Sex-Chromosome-Dependent Ageing in Female Heterogametic Methylomes

Recent research in humans and both model and non-model animals has shown that DNA methylation (DNAm), an epigenetic modification, is one of the mechanisms underlying the ageing process. DNAm-based indices predict mortality and provide valuable insights into biological ageing mechanisms. Although sex-dependent differences in lifespan are ubiquitous and sex chromosomes are thought to play an important role in sex-specific ageing, they have been largely ignored in epigenetic ageing studies. We characterised the genome-wide distribution of age-related CpG (Cytosine-phosphate-Guanine) sites from longitudinal samples in two avian species (zebra finch and jackdaw), including for the first time the avian sex chromosomes (Z and the female-specific, haploid W). In both species, we find a small fraction of the CpG sites to show age-related changes in DNAm with the majority of them being located on the haploid, female-specific W chromosome, where DNAm levels predominantly decrease with age. Age-related CpG sites were over-represented on the zebra finch but under-represented on the jackdaw Z chromosome. Our results highlight distinct age-related changes in sex chromosome DNAm compared to the rest of the genome in two avian species, suggesting this previously understudied feature of sex chromosomes may be instrumental in sex-dependent ageing. Moreover, studying the DNAm of sex chromosomes might be particularly useful in ageing research, facilitating the identification of shared (sex-dependent) age-related pathways and processes between phylogenetically diverse organisms.