dynamics of early TE activation in two Drosophila species with vastly different Y chromosomes of different age. Zygotic TE expression is elevated in male embryos relative to females in both species, mostly due to expression of Y-linked TEs. Interestingly, male-biased TE misexpression diminishes across development in D. pseudoobscura, but remains elevated in D. miranda, the species with the younger and larger neo-Y chromosome. In D. miranda, misexpression of TEs is associated with incomplete heterochromatin formation on the neo-Y and results in more de novo insertions of repeats in males compared...