Résumé
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.