Chromatin accessibility, not 5mC methylation covaries with partial dosage compensation in crows.

The evolution of genetic sex determination is often accompanied by degradation of the sex-limited chromosome. Male heterogametic systems have evolved convergent, epigenetic mechanisms restoring the resulting imbalance in gene dosage between diploid autosomes (AA) and the hemizygous sex chromosome (X). Female heterogametic systems (AAf Zf, AAm ZZm) tend to only show partial dosage compensation (0.5 < Zf:AAf < 1) and dosage balance (0.5<Zf:ZZm<1). The underlying mechanism remains largely elusive. Here, we quantified gene expression for a total of 15 male and female Eurasian crows (Corvus (corone) spp.) raised under common garden conditions. In addition, we characterized aspects of the regulatory epigenetic landscape quantifying chromatin accessibility (ATAC-seq) and 5mC methylation profiles. Partial dosage balance and compensation was due to female upregulation of Z-linked genes which covaried significantly with increased chromatin accessibility of the female Z chromosome. 5mC methylation was tissue and sex chromosome-specific, but unrelated to dosage. With the exception of the pseudo-autosomal region (PAR), female upregulation of gene expression was evenly spread across the Z chromosome without evidence for regional centers of epigenetic regulation, as has, for example, been suggested for the male hypermethylated region (MHM) in chicken. Our results suggest that partial dosage balance and compensation in female heterogametic systems are tightly linked to chromosome-wide, epigenetic control of the female Z chromosome mediated by differential chromatin accessibility.