Plant genera Cannabis and Humulus share the same pair of well-differentiated sex chromosomes

O_LIWe recently described, in Cannabis sativa, the oldest sex chromosome system documented so far in plants. Based on our estimate of its age, we predicted that it should be shared by its sister genus Humulus, which is known to also possess XY sex chromosomes. C_LIO_LIHere, we used transcriptome sequencing of a F1 family of Humulus lupulus to identify and study the sex chromosomes in this species using the probabilistic method SEX-DETector. C_LIO_LIWe identified 265 sex-linked genes in H. lupulus, located on the chromosome that is also the C. sativa sex chromosome pair. Using phylogenies of sex-linked genes, we show that a region of these chromosomes had already stopped recombining in the common ancestor of the two species. Furthermore, as in C. sativa, Y gene expression was reduced in correlation to the position on the X chromosome, and strongly Y degenerated genes showed dosage compensation. C_LIO_LIHere we report, for the first time in the Angiosperms, a sex chromosome system that is shared by two different genera. Recombination suppression started at least 21-25 My ago, and then (either gradually or step-wise) spread to a large part of the sex chromosomes, leading to a strongly degenerated Y. C_LI