Genomic imprinting mediates dosage compensation in a young plant XY system

This preprint has been reviewed and recommended by Peer Community In Evolutionary Biology ( http://dx.doi.org/10.24072/pci.evolbiol.100044 ). Sex chromosomes have repeatedly evolved from a pair of autosomes 1 . Consequently, X and Y chromosomes initially have similar gene content, but ongoing Y degeneration leads to reduced Y gene expression and eventual Y gene loss. The resulting imbalance in gene expression between Y genes and the rest of the genome is expected to reduce male fitness, especially when protein networks have components from both autosomes and sex chromosomes. A diverse set of dosage compensating mechanisms that alleviates these negative effects has been described in animals 2–4 . However, the early steps in the evolution of dosage compensation remain unknown and dosage compensation is poorly understood in plants 5 . Here we show a novel dosage compensation mechanism in the evolutionarily young XY sex determination system of the plant Silene latifolia . Genomic imprinting results in higher expression from the maternal X chromosome in both males and females. This compensates for reduced Y expression in males but results in X overexpression in females and may be detrimental. It could represent a transient early stage in the evolution of dosage compensation. Our finding has striking resemblance to the first stage proposed by Ohno for the evolution of X inactivation in mammals.