We recently described, in Cannabis sativa, the oldest sex chromosome system documented so far in plants (12-28 Myo). Based on the estimated age, we predicted that it should be shared by its sister genus Humulus, which is known to also possess XY chromosomes.  Here, 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.  We identified 265 sex-linked genes in H. lupulus, which preferentially mapped to the C. sativa X chromosome. Using phylogenies of sex-linked genes, we showed that a region of the sex chromosomes had already stopped recombining in an ancestor of both species. Furthermore, as in C. sativa, Y-linked gene expression reduction is correlated to the position on the X chromosome, and highly Y degenerated genes showed dosage compensation.  We report, for the first time in Angiosperms, a sex chromosome system that is shared by two different genera. Thus, 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 (~70%), leading to a degenerated Y chromosome.