Heterochromatin and Toxic YChromosomes

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Y chromosomes of many organisms contain a large number of transposable elements (TEs), which are transcriptionally constrained by repressive chromatin marks. When relieved of these epigenetic modifications, many TEs can readily move from one genomic location to another (toxic Y chromosomes). Yet evolutionarily young Y chromosomes still contain a large number of essential genes that are actively transcribed, and competition between the opposing mechanisms of heterochromatin formation and active transcription can result in incomplete silencing of TEs on evolving Y chromosomes. Our proposal aims to characterize epigenetic conflicts between host-specific genes and selfish genetic elements on evolving Y chromosomes at various stages of degeneration, whose resolution may select for adaptive degeneration of the Y. The accumulation of repetitive elements on the Y chromosome appears to be universal during sex chromosome evolution. We will take advantage of evolving neo-Y chromosomes in Drosophila with varying levels of degeneration, to catalog toxicity of Y chromosomes using expression profiling and chromatin analysis. We will link epigenetic and expression profiles across neo-Y chromosomes that differ in gene and repeat density, to identify whether active transcription of genes on neo-Y chromosomes results in incomplete silencing of TEs. We will establish whether toxic Y chromosomes incur a fitness cost on males, by forming a mutational burden and reducing male longevity, as suggested by our preliminary work. Integrating our results across aims will provide a full picture of how the toxicity of the Y chromosome changes over time, and how epigenetic conflicts between host genes and selfish elements may be resolved.