Maternal X chromosome upregulation in both sexes initiates dosage compensation evolution

When sex chromosomes evolve, Y genes become less expressed than their X counterpart. This degeneration is compensated through various mechanisms in different animal species, that reestablish ancestral expression levels in males or balance males and females. This phenomenon, called dosage compensation, has been observed for some genes in the plant Silene latifolia. However, the mechanism involved remains unknown. Using an outgroup without sex chromosomes as a reference, we show that the maternal X chromosome is hyperexpressed in both sexes in S. latifolia. This compensates for the lower Y expression in males. However, the paternal X chromosome in females maintained its ancestral expression levels, causing global hyperexpression of sex chromosomes in females, which is likely to be suboptimal. Because S. latifolia sex chromosomes have evolved fairly recently, our findings provide insights into the first steps of dosage compensation evolution, in addition to revealing a link between dosage compensation and imprinting.